WMG / Undertale

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     Pre-Game/Demo Guesses 

Toriel is the former Queen of the monsters

We know there are "races" of monsters, because you can exhaust all the monsters in an area by killing them all. The silhouette of the king of the monsters is bipedal, has feet like Toriel, and horns, and is bigger than other monsters, just like her; so is probably another of her race. The monsters all attack you, even when they don't want to, and some plan is in the offing that needs the souls of human children. The only place where human children come from is through these ruins, and the one overseeing the ruins area is Toriel. Toriel has been acting deliberately to prevent the childrens-souls plan from happening, but the monsters don't do anything about it; in fact, they're intimidated by her, and she's a very powerful and skilled spellcaster.

So the theory is: during the war, Toriel is wife of the king of the monsters. Bad things happen, and the monsters get kicked downstairs. The king hatches a plan to get out, involving the souls of human kids. Toriel opposes this, so volunteers to be the one to oversee the ruins and entice children in by pretending to be nice. The kicker is that she actually IS nice and has simply been failing to carry through on her own desires. And if perhaps the king and queen lost a child or two in the war, it would explain both why the king has gone for such a malevolent plan after presumed ages of peace (perhaps with influence by a certain vegetative person), and why Toriel is... Toriel.
  • Absolutely correct! Toriel lives in the Ruins to tend to the humans who might drop in as a contrast to her Husband's methods.

If the above WMG is true, Asgore will be An Ice Person to contrast with Toriel's Playing with Fire.
  • Alternatively, he will also be Playing with Fire, but his Personality Powers symbolism will be the opposite of Toriel's, revolving around fire's chaotic and destructive nature rather than its nurturing warmth.
    • Jossed. Asgore IS also Playing with Fire, but has a similarly nurturing personality, with the exception of his revenge against humans (which is relatively level-headed, and partially reluctant).

If the player spares Toriel, he goes on a quest to make outside the ruins safer in hopes of convincing Toriel to leave them, as well.
  • This could lead to her reacting 3 ways, from being happy with taming said monsters (no kill run) to horrified on what the player character does (kill all monsters).
    • Jossed. Except when completing the True Pacifist run, Toriel never makes an appearance once you leave the ruins. She's only mentioned in relation to her friendship with Sans and status as the former queen.

Toriel destroys the one-way door to the outside once the player character leaves. Alternatively, if Toriel is dead, the ruins collapse with no caretaker maintaining it.
  • Confirmed! While Toriel doesn't destroy the door, she locks it from the inside.

There will be various caretakers in each part of the underworld.
  • Seems to make sense, especially when it could be possible that Papyrus/Sans could be dual caretakers of their own domain.

The red text in the bad ending of the demo and the meaning of the protagonist being "determined" and "finishing the job"
They've decided that they're going to kill all of the monsters that have been sealed underground, which effectively brings an end to the war shown in the introduction of the game.
  • Confirmed. The truth was actually way, way darker than that, but the guess is essentially correct.

New Game+ in the complete version will be an entirely different story
Flowey, now knowing that the protagonist succeeds, will do everything in his power to stop them. Assuming you didn't work together with him.
  • Jossed. Depending on whether you are on the path to the True Pacifist ending or the No Mercy ending, Flowey either gives you the key to getting the Golden Ending by telling you to visit Dr. Alphys if you spared him in the Neutral run or is killed by you in the No Mercy run and in the Neutral run, if you so choose. Either way, he does not make any conscious effort to stop your success.

The concept of determination is a meta-nod to the actual player
Determination, or variations thereof, keep popping out when you save. Flowey mentions that "it is the power to shape the world by sheer force of will." The player is actually being referred to here, as that person can be the one who can know the full outcome via walkthrough or perusing through forums. Whatever they want to do, they can do it, as long as they have the determination to reload at that critical moment and start from that point on.
  • Semi-confirmed. Determination is the power to reload and retry a critical moment, us players do have a part in it since it's us who are reloading the game and it's our determination to finish the game that is the actual source of determination, but it does not have any relation to checking walkthroughs and forums, just trying again and again and trying new approaches each time we reloaded.

Continuing the meta: In the bad path, an entity has taken control away from the player.
When you've met the conditions to simply be "determined" and look in the mirror in Toriel's house, where it would normally say "It's you!", it instead reads "It's me, <name>." It's not the player anymore, it's the protagonist. But then who is the player character, and what's the context for the player now?

Flowey: In fact... You're <name>, right? We're still inseparable, after all these years. Listen. I have a plan to become all powerful. Even more powerful than you and your stolen soul.

The "stolen soul" Flowey refers to is not just the character you play, but you. <name> is an acquaintance of Flowey's. You named the character that way, played the character that way, because you're not in control anymore. After all, you wouldn't normally kill everyone you meet, would you?
  • Confirmed. At the end of the bad path, it is revealed that the character of <name> is the Fallen Child, Asriel's adopted human sibling. They harnessed the power of your human soul and determination to come back from the dead and kill everyone. In a particularly chilling moment, if you refuse to delete your save (which would destroy the world) they say "SINCE WHEN WERE YOU THE ONE IN CONTROL?" and deletes it anyway.

The protagonist is literally a 'fallen human'.

The phrasing used for the naming screen is very precise. They didn't just fall from Mt. Ebott, they fell in another sense. And there's no real indication that it's only the player's control that gives them the ability to SAVE...

Asgore is Toriel's son.

Toriel was worried throughout the whole game that the child would end up being harmed or corrupted by the monsters that dwell beyond the door. Perhaps Flowey played a hand in Asgore's fall from grace as well. And without his loving mother's guidance, Asgore became every bit as cruel as the other monsters in the depths. This would explain why she has such smothering tendencies.
  • Nope. He's her husband. Their son died long ago.

In the final version, nonviolent play will reward the player with better armor and weapons
In the original demo, playing the Technical Pacifist and beating enemies to within an inch of their life and then sparing them allowed you to gain experience and levels ("LOVE") without killing anything. This was so that pacifist players could increase their stats and not be underpowered later on in the longer full game. However, the creator later revealed he was scrapping that mechanic since it "went against the concept of the game" (possibly hinting that ex. points and levels/"LOVE" are conceptualized in this game as some sort of evil bad karma, which may be made explicit at some point in the full game) and that he would need to "get creative" to come up with a different solution to that problem. My guess for how this will be solved is that since they can't gain stats by leveling up, in the final game pacifist players will have their low base stats compensated for by getting better weapons and armor than violent players. Magic equipment that can only be used by someone pure of heart or something like that. The Infinity+1 Sword for pacifist players may even have an effect where, a la "False Swipe" from Pokémon, it will not kill the enemy and will always leave it with 1 HP, allowing players wanting to keep a low or nonexistent kill count to fight less carefully after getting it.

     Sans and Papyrus 
Sans saying he would have killed you makes the No Mercy run canon
Sans says during his conversation with you at the hotel that he made a promise to Toriel that he'd keep you safe. If he hadn't, he would have killed you where you stood the moment you passed through the Ruins. This always struck me as out of character for Sans. Sure, he's an ultra-powerful character due to his knowledge of spacetime shenanigans and the mechanics of saving and the combat system. But he's not malicious, and seems overall to be quite a nice guy. This seems to imply to me that he knows, somehow, that Frisk/the player are dangerous. In his boss fight, he says that 'we' (leaving it to other WMG to speculate who 'we' is) had noticed timelines being erased, jumping left and right, and then, ultimately, everything ending. This obviously refers to Chara destroying the world. Even in the Pacifist run, he can see what could/will happen in the future, and that is why only his promise to Toriel stops him from pre-emptively dunking on you to stop you playing the game altogether.
  • No Mercy is 'canon' in a sense that it was 'encoded' to the game, so technically it is a possible timeline. Sans probably just observed all timelines, it didn't mean that No Mercy timeline 'must' happen. Given the meta nature of the game, the only 'canon' timeline is whatever you played.
  • However, No Mercy is a different ending to the others in that it is irreversible. Once the world is destroyed by Chara, that is it. The only way to get it back is to sell your soul to them, and that taints all future playthroughs. It is not just a single timeline that is affected by a No Mercy run, but ''all'".
  • The reason for Sans killing humans is mostly the same as the other monsters — wanting to set them free. He's just not doing it for himself.
  • It's also possible that he knows the anomaly is trying to get past Asgore. Flowey's been resetting the timeline long before Frisk got there and outright states that Sans caused him multiple resets. It's possible he thinks the anomaly is trying to get past the barrier to destroy everything. Thus killing the final human would prevent the barrier from breaking and the anomaly from getting out of the underground.

Sans doesn't actually die at the end of the No Mercy run
At the end of your fight with Sans, we see him cough up blood, and then shamble off screen. First, aren't monsters made of magic, and can't actually bleed because of that? Second, you never actually hear any kind of death noise after he goes off screen, which is a bit odd. Either Sans is faking his death... or, he could somehow not be a monster.
  • Pretty sure this is false. As seen in this video, Sans does have a death noise after he goes off screen and the player's LOVE goes up from defeating him. Also, he seems to talk to Papyrus, who is dead at this point. Not sure how he bleeds, though, especially considering that he's supposed to be a skeleton...
    • It's stated that Sans likes to drink ketchup, so that "blood" could very well be the ketchup oozing out of his body after you killed him.
  • Well... You definitely hear his body dissolve, and get the level up... But... There's something missing. You don't hear his Soul break. Maybe that's why he shuffles off-screen.
    • That's because he's not a boss monster. Only their souls hang around after their bodies die.
  • Then again, the spare sound and the defeat sound are exactly the same, and the kill counter does not go up. It's important to remember that LV is actually Level Of Violence, which is a perception choice. This means the player can increase their violence levels without having executed anyone, and in actuality as the kill count doesn't go up, the exp also didn't increase. The effect of the LV going up was likely a result of violently smashing Sans's ketchup bottle in his jacket, that he breaks out when he goes to grillbyz with you.
    • EXP does increase when you kill him. Before you fight him, it's at 50,000. After killing him, check your stats again: it's 99,999.
  • Sans might've survived, while still being a monster (although I am a sucker for a good 'Sans is a Human' theory). In the Snowed Inn, the innkeeper tells you that sleeping 'raises your HP above the maximum'. What does Sans do all day? SLEEP! If he slept enough every day since you arrived in the underground (after all, you only slept for 2 minutes, and your HP went up by 10 (at least in the Pacifist run)), then he very well could've accumulated enough extra HP to survive that attack.
  • Interestingly, your kill count doesn't increase after the Sans fight.

Sans didn't bleed
It was ketchup. The same kind he chugged back at Grillby's. That's why he mentions going back there.
  • Unlikely. One of the monsters at Grillby's talks about how monster food changes into energy as soon as you eat it, and specifically wonders what human food is like if we have to use the "toilet" afterwards. Any ketchup that Sans had drunk would have changed into energy also, so whatever that red stuff was, it couldn't have been any kind of food.
  • Then again Sans is reportedly familiar with strange foods the other monsters admit they are unfamiliar with and he is the one providing the ketchup, not grillby, because Sans is the one running the hot dog stand, and the one offering the ketchup when Grillby arrives. He also notably is always packing a bottle. The bottle itself could have shattered leading to it being splattered everywhere. ** This leads to another fun question of What the hell is Sans?

Correction, What the HECK is Sans?
I think he's clearly from the surface given Sans's intimate knowledge of things that other non-surfacers don't know about, including food, the sun, the appearance of humans. All other characters except for Undyne and Gerson don't recognize you as a human at all and they only recognize the character as a human because they've admittedly encountered humans before, which Undyne admits in her monologue, and Gerson is from the war on the surface. Sans retains a clearer memory of the timelines than Papyrus, but the pair were likely once both human, which explains their surface knowledge, and Papyrus's memory of once having skin and hair. Sans also explains intricacies about digestion to the patrons of grillbyz, one mentioning he was curious about trying human foods he heard about from Sans. He and Papyrus are also notably aware of the fact that humans have to go to the bathroom after digesting food, but Papyrus has no memory of the process. Sans also doesn't immediately die, nor do we see him dissolve on screen after his fight, the kill counter doesn't go up. As well, he pointedly makes note to the protagonist on both the true pacifist playthrough, his date with the protagonist at MTT hotel, and the genocide final fight that he doesn't come from the world of monsters, and gave up returning home when he realized to get home he'd have to kill someone. "You must really want to go home, I know the feeling buddo but sometimes it is good to just take what you have.", I gave up trying to go back a long time ago", and "Just give up, I did. You'll never see them again." This means he very well could have adopted Papyrus as his brother in order to substitute the void left behind from a family he previously lost.
  • This is possible, but it is very likely that he's seen enough anime — I mean, "Human History" — to gather how humans work. During the Playable Epilogue, optional dialogue with the main cast reveals that Alphys and Sans know each other quite well. Alphys probably knows just as much — if not more — about life on the surface, especially since she acknowledges that her tapes are not actual human history near the end of her "date." The "see them again" part could be a reference to several things: the photos in his basement (theorized to be from a pacifist ending of Flowey's, as he also has your photo from choosing not to stay with Toriel after a true reset), Dr. Gaster (inferred to be related to him), and even a human family (the basement photos just say that he's with "people you don't recognize," they could just as easily be humans as monsters.) Remember, he has a machine inferred to be a broken time machine. "Trying to go back" could be a reference to past time travel attempts.

Everyone is Sans
In Undertale, everyone is both a furry and Sans. The following is a mathematical proof of this theory. First off, we know that the verified furries include the Dreemurr family. as they themselves are goat-people. This includes Asriel, Toriel, and Asgore. Since Chara is the adoptive sibling of the Dreemurrs, this also qualifies them as a furry. Frisk also counts as one, for both hugging multiple furries and their connection to Chara. This means that Sans is also a furry, as he likes Frisk. Papyrus now qualifies as a furry, being the brother of one. Furthermore, Muffet counts as a furry for being a spider, Undyne counts as a furry for liking Papyrus, and Alphys counts as one both for her love for Undyne and her animal-people art in her home. Since everyone either likes Frisk or one of these characters, everyone in Undertale is a furry. This also means, through the transitive property of furriness, everyone is the same person (monster). Toby Fox has said on Twitter that the best ship couple is Sans, which would lead us to believe that the one person everyone is would have to be Sans. On top of that, I love the characters in Undertale and everyone loves me, even if they don't know me yet. This proves that everyone is a furry and everyone is Sans.
  • I had Sans check your math. He confirmed that everyone is Sans and a furry, but further added that he is still definitely a fictional character. Therefore, nobody actually exists. (Mind blowed)

Papyrus knows the timelines
Despite the memory troubles on Papyrus's end, the pair of them seem to also remember timelines and time shenanigans, with Papyrus very much aware that Sans pranks people through time and space, meaning he has limited teleportation or some sort of time travel. He is also aware of the true pacifist player character, but when running a genocide playthrough, Papyrus and Sans have two entirely different perceptions of the situation. Papyrus is an optimist and believes the player character is a pacifist deep down, and will faithfully protect this belief even if you kill him right then and there. Sans on the other hand, believes the player character is genocidal deep down, which is why his will to murder the true pacifist character is only tempered by Toriel's promise.

Sans is the body of the First Child re-injected with Determination.
Monsters can't handle Determination without bizarre results, and the red-heart coffin is labeled with the First Child's name despite being empty.
  • The coffin of the First Child is probably empty due to the fact that Asriel took the child's body to the surface. Also, we see the First Child later in the No Mercy run, having been resurrected due to their own determination and the player's actions.
  • Flowey implies that Toriel took the fallen child's body to the ruins to bury it.

Papyrus possesses time manipulation abilities like Sans and the Player.

Just, being Papyrus, he either neglects to make use of them, or is simply too good natured to abuse them much. He explicitly isn't trying to cause you serious harm during his boss fight, he seems to be aware of his brother making pranks across space and time, and he's very nonchalant about dying. It could very well be why he's so upbeat and hopeful, even going so far as to request things that only someone who can rewind time can accomplish in some of the neutral endings, with his man child like mannerisms being his either obfuscating stupidity or explaining just why he isn't as intimidating as he could be.

Papyrus is actually about as powerful as Sans

Similarly to the above, he'd just be too damn nice, or oblivious, to actually put any of it to use. During his only fight with you, he's either sparing you right off the bat or completely distracted, still being a very significant threat during the latter. That, and it's hard to see Papyrus actually wanting to unleash entire cans of whoopass on anyone, when he's the only monster in the game who won't even kill you (even Toriel can end you by accident). Both brothers hold back for different reasons, one being laziness and the fact he doesn't care, the other because he just cares way too much. But if given as much reason to actually put up a kill-or-be-killed fight as Sans is during the No Mercy run, if Papyrus actually, truly wanted to kill someone... it'd be different, it's hard to see him actually all-out cheating like Sans, but he'd be just as formidable an opponent.
  • Undyne mentions in her hangout with the player in certain routes that Papyrus is actually "...pretty freaking tough", but that he's too "innocent and nice" to actually survive a combat situation, seemingly supporting this theory.
  • Undyne may say that Papyrus is "...pretty freaking tough", just because even without using any special attacks, Papyrus is still tougher than a lion's share of enemies. He is one of the bosses, after all.

Papyrus's "special attack" (the one stolen by the dog) would have been one of Sans's attacks.
He is completely oblivious of this, of course, and think the attack is entirely his. Sans not only allows it but secretly orchestrated the whole thing, both as a support to his brother and as an elaborate prank.
  • Possibly confirmed, after the January patch, a notation was added to Papyrus' date if you're doing it after abandoning a No Mercy run heavily implying that he would have unleashed an attack on you, and his wording, "Blasted you to bits", suggests he would have broken out a Gaster Blaster like Sans does. Though you don't actually see it because now that the worst has passed, Sans had apparently set up the Annoying Dog to steal it again.

Sans had killed at least one human
  • The thing is, if a human is determined enough, they could SAVE and returned to the last SAVE point. Just how the monsters managed to obtain SIX human SOULs anyway? Either ALL six of them were not determined enough, or there was something that caused their determination to break and give up fighting. For example, if they kept dying during a fight no matter how many times they repeat. Sans's boss battle could easily qualify.
  • That makes his dialogue more chilling.
    "You'd be dead where you stand."
  • Not all humans can SAVE, it's just a power given to the most determined being in the Underground. Flowey had it, and it's not even inferred that the other humans had it.

Sans has killed plenty of humans before meeting you, possibly even all of them
  • Sans's warning to you in the restaurant implies that he has no issues at all with killing humans. The only reason he didn't kill you was because Toriel asked him not to. With this in mind, note that Sans guards the first sentry point after the ruins and has a camera setup in the bushes, so he would be the first to see any humans leave the ruins. Next note that Papyrus has never met a human before, even though he guards the second sentry point immediately after Sans. No human has made it past Sans.

Papyrus is the one who accidentally calls you and sings the "Wrong Number Song".
There was a tumblr post where someone figured out it was him through hacking or something, so I don't know if that was legit, but it fits because:
  • He's one of the few people who actually have your number.
  • He is the sort of person who would sing a goofy song when getting the wrong number.
  • There is speculation that that person was trying to reach Gaster, who is also a skeleton and theorized to be related to Papyrus and Sans in some way.
    • I was actually thinking the Wrong Number Song had something to do with Gaster due to its incidence being affected by the 'Fun' value, like the Gaster followers. It being Papyrus would make some sense. - The Noid
  • Confirmed... perhaps. According to the wiki, if you befriend Papyrus before triggering the Wrong Number Song, the caller id will show up as Papyrus.
  • Jossed. According to that very page, "There is a known glitch with the Wrong Number Song where the avatar of whoever was spoke to last will show up as the ID of the call. Most players have reportedly received either Papyrus or Sans." Plus, even Papyrus's writing is in his font, while the call's text isn't even in caps.

Sans didn't dodge during his boss battle
  • It was weird that he was the only enemy that can dodge your attack. He actually used teleport or some kind of his 'shortcut' technique.

In the No Mercy run, Sans really is the easiest enemy.
  • By this point Chara has such complete control and power that Sans doesn't have a chance. But their motivation isn't just complete destruction. It's also to have fun. And so you must play along until you can beat him on his terms. Once he uses his special attack and no longer has the will to fight, his game has become boring and he is destroyed.

Sans and Papyrus are actually two halves of Gaster.
  • Assuming the previous WMG about Papyrus' own time capabilities was correct, they act as the last pieces of Gaster thaat can interact with the world. When Toby's tweet about how "neither of them could fix the machine," he meant their attempts to reunite. When this failed, Sans became completely apathetic because while he could affect the world, it had no lasting effect thanks to the player. Papyrus, on the other hand, became extemely enthusiastic because all mistakes could be corrected and people could change, which could eventually lead to a reunion. His nonchalance at sans space-time pranks are further support for this. Also, the Snowdin shopkeeper mentioned that the brothers showed upout of the blue, Just as Gaster suddenly disappeared.

Alternatively, Sans and Papyrus are related to W. D. Gaster, and thus, their last names are Gaster.

This makes W. D. Gaster's name being Wingdings (Wing Dings? Wingding? Wing Ding?) make a lot more sense, as well as having Sans's beam attack being called the Gasterblaster- if his last name is also Gaster, it's named after Sans himself or possibly is an ability the entire family tree has. This also makes its appearance and its similarity to the Determination Extractor make a tad more sense- it's modeled after a type of skull, and the DT Extractor is modeled after it, not the other way around.

Flowey told Papyrus to tell you to go to the lab after the Alphys date.
Some people think that that call was Flowey pretending to be Papyrus, but since we see Papyrus' sprite and we know that Flowey has been talking to Papyrus and that Papyrus trusts him, it's more likely that Flowey just told him to call you and tell you to go see Alphys at the lab to progress the story.

Papyrus is actually fairly young.
He's extremely goofy and excitable, has an extreme obsession for puzzles, still believes in Santa Claus, is pretty nonchalant about being killed, and figures out your number by calling absolutely everyone in the Underground until he guessed right. All of these strike me at least as clues to him being a lot younger than he appears.
  • confirmed....ish??? he mentions that his username on the underland social network is coolskeleton95; if 95 is his birthyear, as would be logical, he could be anywhere between 15 and 24 depending on when exactly 201x is.
    • Not really. 201x is the year Chara arrives, not Frisk. The game proper happens an indefinite (though presumably long) time after the fact.

Sans and Papyrus are the animated remains of past fallen humans.
Two fallen humans were brothers. After dying, they willed their essences to persist without souls by combining their determination. This so taxed their minds that they fell into a deep slumber in each other's arms, and awoke long after decomposition had set in and run its course, with no memory of ever having been human in the conventional sense.

When the humans' determination was combined and redivided among them, it was divided quite unevenly. One of them took the psychological aspect of determination - the power of persistence and intentionality - and, upon waking, named himself Papyrus. The other took the timeline-modifying aspect - the power to SAVE and LOAD, or something like it - and named himself Sans. The brothers, despite their amnesia, did at least remember that they were brothers, and that they loved each other very much.

That's why Papyrus is fired up all the time, and has lofty hopes and dreams and the drive to reach them, yet is not very strong - and why Sans is one of the strongest opponents in the game, yet is lazy, impotent, and easily discouraged from really caring about anything. It's also why their eyes glow: that glow is their determination, normally visible in humans as a glint in their eyes, but plainly visible in these particular humans as a burning flame, because skeletons don't have eyeballs.
  • Jossed? Papyrus' body turned into dust after death. Human's body should have stayed after death.

Papyrus' deathtrap on the Snowdin Bridge was supposed to have a Gaster Blaster.
It's certainly the sort of thing you'd put in a stereotypical deathtrap. However, the Annoying Dog, being itself, tried to nick it, and got stuck in the rope, which is why it ended up hanging around menacingly.

Sans is Asgore's equivalent of a special agent/secret police.
Papyrus is only a royal guard in training, but Sans is the last line of defense in front of Asgore. He must have something worth recognizing.
  • Alternatively he was a royal judge or something (Underground didn't seem to have a high crime rates). He judged you even in Pacifist/Neutral runs.

  • At the start of his fight, he questions why nobody ever uses their strongest attack first, right after unleashing a massive, sadistic attack. Yet, as proven later on in the fight, that wasn't his strongest attack; if you spare him, he proceeds to trap you in an unavoidable, unsurvivable attack. If he genuinely wanted to stop you from killing him and moving on, why would he bother giving you the chance in the first place? Why not dunk on you the first chance he gets? Maybe part of him just wants it all to end; he's implied to have PTSD and severe depression, and could very well be using you — Chara — as a means to commit to it.
    • The simplest explanation is: he can't. He can only dunk you after you spare him (and possibly hug him), either because it's a point blank shot or because you get your guard down.
  • A good point about him being suicidal: If he really wanted to stop you at all costs, he could just use an attack like Flowey's in the beginning - surrounding from all sides without escape. That would make Genocide unwinnable, though...

