YMMV / Undertale

  • Abandon Shipping: After the Version 1.01 patch in January of 2016, modified content in the game added stronger implications that Sans is related to Gaster in some way, possibly as family. This has caused a lot of people to jump off the Sans/Gaster ship.
  • Alternate Aesop Interpretation:
    • Due to how surprisingly deep this game is and Death of the Author, many folks have come up with other interpretations as to what the game means. Some feel that how Chara steals your soul at the end of a No Mercy run and how the only way to fix it is to delete your save file gives off the moral, "The only way to move forward is to burn your bridges."
    • People have also said the meaning is either the same or the complete opposite of The Stanley Parable - that the only meaningful choice you can make in a video game is to turn it off and walk away forever. Undertale puts huge emphasis on actions having consequences, but it also points out that it's not meant to have infinite replay value. It doesn't help that the message you receive at the end of the pacifist route rather explicitly aims towards the middle ground of these two - don't stop until you make it to the ending that's happiest for everybody, and then stop playing forever.
    • Some of the routes in the game, between pacifist and No Mercy, have a bit of a brush with Fantastic Aesop: "If you have the power to rewind time, you have no excuse not to do the very absolute best." The likelihood is this is meant to be taken as an elaborate, fictionalized way of saying Comes Great Responsibility.
  • Anvilicious: In part due to Hype Backlash, some people have felt the whole moral of "Actions have consequences" was a tad obvious, and since other RPGs have done it as well, it could come off as tired to them. Many fans will counterargue, however, that while the moral in and of itself may not be terribly original, finding a game that demonstrates it to as great an extent as Undertale does is virtually impossible.
  • Award Snub: While Undertale getting nominated for any Top 5 at the 2015 Video Game Awards is amazing in and of itself for an independent game released late into the year, the games that were chosen over it were somewhat baffling to many of the viewers. "Best Independent Game" itself was won by Rocket League; most of the fans pointed out that Rocket League might be a perfectly fun game, but it is a multiplayer sports game that, unlike Undertale, doesn't try to tell a story, make any artistic statements, nor toy and experiment with the conventions of the game medium. Additionally, Undertale wasn't even nominated for "Best Narrative," which ended up going to Her Story, or "Best Original Score", which went to Metal Gear Solid V: The Phantom Pain.
  • Alternative Character Interpretation: The game is full of deep and multi-layered characters. Therefore, it's just natural that they can be seen in many different — and conflicting — ways. Of course, discussing this ventures into spoiler territory.
    • Asgore: Is he a genuinely good person placed into an impossible situation who only did what he had to do; a wicked schemer who got away with several murders and planning the destruction of mankind; or a coward unable to live up to his promises?
    • Toriel:
      • On relationships: Did she leave Asgore because she wanted nothing to do with his plans, or did she have a problem just with the specifics of the plan and her husband being unwilling to go all the way? How much anger did she share with him over the death of their son, and to what degree has she mellowed out since then?
      • On personality: Is she just as bad as Asgore because, through her pacifism and relinquishing her crown, she essentially condemned the entirety of monster-kind to a futureless, bleak eternity beneath Mt. Ebott?
      • Toriel condemned Asgore for his weakness, saying he could have taken one soul, left the underground, gathered six more souls outside, and opened the barrier that way, instead of waiting for humans to fall in and condemning the monsters to despair. However, if Asgore had done this, it would have led to him exterminating all of humanity. It does make her come across as the more bloodthirsty of the two.
    • Sans: Is he so lazy because that's just how he is, or because he knows everything he does could be undone at any moment and doesn't even bother trying? He even lampshades this.
      • Is he suffering from depression? Is this over his guilt at not being able to save anyone? Is it connected to Gaster in some way?
    • There is an enormous range of interpretations of the first child who fell into the Underground, to the point where virtually every mention of them on this wiki contradicts every other mention of them.
      • It's clear that they hated humanity and wanted to get revenge, but what about monsters? Did the first child care for their adoptive family and want to help the monsters escape the underground, or was that just a manipulative ploy? Were they always a monster, or does the No Mercy path transform them into one as a result of the player's influence?
      • And why exactly did the Fallen hate humanity? Were they just a sociopathic jerk from the start or did they actually have a good reason for such attitude? Considering their official status as an extension of the player, both possibilities could be equally true.
      • Was the first child even human? There are more than a few hints that they were not; that they were something else entirely that was only disguising itself as a human child. Was it even the first child themselves that set everything in motion, or were they possessed by something else, the way Frisk becomes if you play the Genocide route to the end?
      • An interpretation Played for Laughs is that 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 are many hints littered throughout the game that the Fallen Child is the narrator of all the routes, not just the No Mercy route, because of how when the Fallen is in control, looking into the mirror results in "It's me, [Playername]" as the narration text, but when the Fallen isn't, it says "It's you!" or "It's still you, Frisk". If such an interpretation is true, then it adds a new layer to the Fallen's characterisation, as the narrator in the Pacifist and Neutral paths have shown a more dynamic, vibrant, and even sassy personality, and even seems to detest conflict at times, suggesting ways in which Frisk can resolve the battles peacefully.
    • Why exactly did the humans wage war on monsterkind and seal them underground? The cave writings indicate that the monsters have no idea, but considering the fact that many of the monsters you meet are quite clearly not mentally stable by real-life standards, and the fact that there are entire species of monsters, like Vulkins, who don't realize how incredibly hazardous their mere presence is to humans, is it possible that the humans did it out of a legitimate fear for their safety, or perhaps in response to human deaths, accidental or otherwise, at the hands of monsters?
  • Anti-Climax Boss:
    • For those who expect a challenge at the very end, the final battle from the Pacifist route can be disappointing due to two factors: You can not only "dream" for virtually endless healing items, but it's actually impossible to lose the fight since you end up reviving each time you die. The fight can be especially jarring if you've done the No Mercy route first, since the Climax Boss found there is the hardest fight in the game. That being said, the other factors of the fight are not disappointing at all]].
    • While this trope is invoked on the No Mercy route due to the fact that you can kill all but one boss in one hit, the fight with Mettaton NEO stands out in particular. He gains a new form not unlike Undyne the Undying, so in theory he should be a serious threat. In practice, he not only dies in one hit like the other bosses, but he doesn't bother to attack you.
  • Base Breaker:
    • The main character's sprite. Some people think it's straightforwardly adorable, some people hate it for being ugly, and others find it Ugly Cute.
    • So Sorry. Some players absolutely despise them for being a furry Self Insert, and think they ruin the game. Some players think hating them is an absurd stance. Some players admit that a furry self-insert isn't something they want to see, but don't mind that So Sorry exists since they're very hidden and very optional, so they don't have to. Accordingly, every mention of the character is liable to spark a flame war.
    • Alphys. A lot of players find her character gimmick of constant messages and attempts to help more annoying than funny, especially on repeat playthroughs. And even though she has sympathetic reasons for it, the reveal that she's been manipulating the player to feel better about herself doesn't help, especially since she never directly apologizes for it either.
    • For the Temmies, there are three groups: those who love them and think they deserved more of a presence, those who found the gag funny but are glad it was kept to a single area, and those who find it a lame attempt at "lolsorandom" humor that the game otherwise avoids.
  • Best Boss Ever:
    • Papyrus is a pretty awesome introduction to the types of bosses the game will throw at you. It's a very humorous boss fight that seems easy at first, even when he uses his telegraphed "Blue Attack"... but then your soul/heart also turns blue, forcing you to jump to avoid his attacks, which makes the fight harder, yet also makes it more fun and exciting, showing what kinds of curveballs the game might throw at you.
    • Undyne's fight is just as amazing. Her first move is to turn your soul green, which prevents you from escaping but also gives your soul a kind of shield. This shield allows you to deflect her attacks — which come at you from all sides, by the way. They also come at various speeds: at one point, you're faced with an extremely slow moving attack coming at you from one side, only for several quick attacks to come at you from the other side. Eventually, she adds attacks that flip around to your exposed back, requiring you to spin around quickly to block them. Near the end of the encounter, she throws a lot of these attacks at you, making you spin around rapidly trying to block them all.
    • Mettaton EX has Sparing mechanics totally unlike any other boss's, and avoiding his attacks means shooting them to blow them out of the "sky" or make them move in a certain way rather than just avoiding them. It's a curveball of a boss fight to cap off a curveball of a chapter, and everything about its presentation screams "style."
