YMMV / Undertale

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  • 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 Toby Fox not going into detail about what things in the game mean, 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.
  • 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; a coward unable to live up to his promises; or a coward unable to go back on an extremely bad, wrath-induced decision because he can't stand up to his own subjects?
      • Regarding his plan, did Asgore hesitate out of cowardice... or out of pragmatism and intelligence? Had he gone and attempted to take several human souls like Toriel suggested, it would have gotten everyone's attention and despite Asgore's increased power, there was a chance he would've been injured or even killed, especially when you consider the modern day weapons humans would now have to slaughter monsterkind with. By waiting in the underground, he awaited any potential humans who would've attempted to climb or make their way there. While some would call it a long shot, considering that several children have made it there to fulfill the requirements, it still works. Furthermore, no one would bother looking for the missing people since they would be assumed dead. He would've gotten the souls and maintain an element of surprise as well as possibly letting tensions cool down.
    • Toriel:
      • On relationships: Did she leave Asgore because she wanted nothing to do with his plans, to save the children that she knew would fall through the hole on Mt. Ebott, 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. Which begs the question of whether she would have wanted him to go kill humans to harvest their souls, or simply borrow the necessary souls from humans dying naturally; the ambiguity tends to make her come off much more bloodthirsty than him. In addition, when she said this, did she mean she actually supported the general plan, or was she telling Asgore to stop dressing up his actions as noble by saying that if he really wanted to save his people that way he could have done it already?
      • Some have called Toriel on being a hypocritical coward since as Queen, she could've done the same thing using the Souls and rather than do anything, she ran away from her past and abandoned her husband onto a dark path.
    • 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?
      • His awareness of the fact that he exists within a repeating timeline has inspired a lot of fan speculation and discussion in terms of how it affects him emotionally and psychologically, especially since he's intentionally meant to be a character that's difficult to get into the head of.
    • 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 an attitude? Considering their official status as an extension of the player, both possibilities could be equally true.
      • Was the first child even human? Although they have been a soulless entity similar to Flowey/Asriel, were they really human to begin with? At the end of a second Genocide playthrough, they refer to themselves as a demon, but do they mean this literally, metaphorically (similar to how Asriel is dubbed "The Absolute God of Hyperdeath"), or did they end up that way through the player's actions? 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. You can find a very well done in-depth analysis here.
      • This interpretation adds a whole new layer of sadness to the No Mercy route, as the player's bloodlust corrupts whatever is left of the fallen child from someone who helps Frisk through the Underground, despite their hatred of humanity, into someone who assists them in their slaughter of the monsters they once lived with and would kill Asgore, their adoptive father, without hesitation.
    • 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?
    • The main character's predecessors. Some of them are heavily implied to have taken the lives of multiple monsters before dying. It's left ambiguous what their reasons and motivations were, and plenty of people debate whether they killed those monsters for psychotic fun, or in legitimate self-defense since they didn't have Frisk's resetting powers and thus had far more justification to use lethal force in defense of their life, or if they were the aggressors, but did it out of a tragic desperation to escape the underworld and avoid being used as a weapon through which to exterminate their own species. Most agree it probably wasn't a uniform reason between all of them and that between all six of them, all of these reasons and more could have been behind their actions.
  • Anti-Climax Boss:
    • While it doesn't seem like it at first, the Photoshop Flowey battle is actually heavily skewed towards the player. Sure, Flowey's Bullet Hell is the strongest of any boss in the game, but getting hit only cuts your health in half, so you have to be hit plenty of times to actually die. The SOUL sequences (which take up about a third of the battle) are actually impossible to die to, and clearing them grants you a ton of healing and a checkpoint for if you die. Once you call out to all six SOULs, the battle is pretty much over; the now vulnerable boss has a so-so health pool and you're showered with green healing bullets while you attack. While it's very likely you'll die several times until you learn how the attacks work, overall the preceding fights with Mettaton EX and Asgore are tougher despite feeling less so, especially if you have low LV. It manages to be a very cool boss fight regardless.
    • 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.
  • 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.
  • 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.
  • Awesome Ego:
    • Papyrus, partly because he's so funny, and partly because he's a really nice guy.
    • Mettaton. Just about everybody both in-universe and out thinks he's absolutely delightful.
  • 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.
    • Asgore is either beloved by fans who find him a very likable character who was put through a very difficult situation and ended up making major mistakes due to it or hated for the same reasons Toriel hates him. It doesn't help that he's often a victim of Die for Our Ship as well.
    • Sans has become this as his popularity's reached a fever pitch. The amount of fanart and fanfic focused on him dwarfs all the other characters, complete with numerous AUs centered around different versions of him (and AUs of those AUs) sometimes shipped with each other, and even entire original series inspired by the many different Sanses. Various jokes about his astounding popularity, including comparisons to the Once-ler fandom abound. As a result, a backlash is developing among other fans who perceive that he's becoming to Undertale fandom what Wolverine is to Marvel 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, alter their personalities or backstories in order to make Sans look better at their expense, or even have them Stuffed into the Fridge altogether to make more room for Sans when they're not being ignored completely. Adding to this are also people who do like Sans, but dislike the fandom response to him because of all the Canon Defilement and Mary Sue Tropes surrounding him. Not helping things is people thinking the concepts behind many of the broader-encompassing AUs are interesting and have great potential for storytelling, but, barring a few exceptions, a large number of them explore ''only'' Sans.
  • 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. Additionally, calling Papyrus or Undyne in his area will just net a confused "Onion...?" from both of them. Yep, they're just as confused as you are.