At the end of the No Mercy run, Sans sparing you is genuine to some extent.
An extremely idealistic interpretation of it, but if you accept Sans offering you mercy, the resulting death is not entirely a spiteful trick. You're offering him a clean shot at your soul, and he at least decides to make it an instant kill.

Sans' favorite anime is Steins;Gate
We already know that human anime is (if scarcely) available underground and Papyrus is close enough to Alphys to pick some for his brother. Watching would explain how Sans was able to know what a human looks like without ever seeing one, his attempts on a make-shift time machine at home and his ability to tell how many times a time traveler died by their facial expression.
  • I can imagine Alphys trying to puzzle together how humans learned this part of their history, given its nature.
  • Funny coincidence: The Movie revolves around Okabe ceasing to exist as a side-effect of his time travel ventures, with the world going on without the memory of him. Sound like anyone we know?

Sans is trying to stop you in the No Mercy run because he's messed around with timelines before
If you think about it hard enough, some of the dialogue Sans says during the fight feels like it's implying it. "you'll keep consuming timelines over and over, until... well. hey. take it from me, kid. someday... you gotta learn when to QUIT." Maybe he used his powers to go around timelines when he was younger and stumbled across a timeline where everyone was dead. So he decided to stop jumping across timelines because he saw that one timeline. This is just my guess and it may not make much sense.

Sans doesn't believe Flowey to be as dangerous as the Fallen Child.
In the True Pacifist ending, all your friends are captured by Flowey — as demonstrated in the No Mercy run and as mentioned by Flowey himself, Sans could very easily have kicked his ass, because he has. However, he didn't break out with the Gasterblasters and bone corridors because he knew that Flowey is, at heart, a very jaded, sad, and bitter 8-year old, bored and destructive but ultimately not irredeemable, even if he has done some pretty horrible things. In past attempts, Flowey stopped at some point, even if he fought Sans, which is where he and the Fallen Child differ. By contrast, Sans goes all out against the player in the No Mercy mode precisely because the Fallen Child won't stop. It's not just monsterkind or even humanity at stake — it's the whole world, which would be destroyed once the Fallen Child reaches the surface. Stopping an Omnicidal Maniac is definitely something to get off his butt for — a bad-tempered but ultimately childish buttercup throwing a tantrum isn't enough to trigger a Bad Time for Flowey, especially since it's implied that Sans knows way more about him than he lets on.

Sans dunking you is a legitimate act of mercy
Before he offers to spare you, Sans gives a spiel about having memories of the person Frisk should be, a kindhearted individual who wouldn't harm a soul. When you accept his offer of mercy, he realizes that Frisk is fighting back, or the player is having second thoughts, so his killing them is a Mercy Kill, hoping that when they wake up, things will be reset and Frisk will be in control again.

Sans let the Annoying Dog into Papyrus' bone stash on purpose
He seemed a little too quick with the Losing Horns for it to have been coincidental, and it would totally be in-character for him to prank Papyrus like that.
  • Likely. Now, anyone who's seen Sans fight knows those lasers hurt. It's incredibly likely that Sans sets the annoying dog up to steal the blasters both to prank Papyrus and to keep him from possibly killing you because he underestimates the strength they fire with. If you abandon a No Mercy run at him, he implies he can use them and was debating using them before trying to appeal to you with kindness, only losing track of them again after it becomes clear you've abandoned that run and don't present any additional threat, meaning Sans may have set up the dog to steal them again since the worst situation has been avoided.

Sans and Papyrus are literally Helvetica crossovers.
As in, they existed in the comic's world and somehow found themselves transported to a different universe, in the underground. It explains exactly why sans won't ever see his friends and family again. Papyrus doesn't care or notice because he's Papyrus, and possibly because he wasn't a very popular person to begin with. The machine in Sans' secret room wasn't a time machine, but rather an interdimensional portal that never gets finished, due to timeline shenanigans and the general difficulty of quantum physics.

Sans is a previous child who had performed a Bad Time run.
Some of Sans' dialogue in the boss fight strongly suggests that he's a little too savvy about what you've been doing and what you'll do if you're not stopped (what better way to know what comes next than to have gone that way yourself, after all?). He even says "I quit trying to go back a long time ago" at one point, which could tie into this in that he realized he had gone too far, and tried everything he could to set everything right, only to realize that he couldn't fix that mistake. In the end, he's not just trying to stop you from destroying everything; he's trying to convince you not to make the same mistake he did.

Papyrus is the only one who had the right idea.
  • Monsters need human souls to break barrier.
  • Yet, killing Frisk doesn't help because a. The soul is destroyed as seen in game over screen and b. Frisk is determined to just come back anyway.
    • False. The writings in Waterfall explicitly state that human souls persist intact after death. The death animation seen in the game over screen is implied to be part of reloading a SAVE, since the game will immediately reload after the game over text.
  • The only solution to this is actually to capture Frisk alive, brought the child to the true lab, and extract the determination before taking their soul.
  • Papyrus is the closest monster to take Frisk's soul even if he didn't know it. I'd assume Undyne and most monsters have no idea. It's likely that Asgore also knew this, but he was very unwilling to do so. Note that he seemed to know that Frisk had determination.

Sans can stop time.
  • It would explain quite a few things about him. His shortcuts? He's just stopping time and going the normal way. And during later stages of the fight with him on a No Mercy run, his attacks start and stop randomly. Perhaps he's setting up attacks during frozen time?
    • This WMG gave me a weird idea of Sans doing Za Warudo.
    • Also consider how the music that is played when he judges you (The Choice) is literally the background music of the previous section slowed down a lot.

Sans' mercy.
He really didn't have a choice but attacking you, for several reasons:
  • There are only two reasons why the player doing genocide: To seek challenge and prove their strength or because they are curious.
    • If the player were doing it for the first reason: they wouldn't back down until they managed to kill Sans. They won't accept mercy. He knew and expected this, that's why he's prepared to dodge even while giving mercy.
    • If the player were curious: even if there was an alternate ending where Sans was giving them genuine mercy... they would still reload back to the point to see what would happen if they killed Sans. In any case, it would be pointless.
  • Sans being Sans, part of trolling the player and part of making them frustrated enough to gave up genocide. Beside it makes a good pun. 'Sans mercy'.

Sans was once a player character
He was originally a fallen human, and the player took control of him, just like he did with Frisk, and made him do a Genocide Run. This gave him a large amount of power and knowledge. But Sans was able to take enough control of his body and therefore his determination to reset everything. He was able to keep his powers and knowledge but at the cost of his human form. This explains why he knew that Frisk was not in control in the Genocide route even though he hadn't seen him kill anybody yet.

In addition to the WMG about Sans having watched Steins;Gate, Sans has also watched Puella Magi Madoka Magica.
Like Steins;Gate, Madoka Magica also has time travel-related shenanigans, specifically, Homura is in a time loop and is trying to save Madoka from her inevitable death, but fails every time.

The player can perform multiple Genocide runs, and fight Sans multiple times as well, essentially pulling a Madoka Magica. As we all know, Sans is pretty pissed off about you killing Papyrus in the Genocide run, no? In addition, if you pay attention to the fight against Sans, he seems pretty genre-savvy about what's going on...

Sans is a Canon Foreigner from EarthBound

Specifically, he's the skeletal remains of Ness. In the Halloween Hack, the timeline of Earthbound is forked between two possible futures: the one where Giygas was defeated by the chosen four in the past, and the one where the chosen four left to defeat Giygas, but never returned. At the point where those two timelines diverge, the robots are destroyed, and the first difference between those timelines is what happens to the souls of the chosen four. In the good future (ie, the ending of Earthbound) the souls return to their bodies. In the bad future, those souls disappear. The Halloween Hack implies that it's because the souls just migrated to the new timeline, but the people saying that are three of the four chosen: Ness is very noticeably missing from Dr. Andonuts' Magicant.

For whatever reason, possibly the experiments of Gaster, or just dumb luck, Ness was able to remove himself from the wreckage of the robots. And that's not as farfetched as it sounds: Undertale is very clear about what determination can do, and Ness is a powerful psychic whose first PSI power was telekinesis. Andonuts is very clear that turning the kids into robots is a horrible, gruesome surgery that would have had to strip their bodies of all living tissue, because the Phase Distorter can't bring living things through time. So what was in those robots?

Bones. When all was said and done, Ness remained bound to his physical form while his friends went on to the afterlife, even if all he was really doing was possessing his own remains and moving his limbs through telekinesis. It would explain the following:

  • Why there's no Ness in the Halloween Hack (he's in Undertale!)
  • The photo album, broken machine, weird blueprints, and badge in his secret lab. ("Fuzzy Pickles!", the Phase Distorter, Mr. Saturn's design notes, and the Franklin badge)
  • Sans' inherent laziness (just existing requires him to be constantly piloting his own skeleton, of course he doesn't want to do more work than necessary.)
  • His desire to "go back" to someplace other than the surface, which he already knows about.
  • His ability to throw the protagonist's SOUL around by waving his hand (telekinesis)
  • How insanely difficult his boss fight is (he's the protagonist of another game, of course he's a badass)
  • His understanding of time travel and diverging timelines (he's a time traveler)
  • How he teleports (Ness knows that spell)
  • His appearance (he's almost exactly posed and proportioned like Ness's clay model
  • His long-suffering wish for the protagonist to give up their evil ways (that's how he felt about Pokey)
  • Why he's so dedicated to Papyrus and being his big brother, even though they are most likely not at all related. (Ness has a younger sibling.)
  • Why he's so depressed whenever he mentions that he'll never see "them" again (the other three kids) and can never go home, and doesn't actually seem to own a phone: he's afflicted with the homesickness status effect, which can only be cured by calling his mother, which he can't do. This may be why he has such a soft spot for Toriel, even.
  • Why he rides a bicycle in the true pacifist ending, even though Papyrus has a car he could ride in and is himself incredibly lazy. (He had one as a kid, and the only other bicycle is the wrecked one in the garbage.)
  • Sans' name is only one letter off from being a Significant Anagram of Ness. That may have been done to keep the font theme going with Papyrus, or alternately, the font theme was meant to provide a plausible alternate reason for Sans' name.

Sans doesn't teleport. He has the power to stop time.
That's what his "shortcuts" are, and how he blinks out and back in during his boss fight, and how he can occasionally appear to be in two places at once but never within sight of each other (like walking down a path, passing him, and then meeting him at the end of the same path without seeing him move). He's aware of the timeline changes because of the Required Secondary Powers that go with his time-stopping, but he has no control over the timeline itself.

"Sans" is a pun, not his real name.

To be "sans (something)" means to be without that thing. Perfect for somebody who struggles not to forget the things he's given up on trying to recover.

Sans usually has more HP, DF and ATK than 1.
Think about it- everyone in Genocide Route has much smaller amount of DF and ATK than in Pacifist and Neutral Routes, which is why every boss ( except for Undyne and Sans ) goes down so easily. For example, Papyrus usually has 20 DF and 20 ATK, but on Genocide Route, he has 3 DF and 3 ATK. This is because when monsters are close to someone with extreme hatred in their soul ( such as Chara ), they are much more vulnerable to their attacks. So it may be possible that Sans is so vulnerable on Genocide Route because, after all, he is still a monster whose body works the same way as bodies of other monsters. However, if you had to fight him on Pacifist or Neutral Route, he would have higher HP, DF and ATK, but he promised Toriel that he isn't going to fight you and he only breaks that promise on Genocide Route where he has no other choice but to fight you.

Sans and Papyrus have the power to induce sadness.
They do this using their blue attacks. Not only do they make you blue, but they bring you down.

Sans actually never befriends the player.
He never seems to really care about the player. Even in the True Pacifist Ending, he seems to be interested in Toriel more than in them. A lot of his dialogues are very hostile; he even says that he would murder the player if he didn't promise Toriel that he wouldn't. Every time he wants to spend time with them, it's because he either wants to ask them mysterious questions, judge their actions, or simply threaten them. That's because Sans, knowing that the player can reset the timelines, never really wants to become friends with them. Why would he, to be honest?
  • This is somewhat of a moot point since none of the monsters befriend the player — they befriend Frisk. It's Frisk who's smiling with them in the picture in the True Ending, not you. Flowey even asks you after the True Pacifist run to leave Frisk and the others alone to live their lives in peace. Of course, it's hard to determine how much Sans is aware of the difference, if at all, but it's possible that his hostility is aimed towards the player (a.k.a. the anomaly) and in the end, he's becoming genuinely friendly towards Frisk. When he says he's rooting for "you" in the Pacifist boss battle, he's actually rooting for Frisk, and when he offers mercy in the Genocide boss battle, again he tries to appeal to Frisk, or whatever is left of them after everything they were forced to do by the player. In the latter case, he even says that he was secretly hoping they could be friends. (Of course, this might as well be a ruse.)

The monsters use a base of 13 numbers. Or any base higher than that.
Papyrus is "twelve followers away from double digits followers count". This works without a hitch once the numerical base gets high enough.

Sans' fight would be much easier on a Pacifist run.
Sans' could attacks cancel Mercy Invincibility and cause poison damage. Considering Sans' role as a judge and the poison damage being called Karma, it's possible that these attacks work on how much EXP or LOVE the players earned. The more EXP and LOVE the players have, the less time they are given for Mercy Invincibility or the more poison/Karma damage they got on hit. Since in Genocide/No Mercy route, the player has a near maximum amount of EXP and LOVE, the Mercy Invincibility is reduced to the point of non-existent, and the poison/Karma damage racked to a large amount.

In a Pacifist run, since the player gained no EXP or LOVE, the Mercy Invincibility remained as before and there would be no poison/Karma damage. Of course, the fight would be still difficult, considering the Platform Hell and Gaster Blasters.

Papyrus remembers the resets.
Flowey says it took him the longest to grow tired of Papyrus. Because he always reacted differently? Papyrus has a line where he says he'll stand around and think of a few lines to repeat over and over - like everyone else does through every reset, preventing him from being the anomalies person of interest?

At a phone call in Hotlands, Papyrus says he wants to have bishie eyes like Mettaton. Before Mettaton reveals his new form, which is lampshaded by Undyne. At a phone call at one of the bridge seed puzzles, he also mentions a phone than can turn into a jet pack - which also happens later. His line about the True Pacifist ending being the worst ending can be seen on a new light of you look at his sprite when he says it - he's smiling like he's making a joke.

Papyrus is also the only character to outright mention how to control the game - the Froggit has bracketed dialogue, implying that the dialogue about its friend skipping through its dialogue is not actually what's being said and is simply Frisk or Chara providing details on how to play the game instead, and the sign about reading signs is in brackets - something no other sign is, which also implies the sign says something different.

Hell, Papyrus references future events, and he even seems incredibly relaxed over dying, like he knows it's all temporary and will reset soon — something even Sans doesn't have. Sans only mentions the timelines in response to Frisk's response (resetting his judgement, his speeches after dying to him on Genocide, the date with him), but Papyrus just mentions it out of nowhere with no prompting.

Sans knows that Chara is going to erase the world if he loses.
Well, ignoring the fact the player can simply quit and reset before entering the throne room, but they won't get to see 'everything' everything if they do that. Otherwise, there's a good reason Sans actually takes you on in the Genocide route. He has a time machine, and, along with Flowey, knows about the multiple timelines, especially the one where the world has been utterly destroyed by you, and the one where the Souless Pacifist ending occurs. In the other neutral endings, he doesn't attack, even if Papyrus is dead. Flowey must have initiated a Genocide run and fought Sans himself, and Sans knows. He caught onto Flowey screwing with the timeline, and hopes you at least stick with the best ending.

If you beat him, that's it. Game over. Chara wins, he erases the world entirely, and corrupts any future playthroughs, preventing everyone, including Sans, from keeping their happy ending. This in turn causes Sans to pull no punches in the boss fight. Is there any good reason he says "Don't say I didn't warn you"?

Sans' Karma is actually a secret weapon Gone Horribly Right

Basically, at some point, Gaster developed a weapon in anticipation of all-out war against the humans once the barrier was broken. This weapon would basically cause the murderous intent of whoever is exposed to it to function akin to an autoimmune disorder, destroying the subject's soul from the inside. However, it could not be wielded effectively by combat personnel for obvious reasons. Sans was the only one who survived the tests because of his laziness and apathy.

Sans and the Annoying Dog are partners-in-crime of sorts.

Both of them play with the fourth wall in their own ways and have a disposition towards trolling you and Papyrus. Also, the Dog periodically makes a sleeping spot out of Sans's trash tornado.

     Gaster 
WD Gaster's name
WD stands for Wing Dings; it fits the theme with Papyrus and Sans.
  • This is pretty much certain. Not only a secret message from him is written in Wingdings, but Sans owns blueprints with strange symbols and the Ferryman warns you to not trust "the man who speaks in hands".
  • A guide on Steam about Gaster actually raises a good point. If WD stands for WingDings, what about "Gaster"? It's not a font. Unless you taker WD Gaster as "Wing DinG Aster". "Aster" is a font, as well as a flower genus.
    • One of the WMGs for Papyrus and Sans attempts to explain this with "Papyrus and Sans's last names are also Gaster."
  • Alternately, WD Gaster's full name is something like Wilbur Demitel Graham Aster, which he always shortened as G. Aster on documents. Over time, his full name was forgotten, other than by the most dedicated of followers, and was corrupted into W. D. Gaster.

Gaster's speech wasn't garbled by his fate.
Instead, he just speaks and writes some of his more shifty documents in Wingdings, a lost language so far detached from modern languages, even it's accent ends up sounding eldritch and impossible. In public, and on non-shifty documents, he'd speak in Aster, which would be a recognizeable font, and thus, understandable even through heavy accent.

Similarly, Sans and Papyrus speak with their own accents.

Gaster is just another level of meta-commentary.

The 'fun' variable wasn't accidentally broken. It was deliberately left there, "broken" in an obvious way, to attract yet another type of player — the data-miner. Instead of just addressing players who play by the rules of the code, Undertale will also address players who try to get absolutely everything, whether or not it's supposed to be accessed at all. In fact, the grey characters associated with Gaster are already pleading with these players to stop.]]

Grey Kid: Please forget about me.
Grey Kid: Please don't think about this any more.

In the end, Gaster's trail of bread crumbs will lead to a scathing indictment of those players who wouldn't give up looking for him. The game may even consider this act worse than killing everyone — you might not be killing them, but you're still warping their world in unnatural ways, after all...
  • Gaster also has an in-universe reason for not wanting to come back. One of the secret messages asks the player to imagine the world being no different without them. Gaster has learned something he cannot unlearn - that he didn't really make a difference to anyone during his life and that the world is no different without him.
  • Supporting this is that Toby leaves messages specifically for string dumps and spritesheets The first thing you see in both are in the vein of "don't look here, and seriously don't post these anywhere".

Gaster was literally replaced by Alphys on a universal scale.
The reasoning is explained here, and is as follows:

  • In one of the demo versions of the game, there exist phone conversations with a character named "Grandpa Semi" - likely a reference to so-called semi-serif fonts, alluding to his relationship with Sans. One of those phone conversations involves said character calling the player to order a pizza - something that, in the full version, Alphys will do should you reset after performing her friendship sidequest.
  • The backer who co-created So Sorry mentions "the character that would become Alphys" in a tumblr post, suggesting that the Royal Scientist started out as a considerably different person.

Effectively, the Royal Scientist was retooled from being Grandpa Semi into being Alphys... which means that Grandpa Semi had to be cut from the game, and here we are.

Gaster is Sans and Papyrus' father or brother.
After much data mining and investigations, the fans have found certain things about Gaster that point to him being a skeleton. A) His speech is in Windings, which is why the Ferryman refers to him as "The Man Who Speaks In Hands". B) The WD in his name is also a reference to Windings just as Papyrus and Sans are also named after fonts. C) He has an in-game model that can be found by fixing a "typo" in the game's code. That model resembles a skeleton that has been horribly warped by... Something.

While these evidences point to him being a Skeleton, what makes me think he is actually related to Sans and Papyrus is Sans himself. Through some Guide Dang It antics, you can get Sans' room key, which in turn can lead you to find a secret room with a broken machine under a tarp, blueprints written in a way Frisk cannot understand (possibly Wingdings) a picture of Sans and people Frisk doesn't recognize, and a picture of Frisk, Sans and the other main characters. That's a picture that only exists in one of the endings, under specific circumstances, and you can find it even before you finish the game the first time. This plus Sans' Boss Banter during a No Mercy run suggests he not only has a deep connection to Gaster, the same event that lead Gaster's Fate Worse Than Death also allowed Sans to do Offscreen Teleportation, and be a Fourth-Wall Observer.

Gaster is both of the silhouetted figures seen in the True Lab.
In the lab, the player comes across two figures with similar shapes: the figure violently rocking in the shower, and the figure that can tuck Frisk into bed and pat their head if you decide to lie down. These figures are not malicious, and seem to be very different from the amalgamates in that they do not appear to be tangible and seem to appear and vanish at will. There is a possible third encounter in the room with thick smoke. If you walk around the room before activating the wall of fans, you may encounter a solid shape in front of the middle refrigerator. Pressing z/enter to examine this object gives a dialog box to the effect of "you have encountered something 'in the shape of a man.'" This object is gone after you turn on the fans, and the refrigerator is locked (though it does move when the character looks away). We know that Gaster is drifting between dimensions, and can essentially be both anywhere and nowhere at the same time, depending on his whims. However, occasional glimpses of him ARE possible to see in game, and for him to be in the lab would be relevant considering his profession. It would also explain why these figures do not attack the player when all of the others do. Alternately...

Gaster is the person on the phone who calls you in the elevator of the True Lab just before it crashes.
The call comes very much out of nowhere, and the "voice" attached to the text is completely different than any other character's. In addition, the voice addresses YOU, the player, and NOT Frisk. We know Gaster has access to meta and the fourth wall, making it very possible for him to make such a call. Doubles as Fridge Horror with his ending message of "See you soon."
  • That voice is adult Asriel. Assuming Gaster has a voice, he seems to sound like...um...a robot?
  • But at that point in the game, adult Asriel doesn't exist yet. If anything, the use of Asriel's Voice Grunting in said phone call only further solidifies that the phone call is from Gaster: his corrupting presence is causing yet another paradox.
  • Flowey is shown to be able to use Asriel's voice grunting at the end of a No Mercy run without having become Asriel, it's entirely possible for him to have made the call using it.

The 'Wrong Number Song' call was from someone who knew Gaster.
The reason I say this is the call starts with 'Hello, is this G...' before bursting into song about how they got the wrong number. In addition, where the call takes place can be affected by the 'Fun' value in the ini file, which as a rule indicates something to do with WD Gaster, given modifying the 'Fun' value also affects the grey Gaster_Follower NPCs. - The Noid

Gaster is the second fallen human.
According to the Boat Man/Lady, Gaster is known as 'the man who talks in hands'. The weapon the second human supposedly used is the Tough Glove, which is also seen in Omega Flowey's battle. Now, the interesting part is where this tough glove is found. Namely, in the one of the boxes throughout the game, which appear to be connected by spacetime, seeing as any items that are put in a box will appear in any and all boxes throughout the game. This 'spacetime', or in other words, the very code of the game, is exactly where Gaster resides after having fallen into the Core. So if his SOUL was somehow obtained by Asgore after this incident, it would make perfect sense for Gaster to be the second human. Also notice how what is assumed to be his sprite looks more 'human' than any monster sprite in the game.
  • That would be the third. The first was Chara and the second was the girl with the toy knife and the faded ribbon, who never left the ruins.

If there is data for a Gaster fight, it's stored differently than other monster/boss encounters
This is going to be a bit meta and full of programming talk (though with effort to not use jargon and to explain things).
  • Digging through some of the code (namely, the collection of text strings used by the game), the name Gaster appears just after Asriel in what seems to be the list of bosses. All of the enemies in the game have a specific index (the game generates the index for the correct fight when you enter a battle). Asriel's is 255- the highest some programming variables can store. It's obvious that this game was made by people who were very, very good at programming (see also the way it messes with save files). Assuming the fight is there at all, it may have to be reconstructed or have to be triggered in an extremely obtuse way, due to not resembling any other boss fight in the game data-wise.

Gaster wanted to stop the resets for good
Gaster wished to break the 4th wall and stop exactly who was causing their world all this misery: You. He could only manage to escape into the code of the game before his body broke apart, but had he succeeded, there's no telling what he would've done to you.