    • Asgore Dreemurr, King of all monsters. One of the few bosses you absolutely cannot spare and must defeat by force. The battle has no sudden tricks to throw you off or anything you haven't faced before — it's a pure test of your skill at dodging bullet patterns and reacting to blue and orange attacks. Even using tricks to make the battle easier, the fight is still pretty long and difficult.
    • The Final Boss of the Neutral Route, Photoshop Flowey, is an amazing fight. The buildup for it is great, as when Flowey gets the souls that Asgore gathered, the game window closes. Upon booting the game back up, the opening sequence glitches out and your save file has been seemingly hijacked by Flowey. Selecting this file leads to a terrifying battle with the power-mad flower, who has turned into a strange but terrfying Eldritch Abomination comprising photo-realistic faces, mechanical parts, and plants. You don't even get to use your menu commands at first. Instead, you simply have to survive, dodging several bizarre and erratic attacks. To cap it off, he can save state during the fight, which he can later load to catch you off guard. The whole fight in all has been compared to Giygas in its unexpected creepiness.
    • Asriel Dreemurr, from the Pacifist route. Visual Effects of Awesome plus a healthy dose of Awesome Music with "Hopes and Dreams" is one thing. But the fact that it's an inverse of a Hopeless Boss Fight is worked into the story; you can't lose, because your determination to save your friends and the world is strong enough to endure the pain his attacks deal to your SOUL. Even when you can't move, and can't SAVE yourself anymore, then maybe you can save your friends. You enter Asriel's SOUL, save your friends, and even save the boss. Your determination brought you a long way.
    • For the opposite reasons, both of the bosses exclusive to the No Mercy route, Undyne the Undying and Sans. Respectively, Undyne, already mentioned above, basically souped up in one of the most awe-inspiring moments in the entire game and given one of the best music tracks in the game, and a boss who's fully aware of your save game antics, combining awesome music and a hectic, incredibly difficult fight into a thrilling battle, featuring a combination of new tricks and the return of the "blue attack" in a way that will make experienced players remember what they did to Papyrus. Made even better when he turns your own tricks against you or refuses to end his turn so you can't have yours. They're considered two of the hardest bosses in the game in a route where anyone playing knows damn well by now both how much they deserve harder fights than what the rest offer, and that this is exactly what they wanted. For some people, in fact, throwing themselves at these two bosses is the entire point of running No Mercy.
    • The credits sequence in the pacifist ending. One of the most hectic bullet hell sequences in the game (trying to avoid all the names is challenging but fair), set to a fittingly amazing music track.
    • Mad Dummy. His battle style is different from anything else in the game. Instead of fighting or sparing him, you need to lead the attacks of his own Dummy Bots into him. The dialogue is nothing to sneeze at either, with his Large Ham tendencies, his firing all of his minions for their ineptitude, and him being based on one of the most popular and effective villains in the history of manga. To top it all off, the music is a really cool remix of an earlier theme.
  • Best Known for the Fanservice: For the Furry Fandom, regarding all the animal-looking characters. The Ice Wolf and the Hotland Royal Guards receive special attention.
  • Big Lipped Alligator Moment:
    • Onionsan's appearance. He shows up, rambles on as you try to walk along the river, has some dialogue choices that don't really make any kind of an impact, and that's the last you ever hear of him. Don't ask Toby about him; not even he knows.
    • Muffet. She appears essentially from nowhere (her appearance selling spider food looks more like a quick joke than anything else), and unlike other boss fights, hers has almost no relevance to the plot. She alludes to Mettaton's obstacles being an act, but not in a really obvious way. She leaves the plot almost as quickly as she enters it, only seeming to serve as a way to pace the game better. This could be justified in that Muffet was a character that was added into the game due to Kickstarter donation, which could explain why she doesn't fit in. She does, however, have more relevance in the No Mercy Route, where her refusal to leave her spider nest behind creates a route for the player to progress through, where otherwise the player would have been trapped with no way forward.
    • Solving the piano puzzle in Waterfall leads to the Annoying Dog stealing the artifact and flying away while the dog song plays in the background. While he leaves behind Dog Residue that could be used to farm money, the scene to begin with is nonsensical and has no bearing on the plot — especially if it's done on No Mercy.
    • Temmie Village is entirely possible to miss and has no bearing on the game if you do, the Temmies are, frankly, really weird, and their village is about as odd as you can expect, their history is a picture of a Temmie riding on a dragon with no additional context, one of them breaks out in hives only upon mentioning said allergy to itself, and another is trying to incubate a hard-boiled egg.
    • To a lesser extent, Memoryhead in the True Lab. They're the only Amalgamate that doesn't obviously resemble any of the monsters you've previously encountered, unlike the others, and they seem to act like some sort of living glitch (their speech is garbled and blocky, the battle text itself is random, and most of the ACTs are from your non-battle menu). Since their appearance resembles a cross between skulls and Giygas, this has led to people wondering if Memoryhead is related to W. D. Gaster in some fashion.
  • Broken Base:
    • The fanbase cannot agree on what name fits the Kill 'em All route better. The more popular choice of name is the Genocide route, since this is exactly what the protagonist does; however, some of the fanbase feels the name is too much of a Spoiler, and call it the No Mercy route instead. Those who prefer the name "Genocide" argue back that the "No Mercy" name is too vague and fits other routes of the game better (such as the one where all the bosses are killed but none of the monsters, or one where all the bosses and some of the monsters are killed).
    • There are those who are split on whether the game's story and overall message is amazing and refreshing or if it's average at best or poorly executed at worst. This is held especially true for the pacifist route regarding Asriel. On one side, you have people who feel that the story doesn't live up to the message of "don't kill and be kind to people" because Asriel was slain by humans after he tried to bring Chara's body back to them out of respect. On the other end, you have those that feel that the aspect of being and doing good is what matters the most rather than what the end result of it brings and how some people simply cannot be saved no matter what you do because that's how life is.
    • Some fans are divided over the Genocide route tainting your game forever. There are workarounds to purify the game, but some people are annoyed at how far out of their way they have to go to enjoy the game their way. The other camp is annoyed at the first group for rejecting the message of "you aren't above consequences."
    • As of the 1.01 patch, the Gaster Easter Eggs are occasionally accessible without having to edit the game files. Some fans enjoy being able to find them in an easier and more intuitive fashion, while others feel that putting Gaster into the game proper takes away from his story and makes the secrets less interesting.
    • Nintendo contacting Toby about a potential Wii U/3DS release of Undertale has caused one, despite it just being a possibility. The main reasons cited being that there's still content to be finished from the kickstarter, and it would be incredibly weird and technical to make stuff like Flowey crashing the game, the stuff with Gaster, or avoiding the Genocide route's consequences work, since you can't mess around with files in the same way on a console.
  • Counterpart Comparison:
    • Undyne has a lot in common with Adventure Time's Marceline: both are Badass Action Girl, Hot-Blooded, Cool Big Sis, and the list could go on.
      • She's also commonly compared to Vriska and Terezi of the Homestuck fame due to having the former's love of fighting and the latter's hotbloodedness and sense of justice.
    • Flowey can give some players a certain reminding of Kyubey. The grotesqueness of his powered-up form can also remind some players of the anime's witches.
      • It's also noted that much of what Flowey says wouldn't sound out of place coming from The Joker. His obsession with tormenting the player character can also be seen as a mirror of the Joker's obsession with tormenting Batman.
    • Sans is frequently drawn as Jotaro Kujo of JJBA, if only for his ridiculously hard boss battle. More often than not his Gaster Blasters will be his Stand.
    • Most characters of the game have been compared with the characters of Wander over Yonder for having at least one major similarity. The character that is ucompared the most is Papyrus with Lord Hater for both being skeletons with similar character traits, with the only difference being that one of them is evil. Mettaton has also been compared with Emperor Awesome from the same show.
    • The Amalgamates are very often compared to the Gem Clusters from Steven Universe. Ironically enough, Steven Universe also has a music track called "Amalgam" concerning fusion, but the tone between the two is radically different.
    • Chara/The Fallen Child is often compared to Odio from Live A Live. Both are embodiments of evil capable of destroying the universe (and both may end up doing so with the player's help), both invert the idea of a boss battle (choosing Odio's human form Oersted for the final chapter of Live a Live initiates a reverse Boss Rush where you control the bosses while the three No Mercy bosses portray the opponents as being the heroes), both were born humans and are implied to have semi-human status, and the theme for Sans' boss fight (which many attribute as Chara's battle theme rather than Sans') is called Megalovania, paralleling Odio's own theme, Megalomania.