Gaster is trying to scare Frisk away from violence
It's recently come to attention that the first Gaster encounter, in the room hidden in the corridor to Sans' telescope, seems to reduce your EXP. Considering what we know of EXP, it seems that Gaster is trying to warn Frisk, or scare them away from the No Mercy route.

Gaster split into fragments, and two of those fragments became Sans and Papyrus.
(To go with the similar WMG under their entry.)

Gaster was a third symbolic fire user.
Think for a second. Toriel is like a hearth fire, contained and soothing. Asgore is more of a bonfire, being tender, but still dangerous when pushed too far. It could be that Gaster was a fire-wielder who is like an explosion; wondrous and grand while it's there, but dangerous to be near and quickly disappearing as it scatters pieces everywhere.

Gaster's experiment was an attempt to unlock/harness the power to SAVE, and it Went Horribly Right.
Given that Alphys's tasks as royal scientist largely were made in an effort to free monsters from their imprisonment, it seems reasonable that Gaster was given a similar directive prior to the accident. Say that somehow, perhaps after the appearance of one or more of the other six humans to enter the underworld before Frisk, Gaster discovers their power to SAVE and thereby manipulate time, and attempts to replicate and harness this power. He succeeds, but without human determination, cannot control the power, and inadvertently removes himself from time. Some fragments of Gasters work live on however, providing the basis for Sans's limited time manipulation, and inspiring Alphys to try and harness determination, which succeeds in a similarly dire fashion, as seen in the game proper.

Gaster was aware of the player's ability to replay the game and take multiple paths, and intentionally erased himself from the game in an attempt to flee from a potential No Mercy run
W. D. Gaster, like many other characters, was aware of the player's ability to save, reload and reset, and purposefully wiped out his entire existence, fleeing into the game's files in an attempt to hide, fearing the fallen child might decide to take the No Mercy route and wipe out the entire world. No longer even a part of the game's universe, he would be safe from the player's reign of terror.

The "Gasterblaster" used by Sans is actually a fragment of Gaster.
The points that I believe reinforce this: They fly onto the screen from nowhere, resemble skulls, rocket away after firing, and appear in massive numbers. If Gaster split into hundreds of pieces (or more), it would make sense that there's so many. Also, how could the Gaster Follower holding what appears to be a whole face claim to be holding a single one of Gaster's countless fragments? If all the pieces are the face-like Gasterblasters. Adding to the above guess, the Gasterblasters appear during the Sans fight because Gaster's now-cosmic mind realizes how far the Fallen Child is going to go, and lends Sans his aid in the form of his own physical body's energy-spewing remains so that no other universe will have to suffer.

Gaster's erasure DOES have an effect. But no one realized it.
And the reason is simple: The Universe retconned itself because of how monumental Gaster was to the game's progression. Not only was he the royal scientist, it's possible he would've been the one to deliver Judgement in the final corridor, if the filename of Sans' laser skulls being "gasterblaster" is anything to go by.

  • Sans doesn't look like the kind of guy to fill the role of Judge because he wasn't made for it to start with. He was the NPC to explain Papyrus' nature and provide some comic relief throughout the game, same as Mettaton's fans or Monster Kid looking up to Undyne. But, the universe, seeing he fulfilled no primary purpose in any version of itself other than that, reassigned him because he was a skeleton that appeared frequently that was not already Papyrus, and after that point he didn't have much to do. With the knowledge of things he never had to think about before, he fell into an existential depression that he tries to keep hidden when he's being funny.
  • Alphys is so incompetent and nervous as the lead researcher because this wasn't the job she was hired for. She was an assistant to Gaster, content to stay his understudy, not liking the attention such a job would give her. But once again, as she was the scientist who was in the royal lab, she became the royal scientist. As all eyes were on her at all times, she developed a performance anxiety that snowballed into self-loathing and suicidal thoughts as a result of being remade to think she's qualified to do all of it.
  • And worst of all, all of the NPCs that existed to give Gaster more backstory and significance through gossip? They had no purpose BUT knowing who he was. And thus, they were thrown away along with Gaster himself, as everything else was already adequately explained when brought up. When put back into the game, they still fulfil their old role, speaking of Gaster. But they do so knowing that their very existence is trivial, and now share Gaster's desire to be left alone.

In conclusion, Gaster isn't upset because he saw he had no impact on the world. He's upset because his impact was taken from him at the expense of others.

Asgore didn't have to kill anyone. Gaster's plan was to just erase the barrier out of continuity.

It's implied, if not outright stated that Gaster was a brilliant scientist. So, it's not too odd to think that maybe he, like Toriel, was opposed to the idea of killing humans to break the barrier. He urged Asgore to leave his plan to kill humans and harvest their souls to break the barrier as a last resort Plan B, at least until he's done his research into space-time manipulation. The research created the Core, as a gateway to the fabric of spacetime, and may or may not have granted Sans the insight, which he later developed into his signature "shortcut" teleportation. Indeed, it is fully possible that Gaster intentionally dove into the Core to speed up his research, and was torn apart and scattered across spacetime as a result of either poor preparation, safety precautions, or just plain bad luck. Either way, with Gaster's death, the Plan A of deleting the barrier from "spacetime", or the game's files, was lost and scrapped. Thus, the only option left was the dreaded Plan B, which, as we know, was to kill humans who fell into the Underground, harvest their souls, and then destroy the barrier using them.

It is possible that some of Gaster's more able fragments are still running around Spacetime, trying to find either answers, the barrier's files, or just an exit back to the Underground.

  • Adding onto this, it may be through Gaster that Sans, who he seems to know, first learned about meta concepts. Gaster was researching a way to hack the game itself and program the barrier out of all timelines. He had apparently made quite a bit of progress before all evidence of his existence disappeared (perhaps he accidentally hacked himself out instead?), seeing that all you need to do to see Gaster and his "followers" is to change a few variables in the savefile and the .ini.

  • This can also explain some other matters. If we assume that Alphys and Sans were Gaster's lab assistants at the time, they may have been placed in charge of keeping Gaster safe while in the Core...and fell asleep at the wheel. Whether due to general exhaustion with Sans' low health and endurance, or sleepiness brought forth by the poor choice of watching anime at 3AM, reasoning it'll be an easy day at work tomorrow, both assistants are sleepy. Yet Gaster decides to have one of them be on hand to pull him out if the Core gets volatile - either to shake them up a little to clarify why being rested is important, or because it's crunch time, the king is demanding results, and they've got nothing yet. Bad move, doctor. Alphys/Sans falls asleep, unaware to Gaster, who desperately tries to alert his colleagues to pull him back up, and is only noticed once Sans/Alphys wakes the other one up and they both hastily pull up Gaster...or what's left of him, anyway. These events later cause Sans to leave the labs and the scientific life out of grief or fear that he'll be next to dive into the Core, and Alphys, already pressured by grief from creating the Amalgamates, gets even more piled on her shoulders, quite possibly leading her to consider suicide. note 

Speculation and guesses about Gaster's battle
We already know there's battle stats for Gaster hidden in the game's data. So here, we pull out all the stops to chart out potential boss scenarios for the good doctor.

  • Gaster's battle graphics, aside from the good doctor himself, will be exclusively blocky and plain development placeholder graphics, without the usual flair and sound effects.

  • Gaster could be fought on any route, but differs depending on which route you take. On a Neutral or Pacifist route, losing to him is impossible, as he boots you out of the battle once you reach a low enough health. But in a No Mercy run, he becomes absolutely unfair as a boss. Split-second reaction times to move the SOUL to a barely large enough safe zone from a mass of crisscrossing Gaster Blasters, attack patterns getting switched within the same attack, with each pattern overlapping the next one, and not even the otherwise present Wingdings-font warning about the next pattern is there. Not to mention randomly swapping the SOUL color mid-attack without any telegraphing for the event.

  • Gaster's battle would be very similar to Sans's, but possibly with less cheating.

Gaster is somehow related to the Lutece twins.
  • Well, who else is fragmented across space and time, erased from history, and is manipulating the universe to accomplish a goal?

Gaster is the Player.
Sans refers to the player as a 'time anomaly', which is exactly what Gaster presumably is: someone who came unstuck in time due to their experiments. After that, he drifted around unable to affect anything in the world until Frisk fell into the Underground, at which point he tried to guide Frisk, only to discover Chara (who had come unstuck in a similar way) attempting to do the same thing. Thus, either Gaster fights Chara's influence and help the monsters (Pacifist) or, resentful of having been forgotten, attempts to help Chara with their plan only to be betrayed in the end (No Mercy). Since we know Undertale is just a game, it's easy to conclude that the various No Fourth Wall comments are directed at the player, but it's not clear how this is meant to make sense in universe - unless those comments aren't actually directed at the player, but at Gaster.

People still remember Gaster...
...But they don't talk about him anymore, since it's been a very long time since the accident.

One of Gaster's followers states that it took Asgore a very long time to hire a new royal scientist because Gaster was such a tough act to follow. This is, naturally, impossible if Asgore didn't remember enough of Gaster to at least know he was a great scientist. Assuming that the Man who Speaks in Hands is Gaster, the River Person would also have to remember at least something about him.

Gaster fell in, was mourned, and had his story laid to rest. The reason he's never mentioned is because they don't think the human needs to know about the dead Royal Scientist. Keep in mind that the monsters explicitly think of Frisk as a child because of their striped shirt, and people usually don't go around telling children about people who died gruesome deaths before they were even conceived.

Gaster is pulling himself together
As of the January 2016 patch, Gaster's door in Waterfall or his followers in Hotland have a chance to appear randomly even if you haven't edited your game files. While it's possible Toby always intended for it to be like that and the dataminers jumped the gun, given Gaster's nature as something that was Dummied Out it's more likely that it's being treated from an in-universe standpoint and he's doing it because he's becoming whole again. What this means for the future is incredibly unclear.
  • Well, the Dogs sheet's holes are getting bigger. Perhaps this is a sign of Gaster's return?

About Gaster's Followers...
Now a lot of people think Gaster's Followers (The Gray Kid, and the 3 grayed out characters who talk about him) are dead. However, each of the grayed out characters are simply de-colored and slightly modified versions of characters that exist normally. It is possible that the normal characters (including Monster Kid) are Gaster's Followers; how they were normally, but changed to forget about Gaster. I posit that Gaster's Followers are simply the 'memories' of the characters when they were more tied to Gaster. They aren't dead people, they are just dead characteristics of the living people who were forced to forget it due to the incident.

Gaster was the original Final Boss.
Similarly to a few of the above WM Gs, Gaster was far more significant when he existed than he seems to realize in his current state. However, rather than being the original judge in the final corridor, he had Flowey's role instead. His presence would have weighed heavily throughout the entire game, largely through Sans and Papyrus and whispers spoken of him. Once you finally meet him in Hotland, he seems friendly if absent-minded and forgetful. The latter is a front, as in place of Alphys having Mettaton pretend to try to kill you, Gaster is ordering Mettaton to actually try to kill you, as well as giving you bad advice (in the guise of simply being absent-minded) and secretly manipulating the puzzles as well as the CORE. Once you defeat Mettaton, Gaster makes the decision to essentially sacrifice Asgore by letting you reach him and defeat him. Gaster would then appear, grabbing Asgore's soul in some kind of containment device, and absorb the six human souls just as Flowey did, resulting in... most likely a skeletal monstrosity with swarms of Gaster Blasters around it.

Only then would he engage you in battle. Despite being capable of crossing the barrier and harvesting another soul easily, he has taken an interest in your soul in particular due to you surging with determination, and is quite aware of your power to SAVE.

Gaster's motives and personality could range anywhere from a scheming Starscream to simply having monsterkind's best interests at heart, just like Asgore. Gaster is just far more ruthless and motivated than Asgore ever was, to the point where he would allow Asgore to die to see the plan through. (Of course, this distinction would be able to tell you whether or not he would deal the deathblow to Asgore if you spare him or simply move Asgore aside as he takes center stage.)

Ultimately, he started losing interest in this role as he became more and more aware of of the temporal anomaly... AKA Flowey, and became more focused on that than on breaking the barrier.

Gaster made the Temmies
Out of boredom. Why else would they speak in Semi Serif?

Gaster was Driven to Suicide

Gaster wrote the first 8 entries in the True Lab
The first 8 entries are written very formally, with proper grammar, and only focusing on the experiments. Entries 9-21 are written more like Alphys's status updates, tending to be in all-lowercase, sometimes complaining about work, sometimes talking about her personal life, and even using emojis.

Gaster was attempting a crossover.
Ties in with the "Sans is Ness" theory. Gaster was attempting to get around the barrier, rather than through it. Somewhere along the line, he became aware that the timeline of Undertale can be manipulated by someone with the right abilities, and this implies that time travel is possible.

Enter Sans, an actual time traveler, who knows very well that time travel is possible, he just doesn't know how it works. So Gaster experiments with the concept until he realizes that Undertale is a confined universe. The darkness he's talking about in Entry 17? It's basically Behind the Black on a universal scale. The goal was to leave Undertale's universe entirely and go to a different one, something that Sans-Ness would be deeply interested in doing so he can get back home. Gaster tried to bridge the gap, but a crossover requires two narrative universes having a mutual connection, but Undertale's connections to the MOTHER universe are only on its own side. Gaster effectively tried to cross a bridge that he didn't know couldn't exist and now he's lost in the game's files. But there's hope...

Gaster is the one who will return as long as his name is spoken.

The more updates that Undertale gets, more and more Gaster-related content will appear, as long as the fandom keeps speculating on who and what he is. Eventually, he'll be called back to the game proper in a patch or update, and bring all his new knowledge with him. And maybe finally we can all stop crying messily all over our keyboards and save everybody then.

The "darkness" Gaster was investigating is the real world.
He was doing some kind of experiments with time travel, which are naturally accessed in the game world through the "reset" and "save" and "load" functions. These things are naturally occurring in the world of Undertale as expressions of video game mechanics. Gaster didn't realize it, but what he was really trying to study was the reality outside of Undertale, something that he, as a video game character, couldn't actually do. He dummied himself out of the game because he was trying to reach the real world without realizing why it was completely impossible. He basically exiled himself to programming hell.

Gaster and Sans are the same person, and only Papyrus is Gaster's son.
Sans has no effect on what happens in any timeline. In the No Mercy Route he dies after finally trying to change things. In any route where Papyrus dies, he would have failed to save his son. In the Pacifist Route, the only reason things changed for the better was because [[he gave up on killing humans, and did literally anything else]]. The grey NPCs are actually Monsters who died when Sans made sure nobody knew who Gaster was. He remembers all of them, and is keeping his promise to atone for what he did. Sans is a false identity, and Gaster faked his own death after using himself as a test subject for Determination. To prevent any suffering by Papyrus, he assumed a false identity (Sans), and said he was a long-lost brother. He can't reset, but he can remove memories, and the grey NPCs are results of failed attempts to not remove every memory.

  • Building on this, it's possible he can actually remember resets like everyone else, but wipes his own memory because he chooses not to remember.

In contrast to the above, Papyrus is a Gaster fragment, while Sans is Gaster's son.
The experiment that scattered Gaster across the universe left behind a big enough piece that could interact with the world, but not big enough to be recognizable as the same person, even if they shared some qualities. Sans saw his father change into someone entirely different thanks to an accident.

Gaster is (a deconstruction of?) a third aspect of the player, in a slightly different way to the above.
Frisk is the Player Character In-Universe for that kind of a game - a Moral, Ideal(istic), Heroic character who initially gets dragged into a mess by their own personal reasons, but then gets to the bottom of things and solves everything for everyone.

Chara is the Player Character on a meta, deconstructive level, more akin to an MMORPG hero than a singleplayer RPG character - a sociopath who only cares about achieving their stated objective and increasing their stats, and will freely kill or spare NPCs based solely on their role in the game.

Gaster, on the third hand, is the player as a hacker/out-of-universe "entity", watching from another dimension and attempting to break all the game world's rules for... unknown reasons. It's possible he may ultimately be a heroic character, trying to fix the plot and save the NPC's in any number of ways, for any number of reasons - hacking in a new Asriel soul, hacking away the Barrier, hacking a bypass to the Barrier... but it's equally likely that his "shortcuts" in that regard will create a Game-Breaking Bug, thus making him a Tragic Hero that needs to be stopped.

Or even that he's doing it for no comprehensible in-universe reason, and is like Flowey but even worse, warping the world of the game itself just out to see what will happen and what he can find if he messes with the game's coding...

The lattermost option may even make the most sense, as Toby is already noted for "punishing"/identifying hackers (the Dirty Hacker ending), so it wouldn't be out of character for him to create a character that's a reflection of the player as Hacker, and place him into the game saves of people who have hacked, and then force them to fight him and confront what they've been doing, what they've become.

Gaster erased himself in a time travel accident; the creation he fell into was a time machine.

The Determination Extractor had to have existed before Alphys became the Royal Scientist, because by the time she took the job, the only source of Determination in the Underground (Chara) was already dead and gone. Gaster is the only person we know of who could have built it. However, since Alphys created Flowey, that means that the concept of a time reset couldn't have come up until after Gaster was gone. In other words, "Determination" would only have been known to Gaster as a force that can oppose the normal laws of physics.

This is important because Sans and Alphys know each other. Their dialogue in the True Pacifist ending and Sans' awareness of the Amalgamates (as seen in the phone call made outside Alphys' lab) indicates that the two of them are the "we" Sans refers to during his boss fight, observing the changes in the timelines. Alphys even states that alternate universes are a confirmed fact.

So Gaster, having somehow gained a supply of Determination, used it to overcome the Arrow of Time. He tried to go back and undo the war between humans and monsters that lead to the monsters being sealed into the underground in the first place, but rather than becoming his own ancestor or running into Hitler's Time Travel Exemption Act, Gaster ended up creating a time paradox that could only be resolved by removing him from reality altogether, and then editing the remaining reality so that his accomplishments would remain (particularly the CORE, which he invented, and possibly Sans and Papyrus, if you believe he is their father or otherwise created them), but would be put into effect by someone else. To Gaster, and to all other observers, this would imply that everything Gaster did was actually a matter of destiny, that these things would come about no matter who was available to actually do the work, and the individual workers were meaningless.

Gaster was adopted by a Temmie.
He's found in Waterfall, and now all the Temmies make the Grandpa Semi noise. Who's to say? Who's to say?

Gaster was trying to circumvent the barrier by teleportation.
As the royal scientist Gaster was tasked by Asgore to find a way around the barrier that didn't involve stealing souls. The answer her eventually found was not to break the barrier but simply to never have to deal with it at all, his idea was to fold space and completely avoid the barrier. Put simply one theory of teleportation and FTL travel is that space is flexible and with the right math you can force two places in it to intersect and share the same space then when they snap back to normal you end up in the place that was sharing it's space with where you started. Gaster figured if you couldn't go around the barrier or through it then the best solution was to make outside and the underground the same place long enough for everyone to leave so the barrier wouldn't be able to stop them.

Together with his highly intelligent lab assistant Papyrus he stared doing the math and figuring out exactly what sort of power and math would be needed which lead to the construction of the core, with it's electrical generators and massive banks of computers. Then they just needed the help of Papyrus' brother Sans, who while somewhat lazy was brilliant as well, to design the pod that would eventually carry a monster through and out into the rest of the world. When it was finally ready Gaster volunteered to test it on the grounds that if it hadn't worked he couldn't contribute more to the underground anyway. Sans helped him get set up in the core and stayed to observe the test while Papyrus kept track of the effects from the laboratory in the waterfall region where hydro electric power was plentiful and any failure of the core wouldn't compromise data gathering.

The device itself worked, but Gaster didn't account for something in the barrier; it was not simply designed to keep monsters in but to make sure that every monster on earth was in the underground so no resistance up above would exist. So when he was outside, the barrier tried to rectify it by placing him back underground, but at the same time as he was outside, he was also underground because for a few brief moments the two places were one in the same. The barrier, being ancient magic from before the time of any form of quantum physics, basically blue screened trying to figure out how to handle a situation it hadn't been built to comprehend, and moved Gaster "nowhere", as in physically out of existence, while technically not killing him. Not on the surface, fulfilling the needs of the magic, but also nowhere in the underground. Everywhere and also nowhere at the same time.

Sans having been observing the test in person, was not affected by the universe wide memory wipe; the core being the location of the event was also untouched. Everyone else had their Gaster related memories wiped baring omnipotent beings, those whose existence depended on Gaster's in some form were erased from the timeline (that's the gaster_followers), Papyrus had most of his memory wiped because of how much was devoted to things he'd learned from or directly relating to Gaster, which left only a love of the unknown/unexplained in the form of "puzzles" but none of the scientific knowledge to go with it. Alphys, the second choice for royal scientist, was retconned into the position and ended up credited with everything Gaster had done. The Core became the underground's electrical plant, the computers became the monster internet, and the pod ended up in Sans workshop, where for a time he tried to figure out how to undo the accident, but having been exposed to the entire multiverse as a side effect of witnessing Gaster's demise he was constantly bombarded with knowledge of every version of himself failing and gave up, falling into a deep lethargic depression. The only thing keeping him going was a need to take care of his mostly lobotomized brother and knowledge from other versions of himself on what would happen if he died.

So in short, Gaster got splattered across the multiverse, Alphys got retconed into fame and noterioty to fill the gaps where the barrier hadn't been able to erase, Papyrus got magically lobotomized, and Sans became a universal constant sharing knowledge of every possible version of reality.

Depending on which of the above WM Gs get used, even if elements are taken from them, WD Gaster will be a huge Knight of Cerebus
  • To the point that even Sans will speak in proper capitalization and doesn't go back until Gaster is destroyed once and for all.

Gaster is the player in the Pacifist and Neutral Run
Similar to another guess up above, Gaster exists as an entity much like Chara: a disembodied soul who guides Frisk along whichever path they desire. However, the evidence for Gaster being the one in control in Pacifist and Neutral runs is the "Gaster NP Cs". The River Person warns of "The man who speaks in hands". This is believed to refer to Gaster, because he speaks in Wing Ding font, which includes hands.

But we "speak in hands" when we play Undertale - the keyboard and mouse are how we interact with the world and guide Frisk, and both require the use of our hands, so we "speak" through Frisk with our hands.

The third Gaster follower also mentions "it's rude to talk about someone who is listening" before disappearing. Aside from Frisk, we are the ones who are listening to the follower. As a result, we become Gaster in Neutral and Pacifist.

But in Genocide, we are Frisk. One could say we are both Chara and Frisk, but Chara "separates" from us at the end when they revive, leaving us as Frisk, and victim to the consequences of our actions.

On the other hand, Gaster remains to guide Frisk in Neutral, and lets go his influence when Frisk confronts Asriel in "Pacifist Run", where Frisk is in control and free to speak his real name.

     Other Characters 

Asriel/Flowey attempts suicide again in the True Pacifist Ending, and succeeds
Or, in the very least, he repeatedly attempts it until he succeeds. Assuming the timeline never gets reset. Let's be honest. Considering the alternative is a Fate Worse Than Death, it's the happiest ending you can possibly think of for the poor guy.
  • Simply rebooting the game will cause him to reappear to you, no reset needed.
    • Or a more Optimistic theory, the fact you can't kill him conforms that Toby intends to bring him back at some point.

In the sequel, you play as an adult Frisk who is hell bent on recovering Asriel
He asks Frisk to forget about him, but how can they? After making it to the surface and growing up, they discover a way to restore Asriel, and once they do, they return to the underworld for the express purpose of rescuing their adoptive brother. They, however, discover other creatures that need to be rescued too.

Alphys is the top and Undyne is the bottom.
What else do you expect from Alphys and Undyne, whose name starts from U?

There is more to the River Person than one initially thinks.
It's already obvious the River Person is very knowledgeable of the world they're in, as they know Temmie Village's location among other obscure things. But most prominently, they know about Gaster, who doesn't exist. Nearly everyone that knew about Gaster was retconned by the universe, and even Asgore who is absolutely important to the plot, but still had memories of Gaster, got his memory wiped. The only person with any recollection of Gaster's existence is the River Person, and the universe did not retcon their existence, nor erase their memories for some reason.

Asriel's feelings for the Player/First Child were more than just friendship
Regardless of what gender you believe your character to be, it seems that Asriel was hurt far more by the Player's death than anybody else. True, the Player wasn't related to the king and queen by blood, but Asriel was not their blood relative either. But all the events of the game, all of Asriel's actions stemmed from a desire to be with the person he cared about more than anyone else in the world. More than his mother, more than his father. That is much more than mere platonic love.

Undyne is maybe the only successful Determination experiment.

Consider how, provided you don't keep yourself from killing Undyne, she and her undying form will die in a manner unique to her alone: melting, after keeping herself alive through sheer willpower. Consider that only the player and Flowey have had Determination, and yet neither one of them is a monster. It would be surprising that she would be alright after she is mortally wounded... if it weren't for the True Lab. In the True Lab we see numerous Amalgamations that, while not inherently more dangerous than the normal monsters, they ARE all melted and melded together. That's the key word, too: Melted. While all the Determination experiments were on death's doorstep, their proximity to other determination-treated monsters probably made them unstable and caused them to melt. This is not the case with Undyne: while she might have escaped from the True Lab, she didn't have prolonged exposure to other Determination-treated monsters, and as a result, didn't react. That is, until she is brought to death's doorstep by the player, in which their determination goes into one last clash before she breaks down, melts, and dies. In No Mercy, she takes a huge amount of damage in one swing, and while she's starting to dissolve, she stops herself and becomes The Undying, again, in an attempt to take you out with her. However, this time she has no fatigue to her, and that means she still has the energy to mop the floor with you while biding time to get everyone to safety.
  • Alternatively, Undyne managed to create her own Determination. She definitely had the right character for it, and the points where she melts are definitely in the right place for her to start doing it.
  • That raises a significant question of why Alphys would allow Undyne to walk around casually if everyone should know that she's dead. If she was never dead then this raises the question of why Alphys performed Determination experiments on a live subject, and considering it didn't fail, why she didn't keep trying that.
    • It's possible that she was a different kind of Determination experiment. Perhaps before injecting Determination into Corpses, she injected a different substance into Monsters to see if they could produce their own determination. Undyne already had the personality for it, after all. And if you kill her in the Neutral route, she says something about "this is why I couldn't tell Alphys...", just before she gets melty.

Each of the human souls used one of the weapons and armors you find, and died roughly where they are found. Each soul had a certain trait.
Each of the human souls in the Omega Flowey fight uses attacks that look like one of your weapons, and each weapon is found near an armor which is thematically linked to it. Furthermore, the mechanics tied to each color and the text on the colored flags in the golf minigame suggest certain traits each soul had (which seem to match the items).
  • The light blue soul owned the toy knife and the ribbon, suggesting a young child. It had patience, since light blue attacks are avoided by staying still and the light blue flag talks about being patient. Its items are found early on so it must have never left the Ruins (either choosing to stay there, or because staying still wouldn't work against those enemies).
  • The orange soul owned the tough glove and manly bandanna. It was headstrong and always moved forward. Orange attacks are dodged by moving and the orange flag talks about rushing "fists-first trough obstacles", which ties in nicely with its equipment. This human died in or around Snowdin, which makes sense since every one of the dogs has an attack that you cannot avoid unless you stop moving at the right time.
  • The darker blue soul belonged to a ballerina, as clearly evidenced by the ballet shoes and the tutu. The blue flag talks about gracefully hopping and twirling your way to victory. When your heart turns this shade of blue it becomes affected by gravity, forcing you to jump over obstacles.
    • Also, she seems to have been a very violent young girl, judging from the dust that accumulated on her tutu. Was the dust from age or was it monster "blood"?
    • Lending further to that, the descriptions for both items "jokingly" suggest they are quite deadly/protective, which no other items in the game do.
    • Also also, you see the dark blue gravity effect more than once, but only on a No Mercy route when you'd be as or more violent than the soul was implied to be.
      • Adding to the blue soul's violence, it might well have been Sans (or his ancestor) who dunked on her, just like you in No Mercy...
    • This soul died in Waterfall, where several enemies have attacks that are nearly impossible to dodge if you are at the bottom of the screen.
  • The purple soul owned the torn notebook and foggy glasses. These items have low stats but allow extra invincibility, suggesting another noncombatant. The purple flag talks about taking notes to achieve victory despite being trapped, and when your heart turns purple you are trapped on three lines and need to dodge attacks.
    • Adding on to this, if they didn't get killed by Gerson like guessed below, Waterfall would still probably be where they died. They clearly weren't violent, and of the enemies in the area, one can only be spared by flexing (Hollywood Nerd, maybe?), one requires running into the bullets, and one is Temmie.
    • Alternately, you find their items immediately before the two dark rooms. Perhaps this studious, methodical soul couldn't take notes very well when they could not see.
  • The green soul was likely a cook; it owned the frying pan and an apron. The green flag talks about care and concern, and green attacks heal you (and often help end combat) throughout the game. But a green heart also lets you defend yourself from attacks with a shield. Between these useful abilities and the equipment's high stats, the green soul made it to Hotland before perishing.
    • The green heart compared with the frying pan... this definitely brings to mind a more defensive person, blocking attacks with a frying pan repurposed as a shield...
    • Since the orange attacks in this area require taking action, it's likely they died trying to defend when they should have acted.
  • The yellow soul was a gunslinger of some kind, and owned the empty gun (presumably loaded at one point) and the cowboy hat. Its trait was accuracy, since the yellow flag talks about accuracy and the gun is ineffective if you don't hit the center of the target. When your heart turns yellow you can shoot bullets from it.
    • This combat-ready soul may have made it all the way to the end of Hotland before dying. (Note that while much of Catty and Bratty's merchandise comes from Waterfall, Catty says she found the gun in "a dumpster" rather than the junkyard).
    • On the other hand, similar to the dark blue soul, the yellow soul wouldn't be very good at defending against attacks coming from below (seeing as they would have been firing bullets upwards). It wouldn't be surprising if they died in Waterfall as well for that reason.

However, what does this imply for the SOUL-changing effects, like Papyrus' blue attack or the cell phone's yellow button? Perhaps the monsters who killed the earlier humans inadvertently absorbed part of their souls?

Gerson killed, or helped kill, the purple human
This builds off of the above theory that the weapons and armor were owned by the six human souls. Gerson, the shopkeeper in Waterfall, has the items belonging to the purple soul. How did he get them? By capturing that human of course!
  • Gerson claims, when pressed, that he is "not a hero" and "never was", despite obvious evidence that he was the previous generation's equivalent to Undyne. Why? Perhaps he did something he deeply regrets.
  • Judging by the item's stats and general nature this was probably one of the least combat-ready of the six, and may not have even attacked a single monster—the notebook has the lowest ATK of any weapon except the stick. Gerson also says, if you ask him about Fate in the No Mercy route, that he never approved of the king's plan to attack the humans and felt betrayed when the king decided to pursue it. Both of these sound like good reasons for him to regret any part he might have had in killing that human.
  • Judging by that human's trait (Perseverance), the human may have heard the plan for breaking the barrier and volunteered in the hope that it would dissuade Asgore from killing the humans, possibly having taken notes that Asgore was against his past-self's opinion on doing so anyway, and just needed to be shocked into giving up like he was shocked into declaring war. Chances are, this human asked Gerson to do it to avoid putting the burden of taking a life on a Monster that didn't yet have it. Gerson, being physically weak from old age and mentally paralyzed from reliving his past, wouldn't have hit hard enough to kill the human, meaning the human died slowly and painfully, all the while telling Gerson that both of them can do this. The human would have finally died after exiting the shop so that Gerson would not have to see them die. Gerson isn't sparing Frisk out of cowardice. He's traumatized from being an accessory to suicide.

Mettaton killed, or helped kill, the yellow human
Admittedly somewhat inspired by the above. Based on how violent the yellow soul was (they had a real gun, for crying outloud!), they likely did some equivalent to a No-Mercy run. That means by the time they reached Hotland, their stats would have been through the roof! In theory, most enemies shouldn't have gotten a single hit off on them. Mettaton, though, had almost unlimited defense. Additionally, while its clear Alphys was lying about Mettaton becoming a crazy killer robot, he does have some impressive attack capabilities. Plus, Alphys and Mettaton clearly don't hate humans, but who wouldn't try to kill a serial killer rampaging through the kingdom? Plus, Mettaton wouldn't want to lose half his fanbase.

Plus, thematically, Mettaton EX (or Mettaton NEO) is the last enemy a player would realistically equip the Cowboy Hat and Empty Gun against, since the Worn Knife and Locket are right after him. How fitting an end for those items.
  • Why didn't they just keep the box to use against you on genocide, then, since it worked the last time a trigger-happy murderchild went on a spree?
  • It's possible that the kid was killing anything in his way, but not actively pursuing enemies. In Genocide you effectively have to go hunt down every last monster left alive, but if you're only doing it in self-defense you can still rack up a high kill count without turning geno. In that case Mettaton NEO wouldn't have had to been brought out seeing as it was made with stopping a genocidal maniac in mind. So Yellow would've most likely have fought the Met in his indestructible form, because he wouldn't've known where the button to unleash Met EX was.

The other humans had powers other than the SAVE feature
The save ability seems distinctly linked to Determination, which despite being present in the other souls did not seem to be their dominant trait. Rather, their dominant traits were the ones that correspond to their souls' colors in the ball game. So like how red corresponds to Determination corresponds to the SAVE feature, the other souls' colors correspond to the traits in the ball game correspond to different powers.

Further speculah on what those powers may have been is optional.
  • The SAVE button seems to glow with the colors of the other souls, which might lead one to think that the SAVE ability can only be done when all seven traits are present. Six of them are in Asriel, and the seventh is yourself. However, we know from Omega Flowey, that the souls want to help you, so when you remember and realize that they can help you, you reach into Asriel's soul because the six human souls open a door or a window for you to reach your other friends, hence the SAVE button glowing with the six soul colors.

The reason the monsters you encounter can still kill you despite clearly not intending to? They're highly radioactive.
Simply being splashed by Aaron hurts you. Bumping into Tsundereplane hurts you. Even if you ask Woshua to wash you, his white water droplets will hurt you. Why? Because all of them are radioactive. The more monsters you kill, the more immune to the radiation you become, because you absorbed some of it. LOVE is how long you can stand the radiation before dying.
  • That's... not how radiation works. At all. The monsters are explicitly magic, so it's easier to say A Wizard Did It.
  • Monsters are nearly immune to magic, while humans are susceptible to it. It's hinted throughout the game that most of the monsters aren't meaning to hurt Frisk and are doing so accidentally. The hint in the librarby implies that monsters use these harmful attacks very casually with each other such as with bullet-themed birthday cards. Flowey's trick about "friendliness pellets" may actually hold some truth if it weren't for the fact that Frisk is a human.

The yellow human soul belonged to Varik from The Halloween Hack.
The belongings from this soul are an empty gun and a cowboy hat. The cowboy hat is self-explanatory, but for a human to have a gun on them when they stumbled upon the underground, it's possible that they traveled to Mt. Ebott in order to hunt monsters. In the Halloween hack, Varik was praised as a monster hunter.

Burgerpants deserved everything he got
When you kill Mettaton EX, you can walk round the resort and all the other staff comment about what a stand-up guy Mettaton is and how nice he is to them. However, on Genocide, Burgerpants reveals Mettaton made an album of songs about how much he is terrible and plays it repeatedly. Therefore, Burgerpants legitimately is the worst employee ever.

Grillby is the father of the fire girl in Hotland.
They're the only two fire monsters of their kind in the game, and just about every monster that looks similar is family, including the training dummies. Why wouldn't the rule apply here?

The seven main monsters correspond to the Seven Deadly Sins.

  • Papyrus - Pride. He takes much more pride is his work than he deserves and he has fantasies about earning even greater fame and fortune.
  • Sans - Sloth. He is apathetic about most things and he refuses to act until he has no other choice.
  • Undyne - Wrath. She has a very aggressive personality and is easily angered by small things (i.e. breaking the table with a spear when the player tried to stand up).
  • Alphys - Envy. She envies the player and set up a plan to insert herself into their adventure, plus she left Mettaton's body unfinished because she was envious of his popularity.
  • Mettaton - Lust. Spends the entire battle pulling various erotic poses and takes almost orgasmic joy in the adoration of his fans.
  • Muffet - Greed. She is willing to kill someone and take their soul for money, and even during her battle she extorts money out of the player to make her attacks easier.
  • Flowey - Gluttony. "Ate" the human souls and used their power to kill the player over and over again, just for fun.
  • Asgore - Original sin. A fundamentally good person who was driven to despair by the death of his children and vowed to kill every human who came down to the underground, starting the events of the game.

There are other monsters who correspond to the Seven Deadly Sins.

  • Glyde - Pride. Grows destructive with your applause.
  • The Player - Sloth. Frequently commented on as being indifferent to the consequences of their actions, looking to try everything just to see what it's like.
  • Chara - Wrath. Drives the very angry/cruel No Mercy run, as well as the destruction of existence.
  • Flowey - Envy. Wants to feel something, anything, like he did before.
  • Mettaton - Lust. Same as above.
  • Muffet - Greed. Same as above.
  • Annoying Dog - Gluttony. Eats everything and anything!
  • Gaster/Asgore - Original Sin.

The seven main monsters correspond to the seven heavenly virtues.

It's no secret the monsters are pretty good guys. It then makes more sense that the seven people you save at the end of the Perfect Pacifist run are symbolic of virtues.

  • Papyrus - Kindness. Shows good will and trust to you even on the No Mercy run.
  • Sans - Patience. Endlessly waits to judge you and fight to protect the timelines
  • Undyne - Diligence. Takes a beating on all paths and never gives up.
  • Alphys - Humility. Even with all her amazing accomplishments, thinks of others.
  • Toriel - Charity. Saves you and cares for you at the beginning of the game.
  • Asriel - Chastity. Purified and free from the sin of Flowey.
  • Asgore - Temperance. King of underground and weighs his interests with those of his people.

Asgore named Monster Kid.
Self-explanatory.

Gyftrot is somehow related to the Determination Extractor.

Look at the DTE Machine and Omega Flowey. Now look at Gyftrot. See something similar? Maybe the original machine was built around the skull of one of Gyftrot's relatives? OR the machine was intended as a mobile weapon in the war against humans, which never came to fruition because Gaster convinced Asgore to wait until his research was done, which had Asgore mellow out and back off on his declaration of war.

Napstablook was the Dummy from the Ruins (and therefore Mad Dummy was once a Blook).

In Mettaton's old house, the third diary states that there were THREE cousins working at the Blook family farm, but the third ghost went off to become a training dummy. Soon after, Mettaton ran off to become a star, leaving Blooky by himself.

Notably, there's only two ghosts living in Waterfall now: Napstablook, and the Mad Dummy, who's inhabiting... A training dummy.

Now, when the third ghost left, Mettaton described Napstablook's reaction as being "resigned" while (later accurately) accusing Mettaton of going off to become corporeal. Perhaps, when Mettaton eventually did leave, Napstablook went out to see just why did his other cousins ran off to gain physical forms. The Mad Dummy claims that ghosts try to find a body for themselves to make themselves feel whole, which most likely fueled Napstablook's endeavor seeing as how his cousins' betrayals would certainly have broken his heart and left him feeling empty. Eventually, Napstablook came across a training dummy, like what his first cousin went off to become. He tried to make it his new home.

And then Frisk comes along. They either break the Dummy or scare him out of the dummy. If they break it, it's obvious why he would have to leave. However, if any other option is picked, he leaves anyway. The Mad Dummy describes his cousin as "shy." Napstablook is a very shy ghost, often bursting into tears if nobody actively cheers him on (further breaking his "poor ethereal heart"), or he ends up panicking mid conversation and excusing himself ("It doesn't seem much for conversation"). Napstablook is one to avoid confrontation of any kind, which may be why he runs away if you fool around with the Dummy, for fear of anything escalating into something he can't handle. He then decides becoming corporeal is not going to solve his problems and returns to the farm. But the Mad Dummy thinks that you purposely chased him out. He may have forgotten how Blooky would react to certain situations, since it's implied that the third ghost left long ago.

Napstablook tends to either forgive or forget any wrongdoing you (keyword being "you") do to him. If you "attack" him at the Ruins, he'll say nothing of it at Waterfall, and invites you to his house like nothing happened. He'll even be there at a Genocide run, if you haven't exhausted Waterfall's kill count yet. He either forgets the things you did, forgets you completely (the True Pacifist epilogue suggests this may be the case), or thinks that it's somehow his fault you beat him up (also likely, due to that "resigned" comment), but once you visit him he's glad to have met you, and you give him something to be happy about. He decides to stay a ghost, even after his cousins left, since knowing that someone cares about him even a little bit makes him feel "whole." But that initial insecurity may have led him to try to patch the void, driving him to the Ruins in the first place.

(Feel free to explore any holes in this theory. Protip: don't post WMGs at 2 in the morning.)

  • When Napstablook accidentally breaks up the fight between you and Mad Dummy, they don't seem to recognize Mad Dummy, referring to them as 'your friend'. Although it's possible they could be related via Mettaton, or Napstablook feels very alienated from his relations in general after Mettaton left.

Gerson is the oldest non-boss monster alive.
And is also ridiculously old for a monster, and the last of his generation to live. He should have died ten years ago.

He mentions only living as long as he did due to his knowledge - implying all of his peers have died, and he is the only monster aside from Asgore and Toriel who is canonically confirmed to have been around in wartimes. Maybe he has determination like Undyne?

Migosp and Migospel's personalities

As we know, Migosp's personality changes depending on if there's another monster around. If there is a monster, Migosp acts as a hive-mind type of creature and is hostile, even calling the player a "filthy single minder". However, once the other monsters are gone, it gains a cheerful personality and even stops fighting, responding in a friendly (and flirty?) "Hiya~" if spoken to. In this state of mind Migosp is still channeling the personality of another being: Frisk, in their naturally friendly, gentle personality that is shown during a Pacifist run. Migosp's low HP text also reads "Migosp refuses to give up", possibly as a reference to Frisk's determination, and two of its battle quotes read "Bein' me is the best!" and "La la~ Just be yourself~" as a reference to Frisk being revealed as their own person instead of being the nameable Fallen Child.

Likewise, Migospel originally fronts with a friendly, funny, clownlike facade that disappears and reveals that they are actually a rather sad individual; saying things like "Don't let them see me this way", "Laughter hides the pain", "They cannot worry for me", and "Everyone has gone", which can be read as an alternative to "But nobody came". Migospel only appears in Hard Mode where the input name is Frisk, so this could be seen as another facet to Frisk's personality...or perhaps to the original Fallen Child themselves, who has a cheerful look but a dark personality. Migospel also does things like juggle balls made of ants or make balloon animals out of angry bees, similar to how the Fallen Child attempted to make a pie out of pretty, poisonous buttercups instead of cups of butter.

Sooner or Later, Asgore and Toriel will learn of Asriel's fate...
Considering Alphys is determined to be honest and truthful with everyone, and considering Frisk can't possibly allow poor little Asriel to suffer as Flowey forever, Alphys and Frisk will one day tell Asgore and Toriel that Asriel is 'alive', but as a soulless flower due to Alphys' experiments.
  • I don't think Alphys knew just what she had done. In her entries, she just calls the flower... well, "the flower." If she knew that it was sapient, she definitely would have been more considerate, and likely wouldn't have hidden it from the entries since they were supposed to tell the whole truth of her experiments to her friends. Also, Asriel wakes up in the garden (outright stated on a Genocide Run), his "Save Point." He doesn't remember the lab, and therefore is implied not to know how he ended up that way. None of the characters remember the True final boss, so Alphys would still be unaware of his reincarnation after a Pacifist ending.

Undyne dislikes Mettaton because she sees him as romantic competition
I mean think about it, your anime-loving nerd friend seemingly custom engineers a sexy rectanglebot with a genuine soul and the desire to sing love songs, wear cute outfits and cook gourmet meals. What would you assume?
  • Undyne's dislike for Mettaton could be more of a personality/motivation clash. Undyne is all about passion, duty, loyalty, and is the head of the royal guard and has a lot of loyalty for Asgore. Metatton, on the other hand, is passionate and loyal.... to fame and/or his fans. He does anything just to make more people like him, while Undyne focuses on her duty and loyalty to the king first and then her friends after. She might see Mettaton as more of a sell-out just to get ratings, money, and fame. The fact that Mettaton replaced Asriel's statue in the MTT Hotel (which I assume was not originally called the MTT Resort) with a statue of himself probably does not help Undyne's opinions of him whatsoever.

Napstablook survives all Genocide Runs
And I don't mean he simply survives your encounter with him, which he does. I mean that as every other monster runs and hides, as they desperately try to kill you, as they are trying to protect not only monster kind, but humanity as well, and as the world that exists is destroyed by Chara... He's still in house, still existing. Even as everything else dies and vanishes, he's still there, with whatever he can salvage (which would mostly be his music and maybe a snail or two). He is unkillable, no matter what you do!

The dress Alphys wears on your "date" is the old flower muu-muu Toriel only wore once that Gerson tells you about post-game.
She probably left it behind when she left for the ruins, and Asgore gave it to Alphys as a hand-me-down.

There's a monster called Fantasy Froggit hopping around the Core during Hard Mode.

Each of the Human SOULs represents a different way the game (or games in general) could be played.

Because the red SOUL (guided by you) is determination, and determination represents a certain aspect of a gamer (specifically the type of gamer who never gives up until they're satisfied, for better or worse), it stands to reason that the other six humans could also be meant to be taken as another aspect or archtype of gamers as a whole.
  • The Light Blue SOUL is the patience gamer, ready for a waiting-based puzzle they can solve. Alternatively, you could see it as the type of player who plays the game to the end of the prologue (if even), doesn't like it and quits without riding out the rest of the way to see if it gets better (the first child didn't even make it out of the ruins).
  • The Orange SOUL is the type of gamer who just kind of shrugs off the game's message and plays it as normal, they'll FIGHT but may or may not decide to spare the monsters who back off when their HP gets low, meaning that they would kill most major bosses.
  • The Dark Blue SOUL has enough hints that it was likely they'd be the type of gamer to go through the evil path, as it's implied in multiple places (though Fanon blows it out of proportion) that they were on a Genocide or at least a high-kill Neutral run.
  • The Purple SOUL, with their notebook allowing them to write down everything, may be the type of gamer who is the Easter Egg hunter, playing the game to death and cataloging everything different that happens for posterity (think us here on T Vtropes, willing to play the game over and over trying different things to immortalize it on the site.)
  • The Green SOUL is the caring gamer who fully immerses themselves in the world and will always go for the best, happiest route. Green being healing, stopping combat (or both) and creating a shield would lend itself well to someone who doesn't fight back but instead tries to make friends and cares about those they meet.
  • The Yellow SOUL is the type of gamer who tried to be pacifist but eventually got frustrated or gave up at a challenge without an obvious solution (think the type of gamer who would kill Toriel because they can't figure out she can be spared repeatedly), if they just gunned down monsters willy-nilly they'd die pretty early on when they ran out of bullets, they probably saved them for challenges they couldn't surmount without violence.

The Annoying Dog is an indicator that a part of the game is unfinished.

The Annoying Dog shows up to steal Papyrus's special attack, as well as the mysterious powerful artifact. In the January 2016 update, the special attack is elaborated on (Papyrus hints at it being a Gaster Blaster...which has been stolen, yet again) and the artifact was made easier to get (the keys you need to press appear over the statue, instead of making you play the keys by ear). Maybe things that involve the Annoying Dog are going to be continuously updated until they finally become a legit part of the story...at which point you can fight Toby in the secret room.

Carrying on from the above idea of Gaster slowly inserting himself into the game through updates, the artifact is going to do something important when it's fully implemented, and Gaster wants you to have it, which is why the puzzle solution appears now. On a meta sense, it makes an annoying puzzle less frustrating, but in-game it's Gaster making the item easier to acquire. A way to save absolutely everyone, perhaps...?

Flowey has severe unresolved PTSD.

Flowey concludes that lacking a soul in his current form is the reason why he cannot feel attachments to anyone, with this being what eventually led him down the path of villainy alongside sheer boredom and curiosity brought on by constant resets. However, ingame lore admits to the nature of souls being not fully known, so it could merely be folklore that monster souls are what give them the abilities to feel love, compassion, and hope. It's possible that lacking a soul isn't really why Flowey is so emotionally closed off.

Flowey's real problem is PTSD. Many of his thoughts and behaviors align with symptoms of the disorder, and all of these problems happened only after he had suffered a multitude of traumas including but not remotely limited to watching his only friend die slowly of buttercup poisoning, being beaten to death himself, and waking up as a flower later on in his father's garden only to have his screams for help go unanswered. In the immediate wake of all these events, he realizes that despite Asgore eventually coming to his aid and doing his best to comfort him, he can't feel anything for him, and after some time, he runs away expecting Toriel to be able to "fix" him. While this doesn't work either, it's not because of supernatural reasons, but rather just the fact that he is just getting over recent trauma and shutting down emotionally isn't an uncommon reaction while the brain tries to process the trauma.