    • Toriel with, her color scheme and similar clothes, has been compared to Ran Yakumo or, rather, the motherly fanon portrayals of her.
  • Crack Ship: Gaster is a huge launcher of many of them. Including Gaster/Asgore, Gaster/Grillby, and Gaster/Alphys. This is largely due to the fact people are shipping a character who doesn't exist with ones who do.
  • Creepy Awesome:
    • Is Flowey a terrifying character? Yes. Is he also a Dangerously Genre Savvy, Magnificent Bastard who plays all his cards right? Also yes.
    • Flowey kicks it up a notch in the neutral run when he turns into Photoshop Flowey, a form that turns him into a complete Eldritch Abomination. However, his complete mucking around with Save Scumming and the his massive amount of screwing with the interface makes him a pretty badass boss.
    • Then there's the Amalgamates, combining the features of several undead monsters into a highly disturbing something that may or may not be suffering through worse than hell. They are only encountered during True Pacifist in one of the bleakest areas of the entire game, are mostly not quite easy and definitely horrifying to fight, and in at least one case really cuddly.
  • Creepy Cute:
    • Muffet, a murderous spider girl with an adorable design.
    • Some of the Amalgamates can become this once you realize they're just as innocent as all the other monsters. Most notably the dog Amalgamate, Endogeny.
  • Crosses the Line Twice: If you kill Tsunderplane, it starts plummeting down while saying "but I never got to-" before crashing and exploding.
  • Cult Classic: Undertale's popularity on the Internet is enormous enough to warrant it as an example. It was enough for the fandom to push Undertale to a victory at the GameFAQs "Best. Game. Ever." 20th Anniversary Contest, beating Mass Effect 3, Videogame/Fallout3, and various Nintendo powerhouses.
  • Draco in Leather Pants:
    • While Asriel isn't necessarily evil enough as himself, Asriel as specifically his insane God of Hyperdeath form, when he is decidedly still a villain, has a lot of fans who like him more than his true, redeemed self.
    • This can also happen with Chara due to the massive amount of Alternate Character Interpretation. While there's some precedence for this in that they had a (unsaid) reason for wanting to disappear and aren't a world-destroyer unless you, the player, guide them towards being one, some fans portray them as a sad child with an adequate Freudian Excuse who just needs to be shown some love, some even believing that the Pacifist run is a redemption quest for their spirit. Of course, then there are some who only acknowledge ominicidal No Mercy Chara as their version who fall firmly into the camp of Evil Is Cool.
  • Enjoy the Story, Skip the Game: A general consensus among casual fans says this: while the game is worth playing for the story, the actual gameplay is decent at its best and boring at its worst. What makes this worse is that the game itself discourages players from replaying it, especially if they reach the Pacifist ending.
  • Ensemble Darkhorse:
    • Muffet. Her adorable and well-done design, interesting personality, difficult yet fun and interesting boss fight, and great song for her battle make her a lot of people's favorite minor character. There's just as much fan art for her, if not more, than Toriel. It might help that she's designed by Michelle Czajkowski of Ava's Demon fame.
    • Everyman, part of an attack done by one of the Amalgamations, is also becoming surprisingly popular, especially after being given an official name by Toby, despite being no more than a bullet in another enemy's bullet hell. It's even considered an icon of constant speculation source Dr. Gaster.
    • "Burgerpants," the cashier at MTT Burger, has gotten a fair amount of love for his desperate and miserable demeanor and goofy expressions. On top of being one of the very few characters that aren't scared of the Fallen in the No Mercy route.
    • The Royal Guard Dogs in Snowdin, but particularly Doggo (who has been very popular with the Furry Fandom) and the Lesser Dog (for his neck-stretching antics). Even Toby seemed surprised that Doggo became so popular.
    • Ice-E, the mascot character for Monster Kidz Word Search, has enormous popularity in comparison to his relevance (appearing in one scene as a picture).
    • The Temmies appear in only one location, but are widely popular for their cute design and adorable quirks, often being called the Mr. Saturns of Undertale. The shopkeeper Temmie is especially popular for both providing the best armor in the game and its Surprise Creepy moments.
    • Woshua, who's just a random encounter, but is popular among the fandom due to his amusing speech patterns, character gimmick, and some hilariously dark dialogue, which have caused him to be regarded as a self-appointed Memetic Moral Guardian.
    • Snowdrake is another random encounter who's quite popular due to his adorable design and So Bad, It's Good puns, as well as his backstory making him a huge Woobie, as well as being one of the most tangible examples of Video Game Cruelty Potential if you choose to kill him.
    • Rather coincidentally, all three of the "fire elemental" NPCs count. Grillby is loved for being one of the few named minor characters, having a unique design, and there being plenty of room for fans to develop him (particularly with his relationship with Sans). Heats Flamesman is liked for providing a random, hilarious moment, even getting a flair on the Undertale subreddit. And the fire girl in a Sailor Fuku in Hotland is liked just for being adorable.
    • Perhaps the strangest example out of all of these, the Yellow Soul, due to the prevalent 'yee haw' meme on Reddit.
    • Of the side characters who don't have a bearing on the plot, Napstablook is adored for being huggably sad all the time, having a jazzy boss theme, saving the player from the Mad Dummy, and the weird yet awesome moment that happens if you choose to hang out with them.
    • Madjick also has a bit of a following, seemingly for no other reason than it was a running joke that somebody started.
  • Esoteric Happy Ending:
    • The Golden Ending in general is quite positive, but the Amalgamates returning to their families to live happily seems out of place, considering the horror attached to their reveal and the Body Horror they experience.
    • As mentioned under Fanfic Fuel, Flowey/Asriel's fate rubs a lot of people the wrong way, and somewhat diminishes what would otherwise be a literally perfect ending for everyone.
    • A similar, but far more subtle and subjective case could be made about Sans. He explicitly states at the end of a genocide route that it doesn't matter to him if he makes it to the surface or not - he's been so conditioned by Flowey/the player's continuous resets that making it would just feel like a false hope being dangled in front of his face. The question here is a matter of Alternate Character Interpretation; was he only saying that because he was at his Darkest Hour, and has enough faith in the protagonist/player by the end of a pacifist route to enjoy life on the surface like everyone else, or will he spend the rest of his days just as depressed and nihilistic as before?
  • Family-Unfriendly Aesop: While the game promotes a sweet message about not having to hurt anyone, the moral is muddied by characters who are dead set on killing you, presenting an idea of "negotiating with someone determined to harm you can end well."
  • Fandom Berserk Button:
    • Discussing the protagonist's Ambiguous Gender will often lead to some very nasty flame wars on such sites as tumblr and the like, as the mystery is intended to be that way according to Word Of God.
    • There are also many arguments over a certain artwork depicting humanized versions of the monster characters. Some complain that the Tumblr fan artist goes out of their way to make the cast all "special snowflakes" that are usually any race except white (meaning in an attempt to "be diverse" they explicitly leave out one specific race) save two characters (who are coincidentally the ones with the strongest reputation for murder), and assign seemingly random sexualities and gender identities that go against canon or are self-contradicting.
    • So Sorry's existence is a huge sore spot for the fandom (see entries for Base Breaker and The Scrappy) to the extent even mentioning the guy sparks flame wars between those who hate him and those who wish the haters would stop bashing him and move on.
    • Try criticising the game in general or say you liked another game better in 2015 and the heat of a thousand suns will be felt from the fanbase.
    • An especially nasty one that Let's Play-ers often face: Playing a Neutral run, either on purpose or out of ignorance. Expect the comments section to be aflame with fans lamenting the deaths of NPCs and perfectionists demanding that the player restart the game and perform an absolutely flawless Pacifist Run, even if the player made it clear they wanted to play the game blind.
  • Fandom Rivalry:
    • While not as heated (note that OFF is also under Friendly Fandoms), some discussion has also happened between this game and the fandoms of Lisa and OFF, two other indie RPGs that deconstruct the genre of RPGs and put heavy emphases on consequences for actions.
    • There has been some argument between fans of this game and of Homestuck, mostly concerning the music track "Megalovania" which appears in both.
    • Thanks to Toby tweeting about the game's Metacritic rating (a good 95+), there have been heated discussions involving it and the equally rated Metal Gear Solid V: The Phantom Pain.