Even long after everything, when Frisk falls into the underground, he still shows many signs of coping badly with the trauma and suffering PTSD. The main one is how he still cannot feel positive feelings towards others, and generally has a lot of emotional problems going on. He's impulsive and prone to sudden bouts of anger, and in his own words, "became despondent". His entire "kill or be killed" view of the world developed as a way to cope with what has happened to him; PTSD sufferers commonly adopt negative views of the world, such as finding it to be unsafe. He also admits as Asriel that he always blamed himself for all of these events happening due to refusing to fight back when he was being attacked, with self-blame being another PTSD symptom. He attempted suicide in the past, only living on through determination causing him to reset back to when he first awoke as a flower, and later tries to goad Frisk into killing him; those suffering PTSD can begin to have suicidal thoughts or generally self-destructive behavior.

Children also tend to incorporate their trauma into their play, as an attempt to make sense of things when they're too young to be able to fully rationalize everything. Flowey eventually admits the whole time that he's been playing with Frisk—admittedly mistaking them for Chara—and his idea of "play" incorporates things such as mocking them with the same fate he suffered when he first awoke in the garden, and more subtly, using an Echo Flower to taunt them by telling them that Toriel will forget about them once they find another child— something he likely feels she had done with him after his death.

It's entirely possible that Flowey could begin to feel positive emotions again and live somewhat of a more normal life, his trauma will just take time and care to come to terms with. He's just a scared and impulsive little kid who didn't stick around with either of his parents long enough for his brain to process everything and allow a healing process to happen, and concluded that lacking a soul must be why he can't feel the way he wants to feel because he needed answers as to why nothing felt the same anymore. He also seems more like he's told himself that he can't feel and started to believe it more than it is actually true. During the genocide route, he shows happiness that Chara has returned and expresses a desire to simply live on the surface with them, even though dialogue obtained after the neutral ending has him insisting that he could never care about them. At the end of the same route, when he realizes that Chara intends to kill him, he becomes terrified and runs to Asgore, crying and warning him of the impending danger. While this could purely be him trying to save his own skin, he could've very likely done so more effectively by leaving Asgore behind and fleeing. Instead, his instinct told him to run to his dad, sobbing, and though it's true he may have just been trying to gain his protection, it's also entirely possible that something in him also cared about Asgore's fate, even though it was too late to be of much—if any—help.

Getting back to the topic of souls touched on in the beginning, when Flowey gets ahold of the human souls, his behavior doesn't change and he still totally lacks compassion. This could be written off by saying that they were human souls, and the game says that those don't seem to need compassion whereas monster souls are made of compassion. That, then, doesn't fully explain why during the beginning of Asriel's fight, after he has absorbed the human and monster souls, doesn't act too dissimilar from Flowey—attacking, taunting, making threats. It takes saving the souls of your friends, and reaching out to save Asriel himself, before he eventually loses the will to continue fighting and admits that he's hurting. He says that in his soulless flower form, he couldn't care about anyone, and that once he goes back to being a flower again, he once again will lose his capacity to care for others. But then, if you go back after the ending and load up your save file again, Flowey appears. His reason for appearing? To plead with you to not reset everyone's happiness, something he probably wouldn't do if he didn't care about their wellbeing.

Alphys is a baby determination extractor.
More precisely, the DT extractor is made out of a reconstructed-from-monster-dust skull from an elder or several members of what species Alphys is, that can siphon off the 'determination' material through will-breaking magic. A human-monster had to exist at some point before the human-monster war for it to be known about, and a creature that could reduce the overwhelming power imbalance or even better take it for themselves would be second only to ghosts for theoretical human-killing potential (as a human without magic attacks would be unable to hurt a ghost while the ghost could easily hurt them) so it could be known of. As a result of her instinctual but unused power drain ability, Alphys is naturally attracted to people with determination. She is in love with Undyne (who can naturally produce it), stalks, observes, and inserts herself into the adventure of Frisk (who is full of the stuff), and works with Asgore and possibly Toriel (who have enough to persist in death briefly). Because we don't fight Alphys we don't know what her abilities are beyond 'electric bolt bullets maybe' from the true pacifist ending.

Undyne's species is effectively the designated miniboss squad leader of monsterkind.
Other than Boss Monsters Undyne is the only shown non-formally-Boss monster with naturally occurring determination, while boss monsters contain more and enough to persist in death briefly. Fish-people must then be the implied Miniboss monsters, followed by dog-people who compose the majority of the rest of the Royal Guard in Snowdin's area. We see only one instance of super-spider that is Muffet, ghosts are immune to physical attacks unless committed to a body but are often either cripplingly un-determined to fight (Napstablook), require lots of tools in order to be effective at magical attacks (Mad Dummy), or have otherwise not-predisposed personalities (Mettaton), and skeletons seem to be extreme rarities (we see none other than Sans and Papyrus and in secrets maybe Gaster).

Undyne still becomes a gym teacher for Toriel's new school in the True Pacifist ending.
And she can still proudly bench-press seven children, human and monster alike.

The Light Blue Soul wasn't killed by a monster - they were diabetic.
At this point Asgore had already decreed that all fallen humans would die, so Toriel understandably tried to prevent Light Blue from leaving the ruins. Unfortunately, they were too young to understand/explain that they were diabetic (or even what diabetes is), and needed a regular insulin shot. They fell into a coma and eventually died, which is why Toriel never tried to stop the five other humans that fell before Frisk - because she didn't understand what she had done to cause the child's death.

During the Pacifist ending, everyone ditches and "forgets" to tell Jerry that the barrier is open, leaving him alone in the underground.
Hence his absence in the casting call during the credits.

Flowey eggs Asgore to commit suicide in one of the pre-True Reset Neutral Endings
Flowey never shows his face again in the game if you kill him in a Neutral Ending (unless you do a Genocide Run, or a True Pacifist run), or at least until Asgore commits suicide in another Neutral Ending, where Flowey was killed before pre-True Reset. Magnificent Bastard that he is, it could be estimated that Flowey knew the expectation that a player might kill him to prevent him stealing a happy ending from them, so he strikes when you're not looking. Simply convincing Asgore to commit suicide would be the best way to cover his tracks and snatch the ending away from you.

Flowey is not really Asriel Dreemurr
While it's true that Flowey possesses Asriel's memories and essence, he is nothing more than a shadow or copy of the real Asriel Dreemurr that died shortly after fleeing from the outside world and its humans. His SOUL was permanently destroyed and no amount of replacement or temporary SOULs like in the True ending will change that. If you subscribe to the idea of the soul being a person then Asriel is no more.

Also, while it's true that essence can mean what makes a person which would fit just as fine, remember that Flowey is not unique; when Asriel died his dust was spread all across Asgore's garden and not only that, a lot of seeds from the Golden Flowers outside. When Alphys is experimenting on the flower that would become Flowey she states, "The first golden flower, that grew before all the others." That's right, there's an entire bed of Floweys waiting to happen. All it would take is for a little bit of injection with determination and suddenly it becomes questionable who is the true Asriel, meaning that the Asriel we know and love/hate isn't the same Asriel.

The Yellow child was Mike Teevee, or at least closely related to him.
Mike, especially in the 1971 film(Where he's a fan of westerns—remember that the corresponding armor is a Cowboy Hat—and would be allowed a real gun as young as 12 but owns several realistic-looking toy guns before that), would be a dead ringer for that child. He died in Hotland because he got too caught up in being on TV during the Mettaton encounters(wanting to be on TV screwed him over in the film too).

Lest you ask how it's possible to die in one of them when Alphys always bails you out(Mettaton EX only tries to kill you to prevent Asgore from getting the last soul, which wouldn't have been a problem yet for "Mike" so that might not have killed him), maybe Mettaton went the whole hog the first time through to make it more impressive(and less suspicious) when Alphys saved them. When Alphys slipped up and "Mike" died anyway, Mettaton was forced to be much more careful with you.

Muffet is far older than she looks, and far tougher than she looks.
Old enough to remember the surface, and to have fought in the war, and, unlike even Gerson, tough enough to be wounded by a human and survive. She earned a purple heart for her services.

Flowey is a devolved, more intelligent Floran
Think about it. Flowey is, as you already know, a flower. Florans are a race of plant people. Specifically, Ax-Crazy plant people. Florans are known for having very bad grammar. Flowey is what happens when a Floran is taught proper grammar, loses their lisp, and becomes.....less than humanoid.

Photoshop Flowey is a hybrid of three races: Floran(this is obvious), Glitch(the machinery), and Human(the human organs).

     The Player/Frisk/The Fallen Child 
The Fallen Child is a clinical sociopath.
Rather then how the term is usually used in fiction to describe someone who is really F-ed up Chara is an actual medical definition sociopath incapable of feeling emotions. All of the bad or destructive behavior was simple curiosity from Poisoning king Asgore for the sake of a joke to talking his brother into helping him commit suicide then absorbing his soul in a bid for power he wasn't just evil he simply was unable to feel empathy in any form or any other emotions. So when Flowey states he can't feel anything that's Chara's influence. After all they never broke apart, they're still bound together and both suffering from Chara's mental condition. It also explains how Flowey became Omega Flowey with only the already captured human souls, he still had Chara's to work with in the beginning so he didn't need Frisk's just to get Asgore out of the way so he could retrieve the others.
The Fallen Child is, quite literally, the devil.
Even not considering that they're referred to as a demon at times, even discounting allegories of falling from a place of light from above, being called "Fallen", etc, they are still an apparently polite and likeable person, tricks people on their side with no genuine emotion whatsoever, makes pacts and grants great power, deals in souls, is basically all-powerful but requires others to work through, and ultimately wants very bad things for mankind.

Frisk is the Fallen Child's Literal Split Personality, with part of Asriel mixed in.
  • The opening says that the child falling into Mt. Ebott takes place in 201x. There's an "old" calendar in Toriel's drawer labelled 201x. That's the Fallen Child in the opening, not Frisk.
  • The Fallen Child killed themselves via Yellow Flowers to give their Soul to Asriel, so they could pierce the barrier and kill humans. Asriel wanted to free the Monsters, the Fallen wanted to kill humans, but Asriel couldn't go through with it, and the fusion fled the humans, dying. Asriel's dust started to fill up the yellow flower fields. The Fallen's body was originally kept in a special coffin that kept their Soul intact, but their body stayed dead. Eventually, Toriel took the Fallen's Body to the Ruins and buried it. Some stray seeds of the Yellow Flowers were buried also.
  • The Fallen's body did not decay; something Monsters didn't realize was significant, because Monsters turn to dust immediately upon death; they assumed the fact that human bodies didn't mean they stay intact forever. The reason the body did not decay was because having been fused with Asriel, and having the yellow flowers infused with Asriel's dust by his body; there was a very small connection of two Boss Monsters' Souls. (Not enough to visibly age Toriel and Asgore, but it was there.) In time, it was enough for "Frisk" to grow their own soul, a hybrid with benefits of both human and monster; and they materialize above the Fallen's grave, still wearing the bandage from the Fallen's coffin. (Same body).
    • Meanwhile the remains of the Fallen's soul has mostly infected "Flowey" aka Asriel, but the "shadow" still existed in Flowey's body. Just like how Flowey can inhabit any Yellow Flower, if....invited, the Fallen could take over Frisk.

Frisk and Chara (the protagonist and the Fallen) are dual identities of the player, and symbolize how people can give in to evil or be kind.

  • At the end of the Asriel fight, he remarks that "[NAME] left" and the person he's talking to is Frisk. While it seems like demonic possession is in play, it doesn't explain why the Fallen would control Frisk and then let Frisk just be nice. Instead, it seems that Frisk and the player/Chara/the Fallen are the same person. Frisk represents the kindness and good that can exist in humans, while Chara is the innate evil within all humans. The name you choose represents your evil side and the wrong you do in games and real life, because cruelty and hurtfulness are easy to manifest. When you're regarded as Frisk, it's Asriel acknowledging that you're being a kind, new person, not the callous person he was friends with. That's why the Pacifist Run is all about being kind and controlling your desires to just kill, like most games urge you to do: you have to shun your own inner thoughts and whims to become a kind person.
    • The author suggests you to name the Fallen Child/Chara after yourself. While at first it seems to be intended towards a Humans Are Bastards / Take That, Audience! revelation, it may also be a contrast between the different gaming urges: to wreak havoc and to use the game as a plaything, or to get immersed and heed the message of the game. Playing like Frisk is expected to achieve True Pacifism, Frisk is not only nicer, as a contrast against the usual violence in games, but he also represents playing the game as intended (such as when you play Final Fantasy 7 as Cloud and makes the "right" choices) and as a reward the player becomes part of the game world as Frisk. Meanwhile, the Fallen Child, named as yourself, on top of being a Deconstruction of the usual game protagonists who kill for power without consequences, is meant to represent both detachment, disregarding the violence that you cause in-game as "just a game" you are outside of, and the destructive obssession to subvert the game just to see what happens, as well as grind. Just killing constantly is not enough to make you Chara, you must grind to find every single monster and then kill every last one of them, in a game that tries to make you not kill.
    • In the Genocide ending, the Fallen says they only began controlling Frisk as a result of your actions. Only then do they gain an objective: "power." In a Pacifist or even Neutral run, their presence is far more passive. They have no reason to interfere. Naming the Fallen after yourself is meant to detach you from the game once the characters get their happy ending, as the player character is referred to as Frisk from then on, and allow you to move on instead of wanting to take that away and keep playing.

Chara is YOUR canon self insert
  • It's not that they are so horrible, they just see the whole things as a game. Even if you pick pacifist route in Undertale, how many other enemies and monsters you had killed in another game?
    • In Harvest Moon, none. In Professor Layton, none. In Phoenix Wright, none. However this is a game whose the tagline is about not killing, even though it's possible. Chara may just be the refusal to get immersed and care. When playing as Chara, you are not acting like you would as a person that is inside the game world (hopefully), but as a detached outsider, as opposed to Frisk who belongs in it.
    • That's because you can't kill in those game, the same as you can't kill in 'card games', racing or sport games, cooking games and so on. People usually played various kind of games. 'Killing monsters/enemies' are common even in casual games. Unless you ONLY played the non-killing games, my point still stands.
    • If killing was possible in Harvest Moon, then people would do it. The closest thing is Rune Factory. If someone made an Ace Attorney game where you can play as the murderer and hid your crime, I think people would play it. If someone made a twisted game where you need to make a complicated puzzle in order to kill someone, like in Detective Conan...well you get the idea.
    • Well, there's the Deception games, though you're generally defending yourself.
    • See the above. The key word is H O P E F U L L Y.
  • While Chara's name isn't necessarily your name, it is common for people to have an avatar name and used it for several games (or several accounts). It is your character.
  • It is also explained why they suggested to erase the world in the end, and move on to the next (game). Eventually you're going to delete the game files from your PC, admit it.
    • No. I won't.
      • But Undertale won't be last video game you ever play, either. Chara is always ready to follow you into the next adventure, even if all you see is a level number...
    • Sure I did, and hid the game from my Steam library. And now I'm killing all monsters and fish ships.

Continuing from above, Chara is the only one who can do reset
  • Other humans might be able to SAVE and returned to the last point if they are determined enough. However, only YOU can actually reset the world (game) from the beginning.

Chara was once possessed Flowey
  • It wasn't Flowey that had the ability to reset, it was Chara all along. Flowey, just like Frisk once tried to be good and save everyone. But then Chara influenced him, and he tried to kill everyone.
  • After several repeat, Chara got bored because no matter what they do, they can't use Flowey to break the barrier.
  • Chara eventually let the time move until another child fell, and switched their possession to the child. This explained why Flowey stopped having the ability to reset after Frisk arrived.

Flowey sold his SOUL / Chara's true plan
  • As an alternative to the WMG above: At first, Flowey actually had a SOUL. But after being possessed by Chara, he went through reset several times until Chara successfully convinced him to do something similar to the No Mercy route. At the end, he had no choice but to sell his soul.
  • Chara then took control of Flowey, reset the time back before everyone died and waited until the next human fell in to possess them. Repeating the same procedure, Chara would convince them to commit mass murder, blackmail them to sell their SOULs, and then reset time, all so that once someone manages to break the barrier, they'd be in control of them and could wreak havoc from there. They are ultimately successful in Soulless Pacifist ending, where they can finally destroy both the human and monster worlds.
    • Interesting theory, but not likely. Flowey says he wasn't able to feel the emotions exclusive to beings with souls since he first woke up in the garden, no matter how hard he tried. It's always a possibility that he could have lost his memories of a previous successful Genocide run when the world was deleted, but he mentions having "killed everyone" before, so It's likely that he just never had a SOUL to offer since the experiments.
    • It could be that Chara erased Flowey memories, left him so he woke up in the garden 'alone', never knowing his ability nor remember Chara was supposed to be with him. Then he figured his ability 'for the first time', and then started to try 'every route'. Including killing everyone by his own this time.
      • There is no hint whatsoever that Chara is physically capable of this even after the player's genocide run, never mind before the player gets involved in the first place (mind, it is the player who "awakens [them] from death").

The Fallen Child and/or Frisk were possibly abandoned by their parents.
Which would explain why they were wandering around a dangerous mountain by themselves. More on this theory here. It would also explain why the Fallen hated humanity and why Frisk has the option to go live with Toriel in the True Ending.

Ness and Paula from the Mother series are Frisk's parents

It would explain the striped shirt, the nonchalance towards the weird monsters they encounter and why they were traipsing through a mountain top in the first place. Also, if the theory that Ninten is related to Ness is true, then they probably kept it going and named Frisk Wii, making them opt for the gender neutral name.

The Fallen Child and/or Flowey is the one providing the flavor text on the monsters.
  • If Chara/The Fallen Child is the narrator this may explain how the narration switches from second person in the Neutral/Pacifist runs (i.e. describing Frisk's actions to the player) to increasing amounts of first person in the No Mercy run (i.e. describing their OWN actions to the player).
    • There's actually a bit of evidence that the Fallen Child might be the narrator, if you're looking for it. The between-turns narration shuts up after you attack Toriel or Asgore once during their battles, becoming just elipses—they would not be very comfortable with those battles. They seem to know the names of all of the final boss's attacks, which they might have discussed in the past. It makes the "It's you/It's me" line make a lot more sense. And it explains a lot of the weird bits where you know something only the Fallen Child should remember, like the jokes you can tell Woshua (clearly about the Fallen Child and Asriel) and the game over text.

Chara erasing the world is the darkness Gaster saw.

Chara's soul provided power to the True Lab and the determination extractor.
Theory here.

The Game Over Screens
Following the above ideas that Frisk and Chara are somehow dual identities of the same being, or perhaps Frisk is Chara reborn, the game over screens are kind of an extended take on the idea of your life flashing before your eyes when you die. When Frisk dies, they flash back through to the beginning of their life... and then, they flash to the end of Chara's. Namely, Asgore next to their deathbed. The Game Over text is Asgore desperately begging his child to not die. The feeling this gives Frisk turns into enough Determination to allow them to go back to their last save point.

Frisk actually never makes it out of the Underground in a neutral playthrough.
Right before Frisk leaves the Core, Alphys reluctantly tells them that the only way that the barrier can be broken is through the power of BOTH a human AND monster soul, meaning that they would HAVE to kill Asgore. If you spare Asgore, Flowey kills him instead and breaks his soul, even though it would have broken by itself after the body dissolved, meaning that it was done purely to destroy your ticket out of the Underground. If you spare Asgore, after having killed Flowey in a previous playthrough, he goes on the whole spiel that he would have given if you spared him without having killed Flowey in a previous playthrough before conceding that such a dream could never happen, saddling them with the responsibility of delivering the monsters from the Underground, and killing himself so that you can use his soul to leave. You NEED a monster soul to get out of the Underground, and the fact that Flowey doesn't have one at all and your lack of Asgore's soul to use, it means escape from the Underground, to say nothing of a happy ending, is out of your reach in a neutral run.
  • "A human soul and a monster soul" is just a minimum level of power, which Flowey's multiple human souls clearly surpasses. He's quite capable of dropping Frisk off on the other side of the barrier, and when he refuses to fight you he still takes you to wherever his fight normally ends (a room identical to the one on the other side of the barrier). It's a little questionable why he would do that, though, since he wants you to do True Pacifist.

As it's never stated that the other 6 human souls were children, and looking at the equipment they had it's likely that multiple of them were adults, that means that 6 people were defeated by monsters, so the monsters aren't exactly pushovers, so that means that a child got through an army of Superpowered Mooks, and that's not something just any random person could do, if even someone equipped with a gun was unable to best the monsters
  • This would help explain how they are somehow able to take on Omega Flowey and Asriel, both of whom possess the power humans apparently fear so much.

Chara doesn't actually want to destroy the world.
Upon completing your first No Mercy run, Chara destroys the world. But they'll bring it back, if you sell them your SOUL. If you complete another one, they'll bring the world back for free — they won't even make you wait 10 minutes. They also suggest that you should try a different path next time.

Conclusion: Chara wants nothing more and nothing less than for you to achieve the corrupted True Pacifist ending, and then leave it there, so they can do what they want without your influence. Destroying the world was nothing but leverage to get your SOUL, which they needed for this plan to even be possible.
  • That said, their plan, even before the game, was to attack humanity. They can only do that if you destroy the barrier in the True Pacifist ending.

Chara isn't Chara.
Or, uh, less confusingly: The child who fell down in the intro cutscene and plays a large part in Asriel's backstory isn't the demonic incarnation of powergaming that appears at the end of a No Mercy run. Simply put, the child, while clearly messed up and possibly omnicidal, doesn't seem to have any supernatural qualities. The demon, on the other hand, is a fucking demon. They have the same name, by coincidence or not, but it doesn't make a lot of sense for them to be literally the same person.
  • Maybe the child got possessed by the demon at some point and is just it's latest vessel?
  • It's also possible that the demon at the end is what happened after the potentially unstable Chara was exposed to Frisk's overwhelming DETERMINATION, and has simply chosen to apply that power towards a goal they once held.

Chara is 'the Darkness' W.D. Gaster mentioned.
Entry Seventeen mentions an ever-growing darkness that results in negative photon readings, which shouldn't even be possible. What if Chara is this 'darkness' Gaster mentioned, and he only caught a glimpse of the demon's full power?

Frisk is Chara's (non-direct) descendant.
They obviously can't be their direct descendant, since Chara died young, but they could be descended from a sibling or cousin. That would explain why Frisk resembles them so much.

It wouldn't have mattered what Chara's backstory was
On the No Mercy path, the one where Chara completely takes control of Frisk, Sans mentions that he once thought maybe friends, good food, and bad jokes could redeem the timeline-destroying anomaly that Frisk has become. But that didn't work. Merciless Frisk, and by extension Chara, is the kind of person who will never be happy. Even if Chara didn't have the Dark and Troubled Past that Asriel mentions in the Pacifist ending, Chara still likely would have been murderous and merciless, cruel and hateful. After all, they couldn't find happiness leaving the hated humanity behind and living with their new loving family. They're just as quick to throw that family under the bus as soon as they get a chance.

But at the same time, while the No Mercy ending has Chara completely take over Frisk, the Pacifist route leads to Frisk escaping Chara's influence entirely and being referred to as Frisk. If Chara couldn't have been saved by kindness, whatever Frisk went through on the surface that led them to climb that mountain had no hope of destroying their kind, peaceful personality. If Chara is the dark side of determination, never for a second letting up on their ultimate goal of omnicide, then Frisk, the real Frisk of the Pacifist ending, is the light side of that same determination, never letting anything push them away from their goal of making friends with everyone and making peace between humans and monsters.
  • There's something to that as Chara can deal extreme damage to monsters because they're so determined, but needs to greedily comsume things to heal their wounds. Frisk in contrast in the pacifist route, can manfiest healing items through sheer will, increase his defense, and even shrug off fatal damage.

Possible backstory: Chara became jaded with humanity after people in their village fought and killed each other over the power of 'determination' and the ability to SAVE.
Even if only a few humans on the surface possessed that power, if someone with bad intentions got a hold of it they could cause some major devastation. Imagine two or more beings with that ability fighting each other and resetting over and over, and then imagine the other humans around them having enough awareness of the timelines that they know they can't stop them and they become lazy and unwilling to do anything about it (like Sans). Chara was just a normal child, but the constant fighting and destruction made them jaded and angry and caused them to run away and fall into the Underground...where they found that, since they were the only human, they now had that same ability. They may have been abusing that power themselves, killing and resetting so that no one else would know what happened. That's why they weren't worried about killing themselves and possessing Asriel to go to the surface and get revenge on the humans; they figured if something went wrong they could just reset and try again like before. But when Asriel died and merged with the flower, Chara's SAVE powers passed on to Flowey somehow and they could no longer reset. So they waited in limbo until a new possible host arrived, losing what humanity they had left in the process.

Chara fell into the Underground while trying to commit suicide
You have to fall down the mountain in order to get into the Underground. Chara was a kid who had given up on life and humanity and tried to throw themself down the mountain to kill themself. Ending up in the Underground wasn't part of their plan. In retribution, and since they were already way past the brink, they concocted a genocidal plan, goading Asriel to join them, as they ate the buttercups to kill themself successfully this time.

The Fallen Child is not evil to begin with, and only becomes an Omnicidal Maniac as a result of the player's contempt for the Suspension of Disbelief in the No Mercy route
Despite being a fairly toxic friend to Asriel, they seemed to get along quite well, and there's nothing apparently supernatural about the child prior to their death. Based on the fact that the style of the narration in the text boxes don't change in the No Mercy route even as they switch into the first person to indicate the Fallen taking over Frisk's body, it's not unreasonable to think that the first child might have also be the narrator of the pacifist and neutral routes, guiding Frisk's actions in a more indirect way and relaying information about the monsters opposing them that Frisk could not possibly have known. While they were certainly a callous misanthrope in life, all of the other characters in the game are, at heart, decent people, and Undertale as a whole leans strongly towards the idealistic end of the Sliding Scale of Idealism Versus Cynicism. Not even Flowey is portrayed as truly evil, and in the pacifist ending, he is able to find a form of redemption as a result of the player's influence. Perhaps the same can be said for the Fallen, should the player choose not to reset the save and embark on a No Mercy run?
  • Consider: LV is associated with The Fallen Child's assigned name on the status screen and in battle, not Frisk's. Frisk might be gaining LV too, but theirs isn't what's on display.
  • If the Narrator bit is right, Chara is beign on the other routes, even having some standards, like the candy bowl thing.

Chara/Frisk is The Pyro.

After a No Mercy playthrough, Frisk sells their soul to Chara and goes on what they think is a Pacifist run. What's really happening is Chara continuing to "finish the job", as they put it, as an adult. They meet with other humans about as determined, give or take a bit of LOVE as they are, and just as bloodthirsty. But thanks to Pyrovision, Frisk is none the wiser.

Chara is Dracula
  • "It is not by my hand that I am once again given flesh, I was summoned here by humans, who wish to pay me tribute!"
    • They also steal human SOULs and make them their slaves. But perhaps the same could be said of all religions player characters...
    • What is a mon(ster)? A miserable little pile of secrets! (Such as the True Lab.)

Chara only became evil in death
Chara as a child hated humanity, and sees them as evil with no exceptions, not even for himself. After falling into the Underworld, Chara finds a new family, who they love, in the monsters, and wants to see them freed. They don't see a problem with dying, since they hate humanity and themself, and so hatches a plan to let Asriel take their soul and use it to get past the barrier, and to gather more souls on the other side to break it for good. But after dying, their soul embodies their obsession towards humanity's destruction. Spirits are often characterized as violent, even if they were gentle people in life, due to their obsessions over something taking them over. Asriel held back Chara's soul's influence on him long enough not to kill anyone, and dies. Later, Frisk also gets possessed by Chara, who sees them as evil just for being human. They can't accept that there are good humans, and try to make Frisk hurt the monsters to prove it to themself. By the end of the No Mercy run, they have taken full control of Frisk, but by the end of the Pacifist run, they've accepted that there are good humans, and their soul finally finds rest.
  • This is believable. It's common in fiction that if someone died in horrific manner (i.e. with regret, or killed, or tortured/sacrificed) they become some kind of evil ghost entity even if they are just a victim or a decent person in life.
    • Now imagine a child who already hated humanity even before they died, and willingly poisoned themselves to death for the sake of their goals.

It was because of Chara that the monsters left the Ruins.
When Chara fell down and told the monsters they had been forgotten by the humans, who were not planning a full attack of the Underground if they moved from the Ruins, the monsters migrated.

Chara is Horrid Henry.
They both wear a long-sleeved shirt with a single yellow stripe on it, they both have brown hair, they smile/grin a lot, they have a fondness for pranks that can sometimes go too far, they both address the 4th wall and they both hate humanity. It makes a surprising amount of sense.

Frisk isn't just gender-ambiguous. Frisk is actually full on agender.
People refer to Frisk as "they" because they can't tell Frisk's gender by looking, and Frisk refuses to clarify for them because Frisk actually identifies as genderless. Who knows why. Maybe it's because Frisk came from a shitty family with parents that never paid attention to their children and consequently Frisk doesn't even know what gender is. (This would explain Frisk's willingness to openly flirt with people of both genders, and also with creatures of indeterminate gender or which lack the capacity to identify as anything. Of course, Frisk could just as well simply be panromantic.) Or maybe, being so pure of heart, Frisk is well aware of the notion that one's peer groups should be defined by one's private parts, but simply rejects it in favor of indiscriminately and unconditionally being pals with everyone.

I'm tempted to say the same about Chara, but if that's the case, it's clearly not for the same reasons, since Frisk and Chara have very different backstories and personalities.

  • Alternately: Frisk and Chara have Schroedinger's Gender. They are whatever you want them to be (or whatever you identify as, if you named Chara after yourself), and it's simply easier and less clunky to use "they" instead of "he/she". For all we know, some monster species don't even possess biological sex in the first place, or have different ways of discerning gender than we do, and thus don't automatically assume gender based purely on appearance.
    • Another Possibility: Only Chara has Schroedinger's Gender because they're meant to represent the player and is the Player's Character. But because Frisk is meant to be their own character not the player's, Frisk's gender is simply unknown. After all, if the player's not allowed to assign Frisk their name, it makes sense if only Frisk can decide their own gender (or only Toby Fox can anyway). The fact that the cloaked figure ferrying the boat says gender isn't important might support this idea.

Chara is a form of Odio.
The song that plays during the fight with Sans isn't his theme - it's Chara's. And it's a reference to the boss theme of Live-A-Live, Megalomania. Similar to how Odio has possessed various other beings through time, in this game it possesses Frisk.

Chara is the final boss of the post-No Mercy Pacifist run, and Frisk always wins in the end.
Think about it. Chara is you. After you beat the No Mercy run, you can do the pacifist run again for a different ending. No matter how many times you send Frisk through the post-No Mercy Pacifist run, they still obey your every command and jump through all the hoops, knowing they're just going to get a shitty ending where they're possessed. So why do they keep doing it? Because they're determined. And in the end, between Frisk and Chara, Frisk inevitably, invariably has more determination. You know how I know? Because Frisk, being nothing but a gaming abstraction, can keep playing Undertale forever, but you, Chara, being a real human, will eventually tire of it. When that day comes - when you lose the will to fight - Frisk will spare you, and you'll stop playing. Frisk will have their soul back, they and their friends will have their untainted happy ending - which you'll have left too early to see - and everything will be as it should.
  • This brings up a good point. Even viewing the game in abstract, where Chara and Frisk are totally spearate from the player, Frisk should be vastly more powerful than Chara. Considering that Frisk can stick their soul back together after its been shattered, it'd make sense for them to be able to simply tear it away from Chara at will. Even if Chara attacks them, they can simply take them out the way Frisk took down Sans under your control.

The narration with brackets is Frisk thinking, and unbracketted is Chara.
This matches the tow different saving styles - Frisks more whimsical description of being filled with determination to keep moving forwards, compared with Charas flat statement. Second, the first narration is in brackets, which occurs before Flowey calls Frisk by Charas name and wakes them up, before Charas name is even displayed at the bottom of the screen.

Frisk is Chara reincarnated. Even in the pacifist run.
Chara didn't have a bad soul. Their essence was just so bad that it overpowered their soul with pure hate and bloodlust. That's what being a psychopath is, isn't it? There's a chance you could have somehow been a good person, but it was foiled by an incurable neurochemical imbalance.

So maybe Chara's soul was a bad soul when it was telling Asriel what to do with the body. But I think maybe not. I think maybe, in such a sin laden state, a soul becomes devoid of its own intentionality, and just repeats whatever's said to it, like an echo flower. At that point, Chara's soul was just repeating the bad intentions of their essence. It needed some time to recover before meeting its next host.

With Frisk, Chara's poor soul, that had endured so much sin, had a chance to start over, with an essence that wasn't rotten to the core. That's why you hug Asriel at the end, despite apparently never having met him, and why everyone looks at you and is reminded of Chara. It also explains the similar fashion choices and heart color. It's also why Frisk had a dream about meeting Asriel for the first time, even though they've never met.

As for Chara in the No Mercy route, that's their essence that you meet. You gave the essence what it wanted: a new vessel for its soul that's just the same as the old one, except even more determined.

  • This would explain why naming the fallen child Frisk starts Hard Mode. It makes the reincarnation explicit, with all the baggage from the fallen child's life.
  • If you believe that Chara is the narrator, this would also explain why the flavor text when fleeing from a battle is in first person.

Chara and the player are one and the same

The player is the one who gives Chara a name und usually it is their own name. They are the one who truly control Frisk the entire time. Only in the true pacifist run is the player willing to step back and allows Frisk to speak for themselves, saying their real name for the first time.

On the No Mercy run however the player utterly crushes Frisk's mind and spirit. The choice at the end is just another sick game since the player is making the choice, not Frisk. However they do the same thing no matter what Frisk wants. The Post No Mercy runs show that because of this, Frisk as their own person is truly gone and only the player pretending to be Frisk remains.

  • Not very likely given that after a Genocide play through, if you don't agree to erase the world, Chara kills YOU.

Frisk entered the Underground in the 100 year anniversary of Charas death.
That's why Flowey mistook Frisk for Chara - he wanted it to be them so bad. That's also why Toriel was along so early in the morning, and why Chara even attaches to Frisk in the first place. Plus, symbolism.

Frisk grows up to be The Unlosing Ranger

Because determination, general stoicism and maybe something to do with the human SOULs corresponding with the Shadowgram's Hero Energy Sources.

Frisk is an orphan.

Let's face it. Even assuming that Frisk is not merely a vessel for the player's will, they're still a very... unusual child. With that permanent neutral expression and strange lack of fear and restraint, it's possible that Frisk was repeatedly passed over for adoption. Perhaps this feeling of endless rejection caused Frisk to believe that nobody loved them, and that everyone just wanted them to disappear... and so that's why they climbed Mt. Ebott. Not in spite of the rumors, but because of them. This is further supported by the fact that neither Frisk nor Chara ever refer to their biological parents or family, in any capacity. And it makes Frisk's decision to stay with Toriel much more sensible, since almost any child would be too attached to their real parents to live with someone they've barely met a few days ago. Or even a few hours ago, depending on how you interpret the time frame of the story.

Also, if Frisk is a boy, being an orphan would explain why his hair is rather unkempt and long, since he doesn't get much attention, nor can he afford quality haircuts. If Frisk is a girl, it would explain why she looks rather boyish, since she doesn't have access to quality makeup that girls that age are often interested in, and has to make do with boys' clothes. Remember, Frisk is at an age where children are just starting to understand the differences between genders, at least on a superficial level.

Chara and Frisk are full blood siblings.
  • Chara fell at 201x (as shown in the opening). It is unknown at what year the game takes place, but the surface world doesn't look that futuristic. It is possible it only happened within a decade later (enough time for six children to fell down the hole).
  • Frisk heard the story about Chara and either wanted to find out what happened, or decided to follow Chara's footsteps (If the WMG about Frisk being abused or abandoned is true).
  • It also explained how Frisk had determination. Maybe it is hereditary?
    • To build WMG above : Chara was either abused or neglected by their parents. Chara was really named after 'Character' (note the lack of care in naming their child). Chara then went up to the mountain. Chara's parents came to regret this, and went to seek their child only to find Asriel bringing the body back. They shoot him. Later on, they decided to not repeat their mistake. They had another child, Frisk, which means 'Fresh'. Unlike Chara, they loved Frisk and thus Frisk grown up to be a pacifist that loves everything.

Chara fell down in this world's equivalent of the 70s or 80s.
  • Let me explain: Chara fell down after Asriel's birth, which was after the time when video recording with handheld cameras seemed to be possible. Now, this makes a rather decent window of time, considering it was considered a long time ago. Chara and Asriel's clothing does not indicate that they came from ancient times in any way, but they wouldn't look too out of place in somewhat more modern times.
    • Taking in other WMGs into account, it may be possible for Frisk to be related to Chara, as perhaps a niece/nephew or grandniece/grandnephew.
  • Uh, in real world, VHS is popular around 80s-90s. Unless you imply the underground invent their own technology (Alphys' anime source seemed to imply they reverse engineered human technology that was already thrown away). As seen as in true pacifist, it was Chara who fell in 201x (note the single striped shirt as opposed to the one that Frisk used). So jossed?
    • Not necessarily. We don't know that monster calendars and human calendars work the same. Chara fell down in monster year 201x. What is that in human years? Why not the 1980s?

Frisk is nearsighted.
  • It would explain why they're always squinting (especially when Chara doesn't do the same), and why the Cloudy Glasses confer a stat bonus. Wearing someone else's prescription glasses usually blurs your vision instead of enhancing it, so Frisk and the glasses' original owner must have the same or similar conditions.
    • Adding to this, many battles have the unusual flavor text of "smells like [x]" and Frisk can also hear the sound of a music box from several 'rooms' away. Nearsighted people's sense of hearing and smell are usually very strong, to make up for their failing eyesight.

In any route other than No Mercy, Chara isn't actually evil at all.
  • Why would Toby make a game all about encouragement, kindness, and never giving up, one that allows you to never kill a single monster, and then also encourage players to use their real name as the character's name, if the character was just meant to be a genocidal asshole? It seems hypocritical. What if Chara never actually hated the outside world? They fell down purely by accident, and everything they did was an attempt to help Asgore and Toriel (even if it was misguided).
    • Creepy smile? Some people have those naturally, and Chara's smile is only creepy in context; if you had simply met them in the overworld like any other NPC, would you even notice a problem?
    • Laughs when Asgore is poisoned? Nervous, regretful laughter. They genuinely didn't know. Or maybe Toriel once said "don't eat those flowers, they'll make you sick" and Chara took it as "you'll get a bad tummyache or the runs" (or something else within "semi-harmless prank" territory) instead of what ranunculus poisoning actually does, which Toriel certainly wouldn't want to explain in detail to a child.
    • The only poisoning that was deliberate was their own, because they cared so deeply about Asriel getting past the border that it was a sacrifice.
    • And if one goes by the narrator being Chara, the really creepy stuff only shows up after you go out of your way to kill everyone. Chara at the root may have been flawed but never so twisted. In the Pacifist route, Chara observes/abides as your kind nature brings forth their better aspects as Frisk builds up his character (perhaps enhanced by the player's actions). Neutral has the two evened out, and No Mercy has the player's wanton bloodlust corrupt Chara and their darkness creating the menace we see here.

Frisk would have died when they fell into the Underworld if not for the Player
While nobody knows how high the fall down to the Underworld is, it seems likely that you would at least break something in the fall. To make things worse, the very first thing you see after the prologue is nothing but pitch-black darkness. Anyone, even Frisk, would feel slightly panicked in that kind of situation, and the risk of death by infected injuries would shoot higher due to that.

Thankfully, the Player takes control of Frisk at that moment. Regardless of what morality the player is, they have a good motive to keep Frisk alive - can't use someone as a good killing puppet if their body isn't responsive! - and thus force Frisk to move in Flowey's direction, which leads to Toriel's house, and thereafter to Frisk getting whatever treatment they need.

Chara is the narrator for every RPG whose narrator isn't otherwise defined.
"But these other RPGs have a different narration style!" Clearly they're roleplaying, because it's fun to roleplay, and also they need to do something to distract them from forever being a disembodied dead kid whose only plan failed horribly.

Chara was a psychopathic asshole right from the start.
  • Chara had no Freudian Excuse, rather, they hated and abhorred humanity, as they felt superior to them and hated being in their presence. This, along with the need for stimulation consistent with psychopathic behavior, they fled their village, fell into the Underground, and proceeded to fake being a loving, kind child to Asriel and his family, all to use them in their plan to destroy humanity.

Chara versus Frisk. Who would win?
  • We're assuming that they both have separate bodies, each at "full power", i.e. at the very end of their respective routes.
    Now, Frisk is a complete pacifist with very little capacity to violence and pretty much no intention to act on any violent impulse; meanwhile, Chara possesses enough power to literally destroy the world - a level of power Frisk cannot even hope to reach. However, Frisk's Determination allows them to come back from death infinite times... but so can Chara.
    Except that Determination is limited to the most determined "recipent" within a "system" (Flowey loses the ability to SAVE once the Player steps in, as the Player's Determination is even greater than Flowey's). Frisk has been shown to be so determined, they're able to ignore death and keep pushing forward through it, as opposed to Flowey and Chara, who can merely go back to their own save points; they're also capable to reset even the memories of those who possess Ripple Proof Memory. Frisk is clearly the one with the greatest Determination.
    So, this is the end result: Frisk cannot be killed by Chara, since the former's Determination allows them to simply shrug off death; and Chara cannot be killed by Frisk, as the latter doesn't have the will to do so. Nobody can kill or be killed by anyone... which means that, essentially, Frisk wins.

Frisk/Chara in the Tainted Pacifist Ending isn't nearly as dangerous as they seem.
In the corrupted Golden Ending, even though they're now possessing Frisk, they're not actually powerful enough to do more than Poke the Poodle. The prototypical example is to assume The Stinger you get in that ending is the entirety of Chara's exploits — if you stayed with Toriel, Chara... disrupts Frisk's sleep and possibly steals their pie. Or if you said you have places to go, Chara can't do more than vandalize Frisk's memento.

There is actually in-game evidence in favour of this interpretation, if you interpret game mechanics/events a certain way: the LV you see on the battle screen is Chara's, not Frisk's... and it resets to 1 even after the Fallen Child's "Corrupting Reset". Now, sure, a Chara with level 20, full control over Frisk, and the Real Knife might be capable of going full infinite-9s on the World, but a Chara with no EXP and LV to their name? Well, they might be capable of killing a few weak/off-guard monsters, and starting the process of levelling up again... if the human cops didn't stop them. Or if someone else doesn't first... doesn't the Corrupted Pacifist Ending play out like the normal one, implying that Frisk is a separate entity, until The Reveal? Sure, Chara might own Frisk's SOUL, but what about his body and brain? "Despite everything, it's still you" indeed...

This would also mean that the Genocide Route ending is the best victory Chara can hope for.

  • Not outright refuting this, but there is a bit of a complication in this theory. LV (Level of Violence for the uninitiated) isn't really an artificial ranking system, but more based on one's own feelings, or lack thereof. Sans' sums up LV at the end of his judgement as, "The more easily you can bring yourself to hurt others," which is a feeling once can gain through a desensitization to killing or have as Chara did and still does. While Frisk might not have gained any LV or EXP themselves this go around, Chara certainly has a "capacity to hurt," thereby making their Level of Violence high enough already. Just a thought.

Selling your soul to Chara gives him control not only over Frisk, but the game's binaries too.
For one thing, Flowey with 6 SOULs managed to do just that due to how he changed the title, opening scene, and battle system. Additionally, the LV in the menus are supposedly Chara's, which makes his sudden killing intent at the end of a Tainted Pacifist run contradictory to his low LV. This means Chara (with 2 SOULs) is hacking the menu to lie about his LV, just to catch the player off-guard or mock them. And the reason a reset no longer restores Chara's LV to 1 is because he used his powers to rewrite how resets work. This guess could also be a Deconstruction of how many games' New Game+ feature carries over levels, which is exactly what Chara does to his LV.

Frisk soul represents something like freedom or choice.
Frisks soul being determination itself seems far-fetched to me. All humans have determination - why would Frisks soul have more determination than anyone else? That sounds like a pretty garbage soul trait to have.

Freedom - they are the only soul that can move freely about the fight box thing. No other soul can do that. Choice - only when your soul is red do you have the ability to dodge attacks to the best of your ability, or flee the battle. You have no choice but to stay and fight in any other mode.

Chara gravitates toward Frisk initially because of their similar circumstances in life, and in a pure True Pacifist run, is only a passive observer.
Chara hates humanity, which isn't really all that hard to do if you're a small child who's been abused or neglected. Frisk begins the game with a bandage that has been used multiple times, implying that he gets hurt often enough to have learned both how to apply his own bandages, and that he needs to ration what little supplies he's got. In the fight against Photoshop Flowey, we see those bandages are basically just ordinary band-aids, it seems likely that Frisk is neglected, abused, impoverished, or some combination of the three. Chara doesn't interfere on a pacifist run because Frisk is showing him that hating humanity is a waste of time when it's completely irrelevant.

Frisk is Chara's reincarnation, but without Chara's essence.
In other words, the reverse of what happened with Asriel. Flowey is Asriel's essence within the form of a flower and then given the will to live through Determination, without the benefit of a soul. Frisk is the reverse, for Chara: a soul within the form of a human and given the will to live through Determination, but without the benefit of Chara's essence. That's why the No Mercy route calls Chara back to life, the capacity to harm others is the essence of Chara, and why Frisk has no defined personality. Their behavior has no inherent character because Frisk has no inner self but what they earn by going through the Underground.

Frisk is flat out BLIND.
Frisk's eyes appear to be in an eternal squint, but they're actually completely closed. When moving around the underground, Frisk's field of vision is always restricted to a few square meters in an even radius around themselves. The only exceptions to this limitation are when other characters draw Frisk's attention.

During combat, Frisk's opponents are invariably black and white. That's because from Frisk's point of view, they have no colour. They only have colour in the overworld because otherwise the game would be kind of bland to look at. Frisk can only tell where they are and what they look like based on Chara the Narrator's descriptions and the smell and sounds of the opponent. On top of already causing its own share of problems combat wise, it makes Sans' fight even more of a pain than it would otherwise be, as Frisk cannot detect Sans fast enough to be able to strike where he is going to be. When Frisk does eventually pull the finishing move, it's Chara doing it.

So why are things like souls and lasers still coloured? Because it's not physically there. They are, in fact, invisible to the naked eye but Frisk can see them because it's related to souls.

The visual representation of Frisk's SOUL (read: the heart) is a Stand.
A physical projection of the user's mental fortitude and very being, that, when damaged, injures the user's body as well... Sound familiar?

Well, alright, fine, it probably doesn't, but it would sort of help explain Chara's similarity to Dio, in terms of the plot.

Frisk is related to Fran.
They do look kinda similar.

The reason Chara is the way they are now is because they had SAVE powers.
Flowey says that the being with the most determination could save and reset, and that was Chara at one point. Chara was scared of dying in such a horrible way and would constantly go back over and over, trying to savour their life with their new family, but it backfired horribly. They could see how predictable everyone was. How weak and defenceless the monsters were, and began thinking that killing their new friends and family would give them power. Horrified by their thoughts, Chara allowed themselves to die, seeing it as a fitting punishment.

They woke up when Flowey called them, still in this mindset. In a Pacifist Run, they see Frisk befriending everyone and how strength can come from kindness, and start to rethink their ways. At the end of the day, they wonder if Frisk will end up like them if Chara resets and makes them repeat the same day over and over again, and resets. On Genocide, Chara sees their worldview confirmed and believes it, becoming the person we see at the end of Genocide. Even seeing how kindness can change people and free an entire species does nothing to change their mind anymore on Soulless Pacifist, now they see Frisks lack of LOVE, their lack of HP and the fragility trusting brings, only being amused by the irony of Frisk, who destroyed the world and all monsters, now befriending them, and at the end of Soulless Pacifist, Chara gains love. Just not the type Frisk wanted.

You really do sell your SOUL to Chara after the Kill 'em All run, not Frisk's.
Chara isn't possessing Frisk in the corrupted Golden Ending because they own Frisk's SOUL; they're controlling Frisk because that's something your SOUL can do.

Chara deliberately holds back against Asgore in a non-Genocide run
If we go by the theory that Chara is the narrator affected by the player's action, look at the Asgore fight in the Neutral/Pacifist runs; no matter what they have (yes, I've tested using the Real Knife by editing the save file), and no matter how many monsters they kill, they always leave Asgore with an inch of his life, no matter what. They can't seem to kill their surrogate dad, unless the player decides to. They must have genuinely loved him and didn't want to kill him.