    • Thanks to an organized rally by Undertale fans, mostly on Tumblr, to advance the game in Gamefaqs's "BEST. GAME. EVER." poll, resulting it in winning the poll, some Nintendo fans have cried foul after that led to it beating out classics like Super Mario World, Pokemon Red And Blue, Super Mario 64, Super Smash Bros. Melee, and The Legend of Zelda: Ocarina of Time. Part of the issue is that Undertale's initial entry into the contest at all was because of a confirmed bot flooding votes. While the game managed to keep gaining traction on the strength of its fans alone, the controversy surrounding its nomination has caused a lot of genuinely sour feelings towards the game and the fandom.
      • This has come to a head with the Legend of Zelda fandom, when Undertale beat Ocarina of Time for "Best Game Ever." This hasn't been helped by fans of both games being incredibly sore winners/losers.
    • With the Five Night's at Freddy's fandom, as both games became indie cult favorites and have a passionate fanbase. This along with games being released roughly a year apart has led FNAF fans to accuse Undertale of "stealing FNAF's thunder".
  • Fanfic Fuel: A truly massive amount of it, given what other fandom Toby works with, it's likely that there was just enough of a lack of answer to a lot of questions deliberately to give fans stuff to write about.
    • With the True Ending allowing the monsters to access the human world, many fans are already coming up with ideas about how they manage in the outside world.
    • Whoever the six Human SOULs were and what they went through before their inevitable deaths is good fuel for speculation, as well.
    • A popular What If? scenario involves switching Sans and Papyrus' places in a No Mercy run, speculating that if Sans was killed first by the player then Papyrus would have either turned into a vengeful badass who "no longer believes in you" and gives you "a bad time" or a broken skeleton who is still too good-hearted to unleash his full power.
    • The skeleton characters have this in spades, from Sans' Breaking the Fourth Wall and Crouching Moron, Hidden Badass powers to the hidden Dr. Gaster and how he met his fate. Sans' awareness of the fact that he's in a repeating timeline is very open to interpretation as well, with many a fanfic exploring the psychological and emotional effects it would have on him, speculating on exactly how much he's able to remember, and making What If? scenarios in which he (and sometimes other characters) are able to remember them fully.
    • Because the Golden Ending still leaves one character, Asriel, in a tragic situation, a common setting for a fanfic is about abusing the timey-wimey nature of SAVING and LOADING to truly save everyone. The ones that don't have Frisk "fix" Asriel's soul usually involve Flowey getting uprooted, stuck in a flowerpot, and living with Frisk as a sort of "vile houseplant".
  • Fanon:
    • Fans really seem to like the idea of W.D. Gaster being Papyrus' and Sans' father. They do likely have some connection, Sans uses "Gaster Blasters" in an attack, for example, but whatever it is, the game doesn't elaborate on it.
    • Fans seem to have unanimously agreed that the mysterious entity found in the Dummied Out Room 269 (the entity in question is identified as spr_mysteryman by the game's code) is W.D. Gaster. This is not confirmed, but you wouldn't know that going by the fandom. Even the Undertale Wiki goes with it.
    • Papyrus's Voice Grunting noises are nondescript compared to Sans's, but he is frequently interpreted as sounding like Skeletor. Fan voice actors took the idea, ran with it, and gave the fanon voices a sort of Vocal Evolution where they're not straight impressions of the source voices, but still have the qualities of them, with Papyrus being higher pitched with a tendency to roll his r's.
    • Mettaton is also often given the voice of Markiplier, apparently due to the voice of the sole in-game soundclip clip where he speaks sounding similar to his voice, as well as their logos being similar (a giant pixel M).
    • As a stylistic choice, many fan artists depict Sans as literally being chubby, in-spite of his being a skeleton.
    • It's never indicated what order the two skeleton brothers were born in, but most fans portray Sans as the older brother and Papyrus as the younger brothernote . Papyrus being depicted as the older brother isn't unheard of, but it's nowhere near as common.
    • Sans and Papyrus are often believed to have some "Ghost Flesh" of sorts as a way to explain why Sans appears chubby in a way.
    • Papyrus is often associated with the color red or orange while Sans is associated with blue, presumably because that's the color of their outfits. This leads to things like Papyrus's magical abilities being depicted as orange in contrast with Sans' blue (even though both brothers use the same blue and teal colors for their abilities), and drawings with anything non-skeletal on their bodies (blushing, body fluids, the aforementioned "ghost flesh", etc.) being in these colors as well (even though Sans probably bleeds red).
    • As another stylistic choice, fan art of battles will almost always show a human's soul being represented as a heart floating a short distance in front of or inside of them. This is mostly to help such art make sense, as otherwise a full combat floating heart looks awkward at best.
    • A lot of people interpret the protagonist as being completely mute or unable to talk. This doesn't make a lot of sense because that would render them unable to talk to people on the phone, but regardless the idea persists. A common variant of this is to interpret them as being mute by choice. They are often presented as being fully capable of speech, but preferring to communicate with sign language for whatever reason.
    • There have been quite a few works which portray the six other human souls as children just like the protagonist and the Fallen Child.
    • Sans calling Papyrus by the affectionate nickname "Pap" or "Paps".
    • Fans often agree that Papyrus has access to the Gaster Blaster attack. There's not much to indicate he does other than his "non-bone attack."
    Papyrus: HAD I ONLY USED MY SPECIAL ATTACK, YOU SURELY WOULD HAVE BEEN BLASTED TO... WAIT, WHERE'S MY SPECIAL ATTACK?
    • We never do see what Napstablook's cousin looked like before becoming Mettaton, but most fanart depicts them as a little pink ghost with long hair covering one of their eyes, much like Mettaton EX.
    • In the fandom, The Fallen Child is almost universally drawn with red eyes, although they're never depicted this way in-game (in their sole scene, their eyes are brown, just like Frisk's). This is probably done either to represent their demonic nature or to distinguish them from Frisk. This might be because of the Souless Pacifist Ending, where they pull an Eye Awaken with glowing red eyes.
    • Similarly, while Frisk's gender is often left completely open-ended, fandom does tend a little toward interpreting Chara/the Fallen as a girl. A few factors in this are the rosy cheeks on Chara's sprite (which can be gender-neutral but are often associated with femininity), the very name "Chara" being indicated as the proper name for the Fallen and vowel-ending names in English, the game's native language, usually being feminine (on top of "χαρά", in Greek, usually being feminine when used as a name, meaning "joy" or "delight") and finally, Asriel's relationship with Chara. They were obviously friends no matter what, but the events that played out, Asriel's willingness to participate until the end and the context of the interactions we do see all suggest that Asriel's feelings for Chara may have gone beyond just "friendship" - which, given his similarities to his (heterosexual) parents otherwise, makes the most sense in the context of a female Chara. There is still tons of room for a male Chara in fandom, though, especially given Toby Fox's indication that you should use your own name for the Fallen, suggesting that the Fallen's gender should perhaps match the player's own.
    • Sans's sprites have a pair of curved lines just under his eyesockets; what these are are never specified, but fandom tends to agree that they're bags or dark circles.
    • Sans' Offscreen Teleportation are universally called "Shortcuts", based on several lines of dialogue where he says he knows a shortcut immediately before walking you away from the destination or towards a dead end, but arriving at the same place anyway; he never openly associates them as the same thing, and there's no indication he's using his teleportation or actually meaning a shortcut at face value.
    • Sans' mouth is usually portrayed more expressively within fanart than it is within the official game. While the game usually portrays him with a jovial or sinister grin (only tweaking it slightly under certain conditions as to not make it seem jovial), fanarts tend to change the shape of said grin to make it more obvious he's not feeling happy at the moment.
    • Many people depict Sans as rarely opening his mouth while speaking, presumably because he's one of the few characters in-game who doesn't animate while talking (a few others being Undyne under certain situations and Alphys under most situations). Some depict him opening his mouth normally.
    • While most fanart of the protagonist depicts them with the same yellow skin color they have in the game, a large amount of art instead depicts them with a more natural skin color, usually a medium or light tan.
    • Sometimes, the Gaster Blasters are portrayed as the "pets" of Sans and Papyrus. They can be surprisingly cute for floating snake heads that shoot giant laser beams.
    • For all the different ways fans have chosen to design Gaster for their fanart, it's just about unanimously agreed that he has a circular hole through the center of each of his hands. The ambiguous way his sprite is designed could be interpreted this way, but you'd be extremely hard pressed to find one artist that doesn't.
    • Despite Napstablook's overworld sprite depicting them as taller than Frisk, Sans, and quite a number of other characters, fanart will frequently depict them being the size of a plushie.
    • It is very, very common to depict Sans keeping/wearing Papyrus' red scarf as a Tragic Keepsake during a genocide route, to the point where it's pretty much a shorthand way for fanartists to tell the viewer "this is Sans in a genocide route". Such a connection is never even hinted at in-game.
    • Sans' Glowing Eye of Doom during the final battle of the genocide route has inspired some creativity in the fandom. In the game, it only starts glowing when he's using any of his Mind over Matter attacks on you, but some fans depict it acting up whenever he's feeling particularly strong negative emotions. Fans have also taken liberties regarding what the glowy-stuff actually looks like; because of the game's simple sprite design, it only appears to be a flasing blue iris and nothing more, but many fans draw it being flame or neon-like, often trailing or flaring out of his eye.
  • Fan-Preferred Couple:
    • Outside of Canon pairings, Mettaton/Napstablook is very popular, though sometimes people Abandon Shipping when they find out that it'd be Kissing Cousins.
    • People seem to ship Toriel/Sans a lot due to their Odd Friendship as shown from the true end, as well as Toriel being shown to be very angry at Asgore. On the other hand, people do also ship Toriel/Asgore.
    • Mettaton/Papyrus is huge, mostly based on Papyrus' comment about Mettaton and his sexy rectangular body.
    • Frisk/Asriel is incredibly popular, given their interactions at the end of the True Pacifist route.
  • Foe Yay Shipping: The player character has the option to flirt with quite a few of the enemies in the game, though they obviously don't remain enemies for long regardless if you spare or kill them.
  • Friendly Fandoms:
    • With the JoJos Bizarre Adventure fandom, due to Mettaton EX's rapid posing and the Dummy's penchant for throwing knifes being reminiscent of DIO. Some people have also compared Asriel and the Fallen Child to a younger Johnathan and Dio Brando.
    • Most of the OFF fandom as well, due to similar aspects through the games, like graphics, wacky monsters, a mostly Featureless Protagonist, and a shockingly dark storyline and having to face the consequences of killing everything in your path.
    • For the same reasons as the OFF fandom, Earthbound and Mother 3. It helps that Toby Fox had previously contributed to the community.
    • Fans of Wander over Yonder has grown an interest in the game for having similarities with the show, like both having characters who are skeletons, wacky monsters, and for having a similar goal of befriending the enemy instead of defeating them, resulting in a fair number of crossover fanart like characters of Undertale cosplaying the characters of Wander Over Yonder.
    • The Furry Fandom loves Undertale, and not just for "Goat Mom" Toriel. They've even embraced the non-furry characters like Sans and Papyrus.
    • Fans of Steven Universe seem to enjoy Undertale a lot. Tons of crossover art of the two, including fusions and depictions of the characters together or in each other's setting, has been created, and the fandoms tend to get along very well.
    • The Touhou fanbase has also latched onto Undertale due to Toby Fox being a fan of Touhou, The Bullet hell segments in Undertale being inspired by Touhou, references to the games, and Toby even using ZU Ns musical style on a few songs(Most notable in Battle Against a True Hero.
    • Homestuck fans, former or otherwise, make up a large portion of the Undertale fanbase due to having similar (albeit more lighthearted) humor, the same musical inventiveness of Toby Fox, timeline shenanigans in the plot, and three dimensional characters that seem simple at first glance.
  • Game Breaker: Choosing 'Pick on' instead of 'Don't pick on' in a Loox encounter doesn't make them spare-able, but does add 5 extra EXP if you choose to kill them. This bonus stacks, allowing you to potentially get thousands of EXP from one encounter if you're patient enough, making upcoming difficult battles a cakewalk.
  • Game-Breaking Bug: More like "minorly degrading the experience of one boss fight", but on some computers, usually running Windows 7, Muffet's "pet" attack lags horribly, despite the game largely being so simple it could run on a toaster. This is a recurring issue in Game Maker — contrast the entire Omega Flowey battle, which is much more visually complex and yet rarely lags for anyone, because apparently it doesn't invoke whatever it is Windows 7 hates.
  • Goddamned Bats: In the demo, Vegetoids. No ACT will placate them; the only way to spare them is to carefully whittle their HP down until they're willing to give up, which risks accidentally killing them. Additionally, the "Eat" act kills them. This is both obnoxious and wildly out of place — in both the rest of the demo and the entire full game, every monster can be placated without having to Do Well, but Not Perfect at reducing their HP, and no ACT will ever kill a monster. Thankfully, both of these traits were removed from Vegetoid in the full game.
  • Good Bad Bugs:
    • The save file is in no way protected from editing. The very first line is your character's name, in plain text. And unlike when entering your name at the beginning, printing your name during the game has no character limit.
    • You can encounter a bug if you stand on the farthest of either side of the path before Undyne's speech. Depending on which side you're on, you can either get a head start as you go through the cave before initiating the first fight, or immediately start the fight outside of the path and be unable to make progress without restarting as you're constantly chased. Also, the boss constantly respawns at the spot where they land, even if you run away repeatedly.
  • Harsher in Hindsight:
    • During the judgment in a pacifist run, Sans says he would have given up a long time ago if he were in your position. This seems like his old laziness/apathy acting up, but in the final battle, as a lost soul he says things like "just give up. i did." and "why even try?" Everyone else as seems to revert to how they were before you got to know them, but Sans was never explicitly despondent up to this point. To say nothing of the Kill 'em All run where he says that he can't bring himself to put any effort forth, because he knows that everything is going to reset anyway.
    • In a Real Life example, comparing Mettaton to David Bowie. Mettaton is Haunted Technology, and David Bowie died a few months after the game's release.
  • Hilarious in Hindsight:
    • It's hard to tell if it's this or if he knew note  but Toby asking for NSFW images of the game to be tagged "Undertail" so people browsing the game's name wouldn't see stuff they didn't want to was given kind of a humorous spin when the Furry Fandom completely embraced the game. Likewise a line from Sans in Hotland. It Makes Sense in Context as he's selling hot dogs, hot cats, and other such things, but it also sounds like a tongue-in-cheek reference to the inevitable Furry Fandom.
    Sans: ... you really like hot animals, don't you?
    • Within the game itself, there's Papyrus' line during his "date" when you examine the computer where he claims to be extremely popular on the internet. It was meant to be a joke about him being kind of a loser and won't accept it because of his ego, but in the actual internet, well, even he would be stunned by how popular he actually became. Likewise, the conclusion of his date has him lamenting that he's drawn the player into falling desperately in love with him. It's supposed to be a jab at his ego, but ironically he's considered a Mr. Fanservice with a thriving fangirl following in real life, complete with Self-Insert Fic and art. On a somewhat lighter note, during the fight against him, he will note several things that he imagines will happen should he capture you and join the Royal Guard, including Asgore making hedge trim art in the likeness of his face. He actually gets this wish granted in the True Pacifist ending, despite the Royal Guard being disbanded before he can become a member, as Asgore can be seen making said hedge a reality in front of Toriel's school on the surface.
    • Whether or not Toby knew or heard about it beforehand, real-life Glamburgers have existed for over a year, although they're infinitely more expensive and aren't purple, as seen here.
    • After the True Pacifist ending, talking with Mettaton will trigger some dialogue, and among those, talk about having Toriel plushies, but with his face on them. Then a wish was becoming true.
  • Hype Aversion:
    • While Undertale is a great experience, the fandom overpraises it too much sometimes, with many people saying it's "THE BEST GAME EVER! 20/10 " or "LITERALLY MADE ME CRY FOR HOURS!", and they are also very prone to reveal spoilers (especially Tumblr). Some people would rather not play the game just to avoid the risk of disappointment.
    • As mentioned under Hype Backlash, the fact that the game has to be played blind for the full experience is a huge mark against it for a lot of people wary about the game for one reason or another, since most responses to "Why should I give this game a chance?" can be summed up in "You just have to take my word for it because everything I could praise about it is a spoiler."
  • Hype Backlash:
    • Not as fierce as others, but still inevitable with a game that received such a storm of popularity and critical acclaim. Putting aside people who hate it simply for being popular, the most common criticism from players is that the story is good, but the bullet hell mechanics often turn people away. There's also many who don't agree with the way the message is handled in the No Mercy route, with the primary complaint being the game permanently locking you out of the Golden Ending in all future playthroughs is overly harsh. Another portion of this is that every single reviewer for the game recommended playing it first, and many agree that the best way to play the game is almost entirely blind, or completely blind, which becomes a problem due to the advent of message boards and Youtube.