Chara is the reincarnation of Not Important.
Really would explain a lot wouldn't it?

     Meta/Other 

SOUL is also an acronym
  • SOUL is always capitalized in the same manner as EXP and LOVE. Maybe it stands for Source Of Unlimited Love?
    • Or Source Of Unlimited LOVE, in the No Mercy path.

GOLD, too, is an acronym.
  • Since you typically get GOLD by sparing monsters, maybe it stands for Graceful Optimism and Likable Demeanor.

Each SOUL can manipulate its own repsective aspect, but most have yet to use it to its full potential.
  • The Red SOUL, whose trait is DETERMINATION, is able to control time, SAVES and LOADS, and even refusing in the battle against Asriel, the SOUL has grown so powerful and determined that it can essentially rewind time from the brink of death or even at death instantly, without the need for savepoints.
  • The Purple SOUL, Perseverance, is able to control space. The battle against Muffet locks your soul into three lines on the grid, but movement between these three lines is also much faster than how a red SOUL moves freely. In addition to this, the only other time a Purple attack is used is in the battle against Sans in the form of KR poisoning. We also know that Sans has some sort of teleportation ability, which was implied throughout the Pacifist and Neutral routes, but he outright does in the No Mercy/Genocide route. Teleportation would make sense to a person who can manipulate space. This also explains why he has knowledge of the anomaly (besides his possible study into quantum physics), but no memory of past events after the player LOADS.
  • The Green SOUL, Kindness, can possibly control energy, more specifically, the concept of energy in non-modern metaphysics. The ability to heal and create a shield are two common basic applications for energy. Despite this healing, the SOUL can not resurrect dead beings.
  • The Yellow SOUL, Justice, can manipulate magic, becoming yellow inverts the heart which makes it similar to a Boss Monster's SOUL, and monsters can use magic. This is also evidenced by the fact that among all the modes the soul can take, only this color can "attack." I got this idea from a comment on the wiki.
  • The Cyan SOUL, Patience, can control waves, not just waves in liquid, but waves in general, in physics. This is why not moving from cyan attacks can cause you to take no damage, as you are letting the wave pass through you.
  • The Orange SOUL, Bravery, in contrast, controls particles, which is why moving through them does not damage you.
I am unsure what the Blue SOUL manipulates, but I'm inclined to say causality for some reason. Perhaps it controls gravity, as with Jump Mode.

Frisk is the only one to almost completely master their aspect.
Because, despite the previous WMG, Frisk, has a Red SOUL. The other SOULS, while they had the potential to be powerful, died before they could get better at their abilities. The farthest anyone made it was the Yellow SOUL with their ability to manipulate magic, and the Green SOUL which enabled them to heal themselves, but none of the previous SOULS had the ability to resurrect themselves from death as Frisk's Red SOUL does.

Hey, someone had to propose a crossover idea sometime. Chara is actually the new incarnation/host of the Demon King Odio.

  • After defeating Pure Odio, Oersted's final form as the Demon King and final boss of Live-A-Live, he declares before he dies and fades into nothingness for good that as long as hate exists, anyone can become a demon. Cut to a time unbeforeseen in the game, either between any of the other timeframes or in a new universe entirely that spawned after the end of the old one at some unspecified point in time, and the Demon King itself has survived to the present day as a nebulous presence, and eventually sensed Chara's hatred towards humanity and possessed them, thus leading to them hatching their big plan and setting the plot of the game into motion. The Demon King couldn't manifest it's powers for a time, and needed more than simply Chara's (admittedly horrifyingly vast) hatred to regain it's former strength after such a long period of inactivity, leading to Chara's planned death; if the player goes for the No Mercy Ending, Odio's power returns greater than before, and it came back with a massive upspike in savvy to boot after realizing the power of SAVE.
    • Thankfully, it's only in the No Mercy ending that Chara/Odio wins; the final bosses of each route can be taken as Odio evolving into its Pure Odio form, as it did once before back in Live-A-Live, but each being different due to the circumstances. In Neutral, Omega Flowey represents Odio using the flower form itself to manifest, and is possibly the most raw of the three in terms of appearance and sheer malicious behavior during the fight. In the True Pacifist ending, Flowey manages to reclaim his original form as Asriel, but Chara and Odio are having none of that and again possess him, this time allowing the Demon King even greater power than before when combining the power of all the souls in Asriel's body with all the hatred the Demon King has managed to feed on from Chara's soul and Flowey's hate. This is the manifestation of Odio that most closely resembles the original Pure Odio as well as the battle that ends in the most similar way, and when Frisk finally manages to save all the souls Asriel has trapped, as well as Asriel himself, Chara's soul is destroyed at last and Odio no longer has anything to anchor to, banishing the beast from this world until it has fed on a sufficient amount of hatred to again rise. However, the No Mercy Ending is, as said before, the Demon King Odio's victory; roughly equivalent to the Armageddon ending of Live-A-Live with a bit of the ending when one completes Odio/Oersted's final chapter thrown in. Chara has successfully manipulated Frisk into becoming the new host, and thus Odio takes hold of the body of the Fallen Child reborn, and with all it's power the Demon King destroys the entire universe of your game, with plans to move on to another Undertale universe to feed off the hatred there and eventually destroy it, too.
    • Alternatively with the completion of No Mercy, you created under your guidance the enity known as Perfectio, the king of ruin.

MEGALOVANIA is not Sans' boss theme, but your's/the Fallen Child's.
  • Not necessarily my idea, but I figured I should put it here anyway. Sans already has a theme—'Song That Might Play When You Fight Sans'. And it's not like the other Dummied Out songs like Gaster's Theme, Happy Town, Meat Factory, or Trouble Dingle, which are just repetitive, simple loops of music. In every medium that MEGALOVANIA has appeared in, the scene always had a common theme in it: a character (two out of three times the 'protagonist') killing someone in cold blood and not caring, despite the fact that they could've turned around and chose a different path.
  • In most video games you're saving your world from the brink of destruction, however in a No Mercy route it's the other way around and instead it's Sans that's fighting to save what's left of his world. He even fights in many ways someone playing a video game would: big, flashy attacks, dodging attacks (which, even in this game, only the player could do), and even stalling in his turn to buy time. You're the final boss of Sans' game, and MEGALOVANIA is your battle theme.
    • Basically, you're not fighting Sans. Sans is fighting YOU.
  • In the same way, Hopes and Dreams is Frisk's theme rather than Asriel's.
    • It might actually be SAVE the world, considering the context
  • To further support this point, all of the other songs specifically related to Sans—"sans.", "It's Raining Somewhere Else", and "Song That Might Play [...]" share a common riff, but "MEGALOVANIA" is an unrelated track.
Adding on to the previous one, if Sans had decided to fight you in any other path...
That's when you would have heard "Song That Might Play When You Fight Sans". That is his normal boss music, but you never heard it since he promised Toriel he wouldn't fight you.

And combining the previous two with entries higher up on the page...
Given the theme of being Dummied Out surrounding W.D. Gaster, "Song That Might Play When You Fight Sans" could itself be Dummied Out because of Gaster's absence.
  • It's possible that, if Gaster were around to be the "judge" as suggested in previous entries, he might have sent Sans in his stead given their connection and had him test the hero's determination and dedication to showing mercy with an easier form of Sans' No Mercy boss fight. In this case, the dummied out song would have played. Because Sans doesn't fight you unless he absolutely feels like he has to - i.e. a No Mercy run, where MEGALOVANIA plays instead - and he didn't have the push to test you, it never does.

Humans and monsters have different battle interfaces
If 2 humans were to fight each other then it'd play out like a typical RPG battle, the battle's outcome being determined by raw stats, while 2 monsters fighting each other would play out more like your typical bullet hell, with both sides throwing and dodging bullet patterns. After all, the only kind of battle we see all game is human vs monster

The battle system works depending on how alert you are
That's why flowey and Sans are the only characters to have used an unavoidable attack, as at the beginning of the game you weren't expecting flowey, who just tricked you/had been using easy to avoid attacks up until then, to suddenly pull out an attack like that, it also explains why Sans can only do an unavoidable attack if you spare him, as sparing him means you let your guard down and leave yourself wide open, It'd also explain why you get an outline for a number of Omega Flowey's attacks and why all of Sans's attacks are avoidable, as you're alert during those fights, It'd also explain why Omega Flowey can trap you in an unavoidable attack at the end of the fight, the shock of seeing your efforts being negated made you incapable of being alert

Actually, the best ending is the normal ending. With the barrier still standing.
  • The barrier is actually there to protect the monsters. A human can't enter the barrier without a monster's soul, and there's no other monster in the outside world. The only way human can enter is by falling a hole, which would probably discourage many.
    • It was stated in game that anyone could enter through the barrier, it's leaving that requires a human and monster soul. As a result, this wouldn't be an effective method of protection.
  • The game implied that Humans Are the Real Monsters.
    • The first fallen child might be horrible, yes, but they are still a child. What actually caused them to hate humanity so much?
    • In the true pacifist route Frisk is asked why would they climbed the mountain in the first place, if it was rumored people would never returned from the mountain. It isn't far off to guess that Frisk might have fallen down the hole on purpose.
    • Bonus point in the end if Frisk stayed with Toriel instead of family or their caretaker. Alternatively, Frisk said they had places to go. Why mentioned places, plural, instead of home?
  • While most monsters are peaceful and mean well, that doesn't mean they can't (accidentally) harm humans. Anyone who played pacifist route knew this. How many incident would happened now that the monsters had moved above the surface? Eventually it would end in another war.
  • Either there would be some good people accidentally harmed by monsters...or, alternatively, there would be some other people who would purposefully seek this 'random encounter' for free EXP. The No Mercy route only took one kid to do it. Now monsters would encounter the ENTIRE human race.
    • If there were another war against human, monsters can't really expect to win. Not without truly becoming 'monsters' like what Flowey did...
  • Only humans can SAVE (and reset). It is unknown if ALL humans can do it, but it doesn't seem to be a rare ability. What's stopping some children from killing some monsters and do it again? And again. And again. And the monsters wouldn't even be aware of it.
    • Well, judging by how the protagonist "usurps" SAVING from Flowey, there are probably substantial limitations to it - only the Most Determined Person in a given place can do it, say.
  • Even if there were no wars between humans and monsters, it's possible there are still wars between humans.
  • Tl, dr; The true pacifist ending might not be the happiest ending. It is also lampshaded by Asriel that the world above isn't as kind as the underground.

  • This is all Jossed, for what it's worth - Toby's said that the true ending is meant to be taken at face value, and while we might expect some Fantastic Racism, anything beyond that would be dipping into AU territory.

The consequences of final pacifist choice.
  • Choice without consequence is boring, so it's guessing time. Staying with Goatmom is obviously the most heartwarming and most detailed of the endings, as well as the first option. Not to mention pretty much everyone's favorite pick. But is it really the best choice? There are actually a few potential hints that this may not be the case. First of all, the turtle shopkeeper. If you talk to him during the final walk, he informs you that even if you are adopted into the Dremurr family that they will surely outlive you. While it is sad that Toriel will remain lonely if you leave, eventually you are dooming her to once again bury her adopted child. Second the silver key easter egg. The room which it unlocks doesn't contain too much until the final walk of the true pacifist route. During this segment, the room will contain a photo of Frisk and all of their underground friends. This is implied to be the same photo you see at the end if you choose not to stay with Toriel. Well, Sans obviously has deep knowledge of all the timelines, but why would he leave that photo there at the very end? He could very well be trying once again to guide you in the right direction. Third, Frisk DOES have a home. It is mentioned as one of the choices to leave the ruins very early in the game. So, the question is why did they leave? The game gives no answer. It could have been terrible circumstances, abuse, etc. Or perhaps even entirely selfish reasons. Either way it was likely not a happy reason. Whatever the case, this may very well be what Frisk was referring to when if they say "I have places to go." There still may be business they need to settle or come to terms with, and by staying with Goatmom they instead decide that it is better to just run away from them.
The human world above is a Crapsack World
  • It's implied that the anyone could have ability to SAVE if they are determined enough. Unless there were an equivalent of Time Police or something that watched the timeline(s), there would be plenty of timeloops in the human worlds.
  • Some good people might use the ability to reset to save the world. Some others...what stopped a crazy serial killer or a rapist from committing his crime several times, and repeated again?
    • See the above point though, the fact that Flowey loses the power when the Child gains it implies that only one person can have the power to SAVE and LOAD at any time, I don't think they'd have to worry about your average Joe getting a hold of that kind of power. And all this is even disregarding the fact that the ability to SAVE and LOAD may not even exist in the overworld and might have been a result of whatever happened to Gaster.

The barrier acted as time barrier as well
  • The barrier is allowing the underworld to have a linear timeline, no matter how many times a reset happened in the outside world.
  • This explained why you can only reset the time back to when Frisk fall. Given how determined Frisk was, it is difficult to believe they didn't have SAVE point back before they fall, it is just simply NOT accessible to them. If they could, they would simply just RESET the time back before they fall.
  • In No Mercy, Sans dialogue referred to you as an anomaly in time. Why it must be you when there could be any other humans above that repeated the timeline? It's just you're the only one that can be observed changing the timeline in underground, as the human world had their own timelines.
  • This would also explain why Asriel doesn't seem to be too worried about turning back into Flowey as long as he's stuck in the underground, even if he can save and reload again, assuming he can only do it as far back as when Frisk lost the power upon leaving the underground, he can't reset again, only YOU can.

The status of the relationship between Asgore and Toriel?
  • Asriel Dreemurr was revealed to be the child of Asgore and Toriel. The two have since separated. One of the anagrams of Asriel's full name is "Serial Murderer", which references him taking all the monsters' souls for his fight against the player. But another anagram for the name is "Rulers Remarried". This could also be meaningful. Does this mean Asgore and Toriel eventually reconcile? Or does it mean they find other relationships? Of course, this could also be overthinking things. We may never know.
    • Sorry, but Asriel Dreemurr is one R short of being an anagram of "rulers remarried". Nice thought, though! That does, however, mean it's an anagram of "rulers e-married", which can mean only one thing. Some time after moving to the surface, both Toriel and Asgore turn to internet dating. And of course they unwittingly pair up with one another. And then they agree to meet up and find out who they were getting along so excellently with online. And since anagrams are the most powerful force in the universe, they obviously reconcile and get back together. Obviously.

In addition to not having blood, most monsters don't have bones either
Which would make Papyrus correct in his theory that humans descended from skeletons!

Why Papyrus and Dr. Alphys are OK with dating a kid.
It's kind of obvious monsters don't work the same way that humans do, but it goes deeper than the magic and appearances. My theory is that monsters don't gauge age the same way humans do. Instead of looking at how old they are, they look at how developed their soul is. Boss monsters only age when their children do, by giving bits of their souls to them. This makes it seem as if the child doesn't quite have a fully developed soul, and by monster standards, isn't mature. Frisk, however, is a human with a fully developed soul, capable of fighting and such even at a young age. Monsters are more or less nothing without their magic, while humans are very much a physical race. Monsters can't really tell the age of someone by sheer looks, considering how just plain odd they can look. Thus, Dr Alphys and Papyrus aren't creeps, they just don't know what a human child looks like. In Dr Alphys' case, she didn't really mean the date at all anyways. Plus, Papyrus had to read a handbook for dating, and probably doesn't know what it means anyways. As for the monsters who can tell that Frisk's a kid: Most of them are old enough to have been around during the War. Toriel, Asgore, and that turtle shopkeeper could have easily known what human children are like.
  • I thought it was indicated that the monsters assume that anyone wearing a striped shirt is a child. At least that's how that one NPC kid says it.
  • Papyrus also doesn't mention anything about romance unless you flirt with him, meaning that the non-flirt date could possibly be a play date. It certainly fits his childish demeanor. The flirt version has him reject you upon realising that he isn't feeling any romantic emotions, supporting the idea that he doesn't really understand what a date is.
    • Heck, none of the dates veer into sexual territory (Word of God claims Papyrus doesn't really even know what sex is), and the only romantic one is with Dr. Alphys...and only because you're roleplaying with her to try to get her to gather the courage to date Undyne.

ALL the characters are aware of the nature of the game
  • They know they are characters in the game and make comments if you attempt to name the fallen child using their name.
  • 'Underworld' is kinda like a game version of Toon Town. The characters are just playing their roles and memorizing their lines. No time travel involved at all. The player are just sent to the last save point upon 'death', or sent back to beginning upon 'reset'.
  • The 'hard mode' is canon. No characters involved are actually 'dead', they just went off-screen and adding 'special effect' of disappearing. Note they also tried to make the 'game' child friendly, the monsters simply turned into the dust upon death.

Undertale takes place in the same universe as The Gamer

Flowey's Take That toward watchers of a No Mercy playthrough is another one of his plans.
When Flowey talks to the player between Mettaton NEO's defeat and the battle against Sans, he refers to anyone else watching the playthrough as a cowardly sicko. However, this could also be an attempt to bait other humans into carrying out his plans. But instead of tempting them to prove their bravery and do a No Mercy playthrough of their own, he is instead trying to use Reverse Psychology; he hopes that the viewer is horrified by the events they just witnessed and will try to prove him wrong with a Pacifist run, which is exactly what he wants them to do because the climax of that run allows him to claim a veritable treasure trove of souls and regain his true form. After all, if your current pawn has grown so utterly depraved that you start to grow scared of them, why not try to recruit another?

DETERMINATION is another form of Spiral Power and Getter Rays
The Monsters are vunerable to its effects as they are not compatible with it. Overuse and abuse of it can make you more murderously dangerous than the Getter protagonists. Chara was able to use it to remain in existence and attempt to regain an existance by jacking Frisk, and Flowey was engineered to be able to use it.
  • Determination might also have a connection to Digisoul, explaining how you can one-shot monsters who seem powerful. They're made of some otherworldly pseudo-matter that reacts to human emotions.

Bathrooms exist in the Underground, but they're not like human bathrooms.
  • Well, of course they're not. Monster food doesn't require digestion, thus there's no waste product. Monster bathrooms are quite literally bathrooms, as in places where monsters bathe and wash up, because that's all they need them for.

Hard Mode, once it's finished, will be utterly nightmarish, and will reveal that nothing still is as it seems.
Perhaps in the style of Metsu in Irisu Syndrome! — only this time, a happy ending is not guaranteed at all. Perhaps not even if you complete True Pacifist.

What would you expect, when the protagonist and the Fallen Child had the same name, and possibly were the same being? It's not telling you "your life is going to be living hell" for nothing. Plus, as stated above, you the player will likely never be the kind of person Frisk would be.

In fact, one ripe concept for ruthless Deconstruction here could be the very idea of Frisk—the video game hero that you want to be, but never will be. Because it's an impossibility, that you ever will be Frisk, in both meta and real-world senses. Obviously this would plunge the game into the cynical side of the sliding scale, but isn't that the whole point?

In a future version of the game, you will get punished for trying to escape from selling your SOUL.
Meaning that no amount of fucking with your savefile, clearing your Steam Cloud cache or automated patches will help you. The game will detect this, and it will only make things worse. If anything, there's always the registry to hide the necessary infos.

You cannot escape from your sins.
  • There would always be someone DETERMINED enough to figure things out, even if it was in the registry.

Monsters have to be nice or else they will die.
Falling down is the monster equivalent of an incurable illness where the monster loses all hope and dies. If a monster encounters a lot of negative emotions, they run the risk of dying. Therefore, monsters have to be constantly nice. They painted humans as awful and evil to make everyone feel better and act nicer - monsters hate humans and don't want to be like them. On the surface, monsters can be horrible to humans, because humans won't die from being upset over hurt names, and are also ridiculously OP and could literally commit genocide if they wanted to. This is also why the monster population is so low when Frisk arrives - people are losing hope of escaping, Chara and Asriels deaths and any deaths the previous Fallen humans may have caused would all have caused monsters to die off. It also doesn't get better on the surface if you were wondering.

Flowey's SAVE is ruined by multiple No Mercy Endings.

Flowey has, by his own admission, explored every possible route to take through the game prior to Frisk's arrival, meaning that he's killed everyone numerous times. Despite the lack of a Deal with the Devil with Chara, Flowey's SAVE is still tainted by his crimes, and as a result he's unable to ever achieve a perfect ending... which carries over to Frisk's SAVE, as Flowey is the only major character (save for Chara and Gaster]]) who doesn't benefit from the Golden Ending.

Undertale is a sequel to Yume Nikki.
AU where instead of killing herself Madotsuki works up the courage to leave her room.

  • After extensive therapy, she starts to piece her traumatic memories together, and her dreams, while still symbolic, elaborate, terrifying, and bizarre, start making a lot more sense. Undertale is those dreams, the Underground is the new dream world, Frisk is Madotsuki, Chara is a symbol of all her repressed negativity, Asriel is her lost innocence, and Flowey symbolizes all the places she thinks to look for that innocence but doesn't find it.

  • All the other major characters are symbols of influential people or experiences in her life, and they all want to kill her at first because abuse and blah blah blah, but they become her best friends to symbolize her coming to terms with stuff and subconsciously forgiving them for the sake of her own mental health.

  • Gaster is Uboa, of course. Whatever memory he symbolizes, it must have been pretty messed up, because her mind decided it would be best to just vaporize it and spread it thinly across the entirety of her being, just so it would never have to surface again, even if it meant the rest of her mind would become a little more corrupt.

  • The amalgamates have something to do with the traffic accident.

  • Despite Madotsuki being female, Frisk is genderless because Madotsuki doesn't really give two shits about gender. She has no problem using male restrooms, if you'll recall.

Every player is Napstablook.

  • There's a reason the poor guy always feels like trash.

  • That's not to say he deserves it. Getting into the connection this implies between Napstablook and Chara, I think Napstablook is Chara's disembodied soul, which was never allowed into the body because of the body's, um, "neurochemical imbalance." Left to fend for itself, the soul grew away from humanity, grew a layer of protective magic, and thereby became a monster in its own right. Napstablook may not be aware of everything the player is doing, but he gets the vague feeling that he's somehow metaphysically responsible for something horrible (and by extent fucking miserable and full of self loathing) but powerless to stop it. Just like our conscience. As is customary of a lost soul, Napstablook let this grief and negativity consume him completely, until it became the whole of his self image, to the point where he's forgotten what he's even so sad about other than how much he sucks.

Undertale is a multi-generational sequel to The Legend of Zelda

AU where Undertale takes places hundreds, or even thousands, of years after Zelda. There are a lot of similar character archetypes between the two games.

  • Toriel's symbol looks like a muddled up version of the Hylian Crest.
  • King Asgore and Ganon are both relatively similar in that they are both hoofed animals that act as kings of monsters, they both use fire powers and wield a giant trident as a weapon, and they are both Well-Intentioned Extremists (When considering Wind Waker).
  • Papyrus and Sans are skeletons that act as guards... just like Stalfos in Zelda.
  • Muffet is a spider that loves money, reminds me like how the Skulltula Men were cursed for their greed.
  • Dr. Alphys is a lizard/dinosaur person, there's Dinolfos and Lizalfos.
  • Undyne is not only a fish person like a Zora is, but even her appearance resembles a River Zora.
  • Technology is highly advanced in comparison to ours. This could've easily started out from the technology seen in Twilight Princess.
  • There are segmented blobs in both games, Moldsmal and Like-Like.
  • You play as a person who can mess with time travel, much like Link can in Ocarina of Time and Majora's Mask.
  • The hero of both games has an evil counterpart that looks similar in appearance, but is the complete opposite in personality. In this case, Undertale has Frisk and Chara, while Zelda has Link and Dark Link.
  • The main villain, Flowey, is a psychotic, all-powerful being that you meet early in the game and torments you, also has a childish laugh, is represented by a large face, and needs the power of friendship. This also fits the description of Majora.
    • The main villain in both also happen to be innocent characters that got horribly screwed over (Asriel and Skull Kid).
The human SOULs each grant the following abilities:
  • Red SOUL: Determination, for obvious reasons
  • Aqua: Massive size (hence "'Ball' is 'Small'". A large character has less mobility than a smaller one, hence Patience is needed.)
  • Orange: Extremely high offensive power (hence Bravery and "tough")
  • Blue: immunity to the influence the others may have upon you (hence Integrity and why Flowey wasn't effected by their personalities fighting back, unlike Asriel
  • Purple: Extremely high defensive power (hence Perseverance). It also grants the ability to trap enemies. Frisk was completely incapable of doing anything but fight when Flowey attacked him/her, could not escape Muffet's spiderweb, and Sans tried to trap him/her by not taking his turn.
  • Green: Unlimited ability to heal (hence Kindness. Photoshop Flowey did not make use of this ability for obvious reasons.
  • Yellow: Ability to use impossible attacks (such as flamethrowers and nukes with your face on them). It also grants the ability to use Always Accurate Attacks, including Flowey's infamous bullet ring.
  • All of them together: Infinity Asriel has infinite ATK, DEF and HP, his combat background is a wave of cosmic rainbows, and his abilities all have awesome, stellar-sounding names.