    • While most fandoms tend to spread quotes, images and references to their fandom on things that have nothing to do with it, the Undertale fandom is currently one of the most persistent, causing quite a lot of backlash from people that are by now annoyed of all the "X is filling you with Determination" memes, comparing random characters (sometimes even of older series or games) to Undertale characters and similar.
    • This is especially true for Let's Players and streamers, who always come under fire from their Unpleasable Fanbase, some of whom think they should take a certain route, and others who feel they should play the game blind. It's made some of them refuse to Let's Play the game at all. Those that DO play the game seem to regret it later since no matter what they do, it's never good enough. Most start playing blind, but if they don't follow the pacifist route on the first time or miss a small detail, then they're "playing the game wrong". The most common case is when the Let's Player doesn't know how to spare Toriel, so they kill her, and the entire comment section is filled with angry comments demanding the player to start the game over and "do it right this time". If the LP'er does reset, they get further angry comments calling them a "sissy".
      • Let's Players that have proclaimed to play all variants often get yelled at for playing "in the wrong order". While half the fandom thinks that one needs to play No Mercy and Neutral first to really enjoy the perfect ending, others call the player stupid because by doing that they would just not get to see the charm of the game and lose interest.
      • One of the biggest cases regarding a Let's Player clashing with the Undertale fandom is apparently Mangaminx. She got pretty much the whole game spoiled in the comments on her first video despite her explaining extensively in the video and description that she wanted to play blind. She then proceeded to kill every character she could out of pure spite (with missing a few where she was too slow such as Mettaton), only to now amuse herself with videos where she ridicules the comments she gets on both her Undertale videos and her ridicule-videos. That she finally placed it on the top of her "TOP5 most disappointing games 2015" more or less made her a proclaimed enemy of the entire Undertale fandom.
  • It's Popular, Now It Sucks:
    • As mentioned above, ever since Undertale began winning brackets in the 2015 GameFAQs "Best. Game. Ever." contest, it brought complete ire to many people as it beat Super Mario World and several other games that are considered Sacred Cows. It got to the point where it was mentioned by Kotaku.
      • One of the follow-up polls had GameFAQs ask users if they were planning to buy Undertale since it won. The "No, and I refuse to ever buy it purely out of spite" choice came in second place with 24.71%.
    • Sans himself is starting to fall into this, as he is by far the game's most popular and well-known character. The amount of fanart and fanfic focused on him dwarfs all the other characters, complete with numerous AUs centered around different versions of him, sometimes shipped with each other. Various jokes about his astounding popularity including comparisons to the Once-ler fandom abound. As a result a backlash is developing in parts of the fandom by people who perceive that he's becoming a fandom example of Wolverine Publicity at the expense of the game's other characters, especially since so many AUs and fan scenarios feature Sans taking their place in the story or having them Stuffed into the Fridge or altogether to make more room for him.
  • It Was His Sled:
    • Try looking at anything in the game without finding out that there are multiple paths to the game and that the Pacifist ending is the Golden Ending and the No Mercy path is a viable option and changes the game completely, and that the game keeps track of your past playthroughs.
    • The fact that the final boss fight of the No Mercy route is an extremely difficult fight against Sans is difficult to be completely unaware of before playing the game, considering all the memes, fanart, and talk the battle has spawned, as well as the popularity of the battle music. In fact, the fact that Sans is at least much more competent than one would initially be led to believe is almost impossible not to know about if you've heard pretty much any talk about him prior to playing.
    • Mettaton EX is meant to show up as a surprise, but when the vast majority of Mettaton fanart has him in this form instead of his regular one, being unaware that it exists is rather difficult.
    • The mere existence of Asriel is supposed to be a spoiler, but the fanbase doesn't try very hard to hide it. The fact that he is Flowey is starting to head this way as well.
    • The protagonist's true name (Frisk) is an odd case of this. It's included in a lot of fanart and many people know it before playing the game. Despite this, the nature of this spoiler remains fairly well-hidden: people assume that Frisk is just the protagonist's default name and leave it at that, remaining unaware that the character you name at the beginning of the game is actually the Fallen Child. The significance of everything that comes with this revelation is still hidden well.
    • Asgore's appearance, namely that he's the same species as Toriel, isn't very well hidden, due to him having about as much popularity as the rest of the main cast. And because they're the only of those species you see, from there it's easy to guess there's some kind of relationship between him and Toriel, even if the fan-work or art never states it.
  • Jerkass Woobie: Once one learns exactly why he is such a jerk in the first place, it's easy to feel sympathy for Flowey.
  • LGBT Fanbase: Undertale has a pretty decent one, with the prevalence of canonically LGBT characters note  and other characters appealing to the Bara Genre note .
  • Love to Hate:
    • Flowey. Everything he does is to spite you or make you suffer in some way. To the point where fans find it more endearing and less annoying. It helps that he is a Magnificent Bastard, killing Asgore and absorbing the six Souls in the Neutral path and leading a plan to obtain all the Monster Souls by convincing you to go for the Pacifist Route. This gets turned on its head however when it's revealed that Flowey is the reincarnation of Asriel Dreemurr who can no longer feel any positive emotions, which makes him sympathetic.
    • Jerry is disliked by just about everyone in-universe, and fans happily picked up on this.
  • Memetic Badass:
    • Sans, due to being the most powerful monster in the underground despite his laziness, and really showing it on the No Mercy path.
    • Undyne, a powerful, hammy, and relentless fighter who suplexes boulders because she can. She falls even more into this in the No Mercy run as Undyne the Undying, where she is the only boss that can take more than a couple hits from you; not even Sans can do that.
  • Memetic Mutation: Lots, with this game's writing. Now has its own page.
  • Misaimed Fandom: Many fans of the game actually enjoy the No Mercy route, despite it having a goal of making players feel terrible and unsatisfied. These tend to be story-focused fans (the dark, tragic plot can be just as gripping as the normal story, and it shows many characters at their best) and Challenge Gamers (with the route hosting two extremely tough fights that require players to seriously master the game mechanics in order to succeed). It's worth noting that the majority of Sans' popularity comes from the No Mercy route.
  • Moe: Asriel Dreemurr is this to many, many fans of the game, thanks to his adorable design, sweet personality, gentle little voice, and status as a Grade-A Woobie.
  • Moral Event Horizon:
    • In the Fallen Child's backstory, the Thanatos Gambit of eating buttercups knowing it would poison the Fallen, possessing Asriel, and forcing him to be Mistaken for Murderer by humans in hopes of restarting the war between monsters and humans.
    • For Toriel, Asgore crossed it by declaring that any human that fell down into the Underground would be killed and their soul used to break the barrier and kill all humans. What's more, Asgore agrees with her. While he clearly is not remotely happy about what he's doing, he knows full well that his plan to harness the souls of seven dead children to exterminate the human race puts him beyond redemption, and as a result he doesn't allow for any scenario in which he both fails his mission and continues to live. He refuses to let the child spare him, going so far as to smash the mercy button off of the player's command menu, and when the child wins and spares him anyway, if it's a second playthrough and Flowey doesn't show up to kill him, he kills himself because he simply can't live with himself after the lives he's taken.
    • By the end of the No Mercy Run, the Fallen Child has taken full control of Frisk. Why is this important? Because they know Flowey is actually their adopted brother who loved them dearly. And then they use Frisk to violently murder him even as he sobs for mercy, as Asriel.
    • The entire No Mercy run is a cavalcade of horrible, horrible actions committed by the player, but the game always gives them an out, up until the player goes up against Mettaton NEO. Finishing off Mettaton NEO at full strength is the true "point of no return", and only resetting your save and starting over can get you off the No Mercy route at that point. And actually finishing the No Mercy route is an even greater point-of-no-return from which no amount of resetting can truly redeem you.
    • There are a couple other important "turn-back" horizons: You can take Papyrus's offer of mercy, or you can spare the Monster Kid before the Undyne battle.
  • Narm Charm: Yeah, Photoshop Flowey may seem like something out of a stereotypical, low-quality creepypasta and Asriel's powered up boss form might look like an eight-year-old's Mary Sue OC complete with rainbow themed attacks, but it fits, since Asriel is about that age anyway, and seems like the kind of kid who would come up with such narmy material, and adds onto the bosses' cool factor. As the Draco in Leather Pants example shows, even his boss forms have their fans.
    • Sans' warning that the player is "gonna have a bad time" during the genocide route makes for an incredibly well-implemented Meaningful Echo near the end the game, to the point where the phrase is almost as ubiquitous with the character as the battle itself... except for anybody familiar with a certain outdated meme. The line isn't ruined, but it takes quite a bit more effort to take it seriously.
  • Nightmare Fuel: Has its own page.
  • Nightmare Retardant:
    • A possibly deliberate example. The battle against the Endogeny Amalgamation. It's a big, faceless dog abomination... that's still just a playful dog in the end. The fact that the hilarious "Dogsong" starts playing when it's happy enough just seals the deal.
    • Seeing the Amalgamations at the end reunited with their loved ones and being accepted back into the community does a lot to negate their scariness. Once again, probably intentional, as none of them were really evil.
    • The amalgamation that looms over the protagonist while they sleep. While you lie there in the bed, an elongated abomination looms over you... and slowly begins to reach out towards the unsuspecting child with a long, creepy appendage, and then... it tucks you in and gives you a gentle pat on the head.
  • One-Scene Wonder:
    • Muffet the spider girl only appears for a single boss encounter, but she's gained a very large following.
    • Catty and Bratty, despite being easily missed shopkeepers with little relevance to the story, have a decent following. Their uniquely appealing designs and personalities help. Same goes for Burgerpants, considered one of the funniest NPCs in the game even though he's very minor.
    • Asriel-as-himself (he himself considers him to be a different person from Flowey) is very important to the backstory, but only shows up in person for the final boss and what follows, after he leaves, he drops from the plot entirely unless you actively go to seek him out. Nonetheless, he has a disproportionate amount of popularity due to his tragic story, his adorableness, and the fact there's no happy ending for him.
    • Mettaton EX only shows up for a single battle, but he and his battle are so memorable that in many instances, the popularity of his EX form vastly overshadows that of his regular form.
  • Paranoia Fuel:
    • After your first encounter with Flowey in the demo, you know he vandalized the instruction manual. He also knows what you did during the game, and he'll leave more messages in the manual for you. He also knows you can save and reload. He even knows when you stick your nose where it isn't wanted, such as the website's source code. You can't hide from Flowey. Hee hee...
    • During the game, try to walk a screen or so into a room (such as the first save room), then walk back to the entrance. You might spot someone running back out of the room. Didn't we just say that you can't hide from Flowey?
  • Player Punch:
    • The Boss Battle at the end of the demo is not fighting to kill... yet you may very well end up killing them.
    • Let's say you kill Toriel, then reload your save to find a way to spare her. Flowey knows what happened the first time. Killing Toriel and then finishing the game otherwise peacefully will net you more than a few player punches. Not only do you learn that she and Sans had become friends and she had asked him to protect you, which was the only reason he didn't kill you on sight, but also that she was the former queen who lost both her children shortly before the war between humans and monsters began. There's even one more where Asgore, anticipating his own death during your fight, will give some dialogue about how he wants to see his wife and child again. All of that together is just one big ouch.
    • Enjoy being an aloof completionist and seeing every possible route? Well, if you did figure out how to spare Toriel the first time, you'd have to go back and kill her to see what happens, won't you? After all, it's not like it's what you really want to happen, you just want to see everything. Nice try. Once again, Flowey knows what you did.
    • Applies even more strongly to the full game. The only way to replay the game after getting the No Mercy ending is to sell Frisk's soul. Have fun when, upon completing a True Pacifist run, the last shot shows that the Fallen Child has control over them and can ruin everything at a moment's notice!
    • If you kill Undyne on an otherwise mostly peaceful run, the fact that Papyrus IMMEDIATELY calls you up to say that he thinks you guys should hang out some time and Alphys feels no need to watch the battle footage because she knows Undyne is too tough to ever lose a fight are incredibly harsh.
    • For those wanting to make Flowey pay for the suffering he causes, the game won't let you have any satisfaction if you choose to kill him at the end of a neutral run, as his last words are triumphant.
    Flowey: I KNEW YOU HAD IT IN YOU.
    • Killing Flowey on a No Mercy path will give you even less satisfaction, cause at that point he's scared shitless of you. His voice grunt and face changes to Asriel (who he actually is) as he begs you not to kill him... and then the fallen child, Asriel's old friend, brutally butchers him with no input from the player. And you see it frame by frame.
    • If by chance a player wants to see a No Mercy Run, but does not want to actually do one, they can opt to watch it via a Let's Play. Flowey says that these people are "sickos" who are too weak to do it themselves, so they choose to watch it.
    • Papyrus has a particular pained expression that only appears in especially depressing epilogues. Seeing him suddenly become genuinely distressed after spending the entire game so upbeat comes as a heavy shock to many a player.
    • At the end of No Mercy, Sans, at the halfway point of his boss fight, says he senses someone with a kind heart who wanted to do the right thing beneath the Omnicidal Maniac you and the fallen have become, and he's trying to get them to break free of the two of you. While it's implied he's bluffing (he does the same dialogue even if you haven't done a pacifist run), it can easily ring true if you have finished Pacifist and you know Frisk's true nature. And the worst part is, unless you finally decide you can't handle it, Frisk is still stuck as the two of you's unwilling pawn as they kill the last three people important to them.
  • Praising Shows You Don't Watch: When considering for the prevalence and passion of the Undertale fandom, it's hard to make sense of how many fans have actually played the game. Given how the characters deeply overshadow the gameplay, this is understandable.
  • Sacred Cow: Downplayed and Subverted in some cases. While the game is held up in very high regards from fans and even people who've never played it, it hasn't been immune to criticism. One of the biggest complaints among even fans is that you can't save Asriel.
  • The Scrappy:
    • So Sorry, who is one of the game's only Bonus Bosses. Like Muffet, he was included as a backer's Kickstarter reward, but he has received significant hate for quite a few reasonsnote . Nearly any Youtube video of the fight with him is an explosion of disdain that leads to flame wars, and has noticeably more dislikes than other Undertale videos. The vitriol has even spread to the point where the backer has been harassed, requiring Toby himself to make a tweet asking them to stop.
    • In-universe we have Jerry. Abrasive, obnoxious, and unlikable and hated by about everyone, and if you read his dialogue, he confirms every bad thing you heard of him. It gets to the point where monsters actively work with you to ditch him. Nobody likes Jerry.
  • Self-Fanservice:
    • While Undyne isn't terrible looking, fan-art often portrays her as a busty Amazonian Beauty, when she's actually pretty scrawny and flat-chested.
    • Toriel falls victim to this as well. Toriel herself is absolutely adorable in game, but fan-art often draws her with curves (large hips/butt/bust; aka attributes normally associated with more fetishistic mother type characters) she doesn't have in-game.
    • Likewise, Asgore is not only cute, but very manly looking with his Badass Beard and has what is possibly Stout Strength. Fan-art of the character usually has him as either very muscular, very chunky, or both. Like with Toriel, it's a bit tough to determine what exactly his build is like under that armor and the pacifist ending that shows him wearing a pink shirt and blue pants, which entertains the possibility that he has a bit of heft on him, but not enough to fully determine that he is a heavyset guy.
    • Sans in-game looks goofy and harmless, but fans portray him as suave and edgy sometimes having sharp teeth. Because he's a skeleton, however, they've given him the ability to magically conjure blue appendages for sexual pleasure, deriving from his glowing blue eye in the No Mercy Run.
    • Mettaton EX is made a lot more feminine in fanart, having his legs, butt, and hips become larger and having his face made girlier. While in game he's still a Mr. Fanservice, he looks more akin to a glam rocker, ala David Bowie, than a woman.
    • Alphys also has some, fanart tends to downplay her hunchback or remove it entirely and have her teeth look normal (she has a fairly bad overbite). Even though these are small changes those two things can almost make her look entirely different.
  • Ship Sinking: Toby sunk pretty much every ship that Sans launched when he tweeted that he's too apathetic and lazy for any kind of committed relationship.
  • Ships That Pass in the Night:
    • When you first fight Mettaton, one of his questions is "Would you smooch a ghost?" and all the answers are "Heck yes!" Thanks to that, Mettaton/Napstablook have a fair amount of followers on Tumblr. Some even draw Napstablook as a robot called "Napstabot", with a similar style to Mettaton. The real reason is probably that Mettaton was created by merging the soul of a ghost with a robot body, something not revealed in most endings. Ditto for why Napstablook's call-in in the bloodless conclusion to his boss battle starts to sway him. Of course, the fact that they're cousins has deterred quite a few people.