All of the previous humans could save.
Omega Flowey has six save files for each soul he absorbs, and Toriel says that all the humans that fell down before are familiar. No one would be able to make it to Asgore without dying nice, it's near impossible.

Alternative to the above,it is a meta stat boosting
  • Red : Ability to SAVE
  • Aqua : High DEF
  • Orange : Either high STR or SPD
  • Blue : High ATK
  • Purple : Either high WIS or INT or LUK
  • Green : High HP
  • Yellow : High ACC

Soulless pacifist isn't that bad
  • If you decided you had places to go, Frisk/Chara simply crossed out the photograph. If you decided to stay, Chara simply possessed Frisk. It doesn't necessarily mean that Chara (will) kill everyone.
    • And you know that Chara hates photographs from True Pacifist.
  • It isn't that bad... well, except for Frisk being soulless. But hey, Flowey also didn't have a soul and he's still alive.

The game is a prequel to Animal Crossing
After the return to the surface, humans and monsters once again live in harmony. The monsters establish settlements which are open to humans, these being the Animal Crossing towns. The Reset Surveillance Centre is established to prevent the abuse of SAVE, staffed by the Resettis, who can perceive timeline changes. Gross changes to the timeline are overseen by Rover, who, like Flowey, advises you against such things.
  • Worth noting that the Animal Crossing WMG page has an inverted take on this theory: that your player villager is a fallen human kept blissfully unaware by the NPCs who are simultaneously keeping you hidden from Asgore and his subjects.

[[Most monsters don't know what sex is]] Because they don't need to have sex, and instead get all freaky and meld their Souls together. Sans knows because he found out about it in the surface and remembered it the next reset. Monsters don't need to know and would probably be really confused by the idea, and shake their head, regarding it as a bit of human hilarity, remembering it fondly along with other great laughs humans have providing them, such as human food and humans interpretation of monsters.

The Artifact that the Annoying Dog absorbed would have given you magic
  • There are Dummied Out files that indicate that, at one point, you would have gotten magic. Though it's not certain why it was removed (possibly because it would have encouraged one type of run over another), it's possible that the artifact from the piano puzzle in Waterfall would have given you this magic (or even just a spell). The reason the room wasn't just removed was because it already appeared in a trailer, and the reason for the Annoying Dog arriving was because the artifact no longer had any purpose, so Toby didn't want you breaking the game by getting it (as in crashing, not getting something you weren't supposed to have). After all, the dog is Toby's avatar (supposedly).

The No Mercy run is an inversion/deconstruction of Earthbound's final battle.
  • In Earthbound, the protagonists use the prayers of everyone in their world to defeat Giygas' final form, but this eventually stops working and only starts working again when you, the player, contribute to it. In this game, Frisk is naturally predisposed to go for the True Pacifist ending, which means that any deviation from that (even if it's just to go Neutral) had to come from you, the player. In one scenario, the player saves the world from the outside, and in the other, the player destroys it (or at least, leaves it in a less-ideal state than if they had just let Frisk act naturally).

All Monsters possess DETERMINATION.
  • Think about it. Throughout the entire game, if you've reloaded a save and replayed some events, monsters can gain a sense of deja vu upon doing certain reactions all over again, such as Papyrus finding you familiar, or Toriel being able to guess whether you prefer cinnamon or butterscotch. Sans knows about humans beings able to manipulate time, and during the No Mercy route knows that the reason he feels that he's fought you before is because he has fought you before, and is even able to count how many times he's killed you or if you've killed him, because of his own DETERMINATION and research. Now, of course, they don't have enough DETERMINATION to manipulate time, or for their souls to persist (very long) after death, but they do have enough to gain that sense of deju vu, and have the drive to keep going. Monsters with more DETERMINATION than other monsters melt partially upon death.

Monsters experience the afterlife when their soul disappears.
  • When Sans dies, he begins to talk to Papyrus as if he were present. This could just be him losing consciousness, but it could also be him fading to the other side.
  • A more obvious point is that the underground is populated by ghosts and skeletons.

The game will eventually be patched so that it's able to notice when its files have been manually tampered with
  • So that there can be NO way around the consequences of a No Mercy run
    • Isn't that kinda pointless? If you wanted to make the game to NOTICE if the files had been tampered...that means tampering the game itself is possible in the first place.
      • The game already can be messed with that way, and hackers can always find a way. Allowing the game to notice is similar to the Genocide run's normal consquences- nothing is stopping you from doing the crime, but the game will do its best to make sure you also do the time.

Alternatively, the future game will encourage you to access the files
  • To break the fourth wall further so the player would be directly involved in the plot. Basically, something like Oneshot

Some parts of the game were inspired by Fallen London.

Speculation about a potential sequel.
  • Gaster will be either the Big Bad himself, or half of a Big Bad Duumvirate, the other half being the Fallen Child.
    • If the latter is the case, one will double-cross the other.
  • Whoever the Big Bad is, he, she, or they will exploit the fourth wall for his, her, or their plans.

Napstablook, Mettaton, Sans, and Papyrus are not undead/Blooky's depression.

  • We see what happens when a monster dies. Their body turns to dust, and they cease to exist. But Napstablook and Mettaton haven't turned to dust— they were just somehow always ghosts. All four never mention a past life as a human or other monster. Blooky's depression isn't a result of anything in a past life— their personality is just meant to be a contrast to their cousin. Mettaton is vain and extroverted, Blooky is self-deprecating and introverted. Similar to Sans and Papyrus, they're foils for each other.
    • Alternately, Blooky's introversion is meant to go hand-in-hand with their passion for creating electronica. Many musicians in that genre, like Daft Punk, deadmau5, Phuture Doom, etc. are all anonymous, refusing to unmask or give out their real identity. Maybe they have an online persona that is mysterious and cool that releases their tracks under a pseudonym. Mettaton thinks highly of their cousin's work, and keeps encouraging them to start playing shows.

The game is set in the (very) distant future of the Dark Souls universe.
  • By now the cycle of light and dark has gone on for so long that neither has the strength to cause catastrophic damage to the world anymore, and most human souls have light and dark in roughly equal measure. The monsters are distant descendants of the Bed of Chaos, having also mellowed out after hundreds of cycles, except for Asgore's family (who may be descended from Gwyn) and the skeletons and ghosts (remnants of Gravelord Nito). This also creates a sort of equivalence between determination and humanity/dark; both can only safely be used by humans, both cause non-humans to melt when they're exposed to it (for example, compare Knight Artorias to Undyne the Undying), and both play a role in the ability to return after death.
    • Asgore's Royal Guards use heavy black armour with horned helmets, as do Gwyn's Black Knights. The Black Knight armour resists fire, and the Royal Guards are encountered in Hotland.
    • This would explain Dr. Gaster's experiments; if determination is really humanity/dark, then that's where the negative photon readings came from.
    • As a corollary, Sans is a member of the Blades of the Darkmoon or whatever order they evolved into. His knowledge of the timelines doesn't come from Gaster's time machine (that's broken beyond repair), but because he has a Blue Eye Orb (maybe the orb is his actual blue eye?), which lets the user invade the alternate worlds of sinners to exact vengeance. Sounds a lot like what Sans does on a No Mercy run, no?

The Annoying Dog is Amaterasu

Because why the hell not? It is known in Ōkami that most people see Ammy as a white wolf... and since she is a god, there's no reason why she can't go in and out of the barrier at will. She took that artifact in the secret room and messed with the other characters because... well, she's a troll. She does all kinds of crap to Issun and some of the others, so I can see the Annoying Dog being Ammy.

(Could someone help me with linking things here... I don't think I know what I'm doing very well...)

Fixing the Genocide save corruption

What if this is still running off of Determination? A theory was that you couldn't fix the Genocide run taint because Chara now had more Determination than Frisk/The Player, so it wasn't in your power to fix it by yourself. Unless, of course, you're so determined to fix your mistake that you literally directly edit the reality of the world via the game's data files. Determination is defined as the power to shape reality using sheer force of will, so if you, The Player, go into your save file and remove the corruption, you're effectively out-Determination-ing Chara and thereby regain your control over the saves. This goes doubly so for the Steam version, as you have to do it every time you want to update as the Steam Cloud will replace the deleted files.

The screaming human face on Omega Flowey's monitor during the fight with him...

Betcha dollars to donuts it's Toby Fox himself making silly faces.

Unknown to most of the monsters of Underground, there are other monsters somewhere that use portal devices that resemble closet doors to scare out human children and collect fuel from them.

The idea seems to be a great crossover fuel.

Impossible or not, there were some monsters that seem to have managed to escape the barrier without killing a human SOUL

In particular was a bear trio stack consisting of a grizzly bear, a panda, and a polar bear. And now they are finding their place in the human world and the wilderness outside the mountain.

After the humans won the war thousands of years ago, the overworld was invaded by cosmic gem beings, and was nearly annihilated if not for the efforts of the rebel gem group sworn to protect the planet

Perhaps for the monsters, being trapped in Mt. Ebott would have been Cursed with Awesome. And it would have been safer if they stayed a little longer there until the crisis is averted.

sans is also an acronym for Steven After Not Surviving
There is this one theory that came from Twitter that sans is actually Steven Universe who died from either cancer or a battle that didn't go well. The similarities might be uncanny given that both Steven and sans have the same height and smile a lot.

The barrier
  • The barrier worked similar to muggle repellent magic from Harry Potter. Only, it drives away ALL humans from climbing the mountain.
  • Somehow, children are less affected by this magic. Which explained why the souls belong to children.
  • The same magic also affected the mountain appearance. If someone tried to take satellite image of the mountain, it would only result in normal picture.

The other SOU Ls had PLAYERS too
  • Genocide run revealed the source of determination is the PLAYER.
  • Other souls have players too, they have ability to SAVE and RESET like Undertale Player. However, they either rage-quitting in between the game, or simply lost motivation playing, so they never finished the game. The souls are never revived and taken instead.
    • Alternatively, it was a prequel where the player had no choice but to let the character they control die because of the plot.

The yelllow SOUL came from Linkara
  • He uses a magic gun and his hat kinda looks like a cowboy hat. He also has a pretty strong sense of justice, and can use magic in accordance with that SOUL power wmg above.

Theoretically, if Toriel and Sans were to become an official couple after the true ending...
...their child (not necessarily biological) would be named Arial.

The light blue soul is a Shout-Out to Alice in Wonderland.
The color and the ribbon references the Disney version and the knife, the game version.

Chara is the REAL final boss.
  • Chara is actually a boss that can be fought and beaten. But since there is no battle screen, not through conventional means. No, Chara, Sans, Flowey, everyone completely underestimates how much Determination you have and what you can do with it. While some can just save and load with it, you have the power to control the entire essence in which that world runs on. The way to defeat Chara is by deleting system_information_962 and system_information_963 from your App Data folder.

There's something else affecting the timelines.
Sans mentions that there's an anomaly in the time-space continuum, with timelines "jumping left and right, starting and stopping, until everything ends." We know there's at least two people capable of this, Flowey and the player, due to the SAVE mechanic. The thing is, SAVE files don't actually cover that. You can't switch tracks in the middle of a run. The best you can do is ruin a No Mercy or Pacifist run and start going for a Neutral run instead, and since you only have one save file, you can't switch between them to take different paths for each. In short, there's no "left or right" because whatever path you're on, you're on it until you either finish the game or reset. Flowey has been through all the timelines he can bring about himself, but he's in the same boat the player is. Added to this is the fact that Sans still makes that assertion even if your first run was a No Mercy run and you've never tried to steer off that path. It implies there is or was a third entity involved, capable of affecting the timeline in a different way.
  • Additional guess: What Sans and co. were detecting was Photoshop Flowey. They didn't realize it, but they were detecting the multiple timelines created over in the Neutral Route by Photoshop Flowey's constant Save Scumming with the six SOULs. He was essentially "jumping left and right" in SAVE files to cheat on his fight with you.

There are already monsters living on the surface
In the True and Corrupt Pacifist endings the Monsters of Ebott seem to seamlessly integrate into the existing culture. But in fact it was only one kingdom of monsters cut off from the world.
  • Makes sense. It's highly unlikely that every civilization on Earth would be warring with Monsters at the same time. It also explains why Frisk was totally unfazed by everything they came across underground; they'd already seen things like that plenty.

The Temmies set up a new village somewhere with Tem in the name
Such as the River Tem in England or Tempe in America.
  • Yeah, I live in England and I know that there isn't a river Tem. There's the Thames which is PRONOUNCED "tems" and the river Teme, which is pronounced "Team", but no river Tem. Sorry.

Toby will release a deliberately bad sequel.

Any future updates/expansions/unofficial sequels will be based around the game breaking due to "glitches"
After the first major content "patch", there are few clues that may mean a future change.
  • The new text in the Annoying Dog's room says it will take more than one patch, since "new holes keep popping up".
  • Most of the new text from Sans and Papyrus has some character reacting in surprise, like even they didn't expect the change.
  • Being able to access Gaster and his content without hacking officially turns him and his followers into Missingno. If you don't get how that connects, check that page.
  • Admit it, with all the hacking and data mining the game takes advantage of, isn't it odd how there are almost no references/responses to glitches?
Mostly speculation, and easily jossed, but it's slightly more plausible than some WMG's.
Undertale's Neutral Route, where Flowey runs away, is Touhou 0
Flowey is the one who taught Marisa how to use star spellcards, and how to focus her LOVE to make her signature Master Spark... Before Marisa betrayed Flowey like Chara does in the Genocide Route. She used Flowey's uprooted 'body' to fly and boost her magic in Touhou 2. The barrier? It expanded with time. Notice how Touhou seems to take place in the future, as hinted in the games and official mangas. The Hakurei Barrier might just be patching the original barrier. I'll update this WMG later with more thoughts and potential evidence.

The non-Golden Ending routes are doomed timelines.
Of course, anyone familiar with Homestuck mythology knows that everyone in a doomed timeline ends up dying sooner or later. Genocide Route leads to a "sooner" scenario, and the Neutral Routes lead to—for some to most—a "later" scenario. True Pacifist Route is, of course, then the alpha timeline.

It is completly possible to rescue Asriel.
You just need a mod. It's excercising the player's will as The Anomaly and their ability to change spacetime!

The way you obtain the Temmie Armor is a Take That toward the idea of Bribing Your Way to Victory.
Considering that Temmie is the Author Avatar of one of the game's developers, and that you have to fork over a lot of gold to pay for both her college tuition and the armor itself, it's possible that this is a clever jab at the idea of paying extra real-life cash to a game company to get special powerups that make the games easier.

Frisk lives somewhere in Pelican Town.
Knowing that their town has been slowly decaying and is having doubts about a soda corporation would overtake the town, Frisk decided to go into the mountain to ask the spirits for help despite the townspeople's warnings of children disappearing. Upon there, they stumbled into the cave, and you know the rest. By the time Frisk and the gang came back to the surface, the old farmer's grandchild had arrived and would have revitalized the town by now.

A sequel would have the fact that many players erased the consequences of a Genocide run as a plot point
Many players figured out how to avoid consequences after doing Genocide by modifying game files. However, that ends up leading to unforeseen consequences, making things MUCH worse, somehow.

Chara is related to the Forgotten from Gemcraft.
They're both demons who have impressive offensive powers but require help to break large magical walls, who rely on others to fulfill their ends by possessing them and live in worlds full of monsters.

The high contrast screaming human face on Flowey's monitor during the final boss fight with him in the Neutral Route are representations of the human souls he has absorbed.
  • Flowey is currently using 6 human souls to power himself and ascend to godhood. This means he has seven "minds" within his body all at once and can feel all of them (he comments that he can "feel them wriggling"). However, due to their incapacitated nature, they are unable to do anything to stop him unless he himself is significantly weakened, only able to aid the protagonist during the short windows of time Flowey attempts to use their individual powers to torture the protagonist, which is the time at which he is weakest. The screaming human faces on the monitor are actually an amalgam of those of the humans trapped in Flowey's body (hence the disturbing Uncanny Valley appearance), screaming silently as they fight Flowey to gain control for a brief moment.

In the "Anarchy" ending, a new ruler is eventually found, and it's...
Jerry.
  • Dear god! Not even a genocide player would be so cruel as to let that happen!

The yellow SOUL is a Shout-Out to Puella Magi Madoka Magica's Mami Tomoe
Let's compare. Mami's associated color is yellow, just like the SOUL. The yellow SOUL is connected with Justice and Mami's theme is "Credens Justitiam", which translates to "Believing in Justice". Also, the associated weapon with the SOUL is a toy gun and what does Mami attack with? Guns!

The Human Mages Died to Create the Barrier
It’s literally their souls’ remnants that make up the barrier itself, equaling a bit less than the full power of seven ‘’live’’ human souls, thus justifying the incredible power needed to break it. (Also see my “Your actions…” theory below for more thorough discussion of the Barrier.)

The Human and Monster Souls Don’t Have to Share a Body to Pass the Barrier
A co-mingling of DETERMINATION and love could be truly all that is needed to allow passage from the magic there. If so, a deep love (either familial or romantic), would be enough to allow passage. Presumably, Chara and Asriel could have walked through, with enough confidence. (Again, see below for more on the Barrier itself.)

The Offspring of a Human/Monster Pairing Could Pass the Barrier
It would have a soul containing both monster and human elements (DETERMINATION and love), which in this theory, is enough to cross the gate. (Also, see below… yet again.)

Your Actions as a Player Represent the Attitude of the Society Frisk Comes From
While it’s taken as canon that Frisk’s own personality and desires match those of someone on a True Pacifist run, this theory is that he’s ‘possessed’ by the overall spirit of Humanity when he falls through the hole into the ruins. Presumably the Barrier actually extends over the entirety of the Underground, and Toriel’s not preventing her fellow monsters from building a ladder and climbing out of an open hole. The Barrier, being made of human souls, is probably pretty sensitive to the feelings around it, and would imbue any human passing through it with said attitudes, overriding their own. This happened with Chara, being filled with humanity’s self-loathing (think about how often disgust is shared nowadays about this or that portion of society, or oneself, etc. Then add in all the injustice and rage against it. Filter that down, and it comes down to a pretty solid hatred) at the time of his falling, and that fueled the plan he made with Asriel. By the time Frisk falls, things could have changed, leading to a True Pacifist run, be roughly the same (various types of Neutral runs), or have gotten worse (Genocide). This is also why the True Pacifist ending would really be as good as the cutscenes imply, if most of humanity is as loving as Frisk, then the monsters really would have nothing to fear. Yeah, there will probably always be pockets of hate and racism, but the monsters would be almost completely safe.

Memoryhead is the Result of a Failed Attempt to Bring Back Dr. Gaster
It seems pretty obvious, given the lack of recognizable monster parts in its battle sprite, and the name “Memoryhead,” that said amalgamate has something to do with W.D. Gaster. He only exists as a memory, and then only to some. Perhaps, before his memory faded too much for anything to happen, Alphys tried imbuing whatever dead skin cells, DNA, etc. that remained on his old possessions with DETERMINATION, but didn’t see any results. It was only after the other failed tests, that some of the remaining monster dust intermingled with the microscopic remains of Gaster, and his presumably great magical power, to create Memoryhead.

Humans don't have magic at all; they have sufficiently advanced technology and the barrier is actually a temporal displacement field.

Which is why Determination, a human characteristic, confers the power to manipulate the timeline. The underground is physically underground, but the barrier works by taking the stretch of time between the monsters being sealed and the monsters going free, and separating it from the rest of space-time. The "way in" from the mountain is actually a time portal.

The "Empty Gun" is actually a toy gun.
If we're going with the general consensus that every human that fell down and was killed was a child, this makes more sense. It could be an older toy gun that looks a lot more realistic than toy guns do now, hence why Bratty and Catty can't tell the difference. Or maybe they wouldn't be able to even if it was a more colorful one, if they'd never seen a real gun before finding the Empty Gun. In the latter case, depending on how old Frisk actually is, he/she probably might not know any better either and would just take their word for it. There's also the meta-excuse of preventing repetition by having two "Toy X" items(this and the Toy Knife).

You could make a similar argument for the Burnt Pan, but it's less of a stretch to imagine a child being allowed to play with cookware than with an actual gun.

Some of the monsters chose to return to the underground.
Most of them lived their for their whole lives. Is it such a stretch to think that some of them might of chosen to stay with the Good old Ways?

On the surface, Monsters win fights through a combination of interface screw and pacifism.
All ancient warriors who possessed the knowledge of fight mode are dead and buried. Modern weapons are cold iron free and don't harm monsters. As a result most soldiers will be really confused and choose to flee.
  • That makes zero sense. First, a child can beat monsters with a stick, cold iron never came up. Second, how does pacifism come up at all. The way you explain it, monsters can basically slaughter humans with impunity, thus making the pacifist ending the WORST ending.

Speculation for an Updated Re-release or Video Game Remake on consoles (or otherwise):
  • Graphics either redrawn at a higher resolution or in some level of 3D, either full or as a Sprite/Polygon Mix (either way, Photoshop Flowey would be in full 3D to emphasize the "wrongness" in his design).
  • Rearranged soundtrack.
  • Battle backgrounds beyond the occasional boxy shapes (except for, again, Photoshop Flowey, because you and him are the only things in the world at that point).
  • Muffet becoming a major character, for multiple reasons (for consistency with other SOUL users, for representation in the Asriel fight, to serve as the boss of Hotland so it doesn't have a disproportionately high kill count in a No Mercy run, sheer popularity).
  • Regarding Flowey and Chara crashing the game:
    • Flowey: Instead of the game window closing after he absorbs the human SOULs, the screen cuts to black, several glitchy noises play, and the game appears to restart until the intro screws up and you're taken to the "My World" file. Losing to Photoshop Flowey at any point simply takes you back to the file select.
    • Chara: Again, since there's no game window, the screen of "9999" simply shakes from side to side a la a dying monster, and the black screen with wind appears immediately. The wait time before Chara makes their offer is drastically reduced (for battery life/patience reasons), and they will automatically reappear to make their offer given the same amount of time. (On the 3DS, Chara's attack if you refuse to destroy the world could take advantage of the system's 3D effect...)
    • In both cases the crash would work as in the default game on PC.
  • Of course, meta commentary on the game being a remake, particularly regarding the added content.

A theory about Chara's motive for potentially going through multiple resets but only ever doing pacifist runs.
  • If you keep resetting but only doing pacifist runs, Chara's motive for resetting so much is that they still haven't given up on saving Asriel. Despite being a reference to the player, Chara has repeatedly shown that they have a personality beyond just being the player so they would need some kind of motive to reset unless your influence over them is just that strong.

The pacifist ending leads to another war.
Think about it. Nearly every monster attacks you on sight, never stopping until you are dead or you convince them to stop, most likely by them getting confused at you refusing to defend yourself long enough to escape. Anything saying that they don't intend to kill you is really just Frisk trying to convince themself of that, they really want you dead. At the end, the barrier is opened. However the war goes this time, it's ending in extermination. The ending you see, just another one of Frisk's delusions.
  • Word of God confirms that the pacifist ending is exactly as it seems.

All monsters are ghosts that became Corporal.
Think about it, the underground is implied to have a large ghost population and also many skeletons, the rest of them taking on animalistic chararistics. Eventually ghosts that posess remnants become a fusion of the object that they have taken.

Consider that the monsters were banished underground a very long time ago; if they did eat human food, they presumably ran out after a while, and so they've most likely been exclusively eating monster food for a long time. Since monster food digests completely, they don't need toilets, but present-day monsters still have a concept of "going to the bathroom". In their case, "I have to go to the bathroom" may actually mean "I have to take a bath/shower". Consider also the two monsters who use this excuse: Papyrus is mentioned to take showers in an earlier bit of dialogue, and after Alphys gives the excuse, she hides in an elevator to the True Lab, which does feature a shower/bathtub in one room.

The former Humans aren't as bad as the Monsters thought.
So supposedly they go to war to capture monster souls....but when they win, they seal the Monsters away. This isn't to say there weren't racial tensions on both sides, but those actions don't match.

http://tvtropes.org/pmwiki/pmwiki.php/WMG/Undertale