    • Mettaton/Papyrus is also quite popular, due to a single line where Papyrus mentions that Mettaton is his "favorite sexy rectangle".
    • Nice Cream Guy/Burgerpants. Probably because of their opposite personalities, and that an update to Burgerpants' dialogue trees confirmed that they are slightly friendly with each other.
    • Undyne's line to Papyrus about "extra private one-on-one training" has raised a few eyebrows, but her relationship with Alphys usually takes heavy precedence and her later interactions with Papyrus don't imply anything further.
  • Surprisingly Similar Stories: So a kid falls from grace and hates humanity, gets adopted by a noble family, and poisons the dad. Finally, they attempt to take control of their stepbrother's body and transcends humanity to become something else. Are we still talking about Undertale or are we talking about Jojo's Bizarre Adventure?
  • Suspiciously Similar Song: "Bonetrousle" sounds a lot like "Unfounded Revenge" from Mother 3, and "Metal Crusher" bears a resemblance to the Tetris "Type A" theme.
  • That One Attack:
    • The very first attack thrown at you by Sans, who is only fought in the No Mercy route, is sadistic, starting out as a very fast corridor of projectiles you must squeeze through and afterwards becoming a gauntlet of large lasers which fire in multiple directions. What makes this worse is that Sans can fire this attack randomly during his introduction speech if you don't beat him on your first try, which makes it easy to get caught off-guard. This is Lampshaded by the attacker, who wonders why other enemies don't use their strongest attack first.
    • Undyne's golden spears circle around and behind you right before they hit, making them fairly challenging to block because you need to turn your back on them. This is not insurmountable when there's just a few of them, but eventually she chucks a long volley of nothing but golden spears. Good luck with that.
  • That One Boss: Has its own page.
  • That One Level:
    • The Core in a Neutral/Pacifist run. While not a particularly long level, the puzzles there can be quite frustrating, and enemy encounters are scripted to happen in certain spots besides just happening regularly. The enemies deal a lot of damage and their attacks are difficult to dodge, and figuring out how to spare them is an involved process that takes multiple turns per monster. Even a Neutral player who chooses to kill them has no solace, as the monsters have weighty defenses and loads of hit points. All of the above is compounded when multiple monsters show up per battle, which is common. And when you finish the Core, Mettaton is waiting at the end.
    • Likewise Hotland, especially on repeat playthroughs. The various puzzles and scripted events combined with Alphys's frequent Stop Helping Me! moments cause the zone to drag out compared to the previous zones.
  • That One Puzzle: The piano puzzle in Waterfall is pretty challenging, but not because of a confusing solution. It's actually because, to some people, the correct tune sounds rather different from the music box. Toby seems to have recognized this, because the January 2016 update made the solution reveal itself directly once you activate the music.
  • They Wasted a Perfectly Good Character:
    • Mettaton has a form you can only see in a No Mercy run, Mettaton NEO, that comes complete with its own theme tune. He still dies in a single hit, just like most of the other bosses in a No Mercy run.
    • A lot of fans would have liked to see Muffet's character explored in more detail. More than a few fans have argued that her design and concept alone would have been enough to warrant a larger role in the story. Considering that she's a backer character, her lack of presence is justified, at least.
    • Dr. W. D. Gaster. Everything about him (role, fate, relationship with Sans) sounds really fascinating, but you will only get to find about all of this by searching Dummied Out content. Then again, this seems to be the whole point of his character and a possible Sequel Hook.
      • As of the 1.01 patch, Gaster is no longer Dummied Out and both his followers and "mysteryman" have a small probability of appearing in any given playthrough.
  • Toy Ship:
    • Asriel/Frisk, due to their interactions in the True Pacifist Ending.
    • Asriel/Chara is also a somewhat popular ship.
  • Ugly Cute: The majority of the cast falls into this. An adorable old goat lady, her equally adorable husband, a pair of wacky skeletons, a fish lady with a Slasher Smile, and a nerdy lizard-like mad scientist. Inverted with Flowey, whose cuteness lasts all of 5 seconds after your first encounter with him. Of course, with the game's relatively simplistic art style, exactly how "ugly" the characters are is debatable.
  • What Do You Mean, It's for Kids?: Toby Fox states the game is essentially family friendly, and for the most part it is. However, some certain parts near the end of the game involving Photoshop Flowey, the True Labs, and the truth about the Fallen Child, (the latter especially on a No Mercy playthrough), are meant to be horrifying and are played so well that some may get the idea that he hates children, considering that many adults have gotten actual nightmares from the game. At least there's no attack called "PSI Bitchkill".
  • Wild Mass Guessing: Anything to do with Dr. W. D. Gaster, due to the difficulty of finding him, his nature being shrouded in mystery, and the fandom having practically nothing to work with.
  • The Woobie:
    • Flowey manages to get hit with the jerkass variation. Driven insane through lacking a soul, and thus not knowing love or mercy, he slowly realized his world was just a game where everything happens the same way. And on top of that, he gets slaughtered by his former friend at the end of the No Mercy route.
      • Asriel. His kindness and love led to his death and unholy reincarnation, due to the manipulation of the first child. He has suffered more deeply than any other character. Literally every scene with him is a Tear Jerker.
    • Asgore, as well. Fighting a war he started out of sheer rage over the deaths of his son and adopted child, then couldn't end when he cooled down, he was abandoned by his own wife over his actions and ends up spending his days confined to his palace. When you do finally confront him, the way he says that he only wishes to be reunited with his wife and son suggests he has broken under the trauma and become a Death Seeker.
    • If you know of certain revelations only shown in the True Pacifist ending, the player character in the No Mercy path is perhaps the most tragic figure of all: they're Frisk, a kindhearted child whose personality and desires are actually independent of the player and the Fallen Child. They're not even being manipulated like Asriel initially was, but flat-out forced to kill absolutely everyone against their will. While most of the main characters have a Dying Moment of Awesome, Frisk's SOUL just fades away over the course of the story as it's replaced with that of an inhuman monstrosity — and, for whatever time they have left, the other characters won't know Frisk as anything else. The one person that comes even slightly close to realizing it- Sans, who asks the Fallen Child about whether or not there is anything gentle left in them during his boss fight- gets just as brutally slaughtered as everyone else. And the total kicker? If the player finally decides to at least spare the world itself, they get instantly killed, too.
    • Napstablook is a pitiful character who is very passive and obviously depressed. When one finds out their friend (Shyren) has become more reclusive due to her sister "falling down" and their cousin (Mettaton) has abandoned them to pursue stardom despite his promises not to, you can see why they're so depressed.
    • Snowdrake's Amalgamated mother. She is corrupted almost beyond recognition, barely able to attack or speak, is accompanied by a heavily distorted version of the peaceful "Snowy" music, and manages to hang onto just enough of her original self that she speaks of her missing son. Everything about her is played for tragedy, like Asriel.
    • Sans. After gaining awareness of the fact that he exists within a world where time can be reset and manipulated at any point, he tried in vain for who-knows-how-long to fight it until he devolved into a pit of despair and nihilism. He can't bring himself to fully enjoy anything except the company of his dear brother (who you can murder in cold blood, by the way) and drinking, so much so that even making it to the surface doesn't really appeal to him anymore; why bother if it's all going to be reset anyway? The way he acts during the battle, smiling and chatting dismissively despite the pain and exhaustion in his eyes, makes it seem like even the idea of dying barely fazes him anymore. On top of that, assuming he has subtle memories of previous timelines like most monsters do, his mind could potentially be flooded with half-formed recollections of anything from losing everybody he loves and dying with the knowledge that he failed, to finally achieving his happy ending only to have it be snatched away like a toy, except unlike everybody else, he knows that they actually happened. The worst part about all this? Unlike every other example on this list, the average playthrough would give you practically no indication of this. Aside from a few tiny hints that are either incredibly subtle or incredibly hard to find, Sans never gives the impression that he's anything but the friendly, pun-loving, perpetually-smiling skeleton he presents himself to be. It's only at the end of a genocide route, where he's been pushed to the absolute end of his rope, does he finally bare his soul in a last-ditch attempt to get the player to go back on their evil ways and reset. Somebody, please, give this poor guy a hug...