    • 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.
    • The... whatever-it-is that happens when you lay on the floor of Napstablook's house. While pretty and ultimately very soothing, it's a bit out of nowhere and never explained in-game.
    • 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.
    • Discussing Mettaton's gender is often fuel for flame wars on a massive scale; while Mettaton is referred to as he in the game (unlike characters like the Human Child and the ghosts, who both in-game text and characters are content not to specify and the latter of which are actually the same species as Mettaton, making him the only ghost who's gendered in the game), his effeminate behavior and rather... flashy EX form have lead to many an instance of Viewer Gender Confusion among more casual fans and been played in arguments over just what he identifies as.
    • Relating to this, there's whether Mettaton "counts" as being transgender or not. His change from being a ghost to possessing a robot body is referred to in his diaries in similar terms as a gender transition might be, enough that if nothing else there's a case for him being a transgender allegory or trans equivalent of the Discount Lesbians trope of sorts, and is combined with his pronouns changing from a gender-neutral they/them/their into he/him/his - for some fans, this is enough for them to feel that he canonically counts as transgender representation, while others actively deny that gender is even an issue with his character. People on both sides can be incredibly aggressive about the whole thing.
    • There's a decent point of conflict within the fandom over Asriel and his ultimate fate in the Pacifist Run. Some fans argue that his actions as Flowey don't make him worth having his own happy ending, or believe that him staying in the underground as a redeemed but still emotionless flower is suitable penance, or that there needs to be one last moment of bittersweet to keep the ending from being insufferably Tastes Like Diabetes. The other side... well, the amount of Fix Fics that have Asriel saved, one way or another, should tell you that there's a massive crowd who thinks absolutely everyone should get their happy ending, even him.
    • Mentioning a sequel is usually a surefire way to get the fanbase fired up. One side believes that the game's Golden Ending should remain as is (thus no sequel), while the other side feels that the game can expand upon a sequel and that there's room for it.
    • Is Toby a genius for using Recurring Riffs and recycling songs (often speeding them up or slowing them down by large magnitudes, to the point where it's not obvious), or is he lazy for not coming up with more original tracks?
    • The role of the Fallen Child, "Chara". With their backstory left vague and completely unanswered by the end of the game, fans have argued whether or not they are the true Big Bad of Undertale, Frisk's Mission Control and the game's Lemony Narrator, or even the Anomaly and the true Player Character themselves. Each theory has varying levels of in-game evidence, but each also raises several questions, meaning none are "right," but this doesn't stop fans from arguing theirs as "the most right." To make the situation all the more complicated, each theory came out chronologically, meaning Genocide Chara Theory has been around the longest and has become the base for both TV Tropes and the Undertale Wiki (meaning newcomers will likely see it first), Narrator Chara Theory has been around for a shorter time but is quickly gaining supporters across social media and gaming websites, and Player Chara Theory is only held by a small minority.
  • 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. Plus they have very similar clothing: a jean, a dark tank top and red boots.
      • 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 and desire to goad them into killing can also be seen as a mirror of the Joker's obsession with tormenting Batman. Even one of Flowey's Nightmare Faces looks like the Joker's face.
    • Sans is frequently drawn as Jotaro Kujo of JJBA, due to his ridiculously hard boss battle, association with the color blue, and one scene where he seems to have the ability to stop time. More often than not, his Gaster Blasters will be his Stand.
      • Aided by the fact that their games are similar and have a lot of mutual fans, many people have also noted a lot of similarities between Sans and Pablo, a.k.a, The Judge; both perpetually sport toothy smiles, are friendly but have strong moral codes, have younger brothers that they love dearly, are sent into a Heroic BSOD upon said brother's death, and confront the player for their evil deeds at the end of their respective games, potentially serving as the final boss. Sans' final confrontation is even referred to as "[being] judged", thus making his unstated title the same as Pablo's.
      • If you consider the game to be a modern-day version of Alice in Wonderland, Sans is very much like the Cheshire Cat. After the child falls down the hole, he greets the child from the woods; he is friendly, but sometimes acts as a distraction, to the point of trolling; he has a fondness for wordplay, can move from place to place in the blink of an eye; and of course his perpetual grin. The player's final confrontation with Sans (in a castle hall just before the battle with the monarch) is eerily similar to the Cat's death in American McGee's Alice, except that if Sans dies, it's at the child's hands.
    • 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 compared 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. Another Wander character, Lord Dominator, gets this with both Undyne (with strong Good-Evil Counterpat vibes) and the Genocide Run version of the Fallen Child.
    • 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.
    • 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.
    • The Fallen Child. In the Genocide Route, they're a cute, smiling child that's become an unrepentant mass murderer. Doesn't help that a lot of fanart portrays them with various degrees of aging-up and physical attractiveness.
  • Crosses the Line Twice:
    • If you kill Tsunderplane, it starts plummeting down while saying "but I never got to-" before crashing and exploding.
    • Jerry is made obnoxious in so many ways that he wraps around into being a Love to Hate character instead.
    • In a masochistic sort of way, getting dunked on by Sans is likely to be frustrating and unexpected for new players, and having your trust be betrayed by the game is extremely startling in a meta sort of way. But the fact that the Non Standard Game Over is accompanied by Dog Song, one of the silliest songs in the game sped up to be even sillier, takes a bit of sting out of the wound and instead makes the experience Actually Pretty Funny.
  • Crossover Ship: A few, but two that stand out are Frisk and/or Chara with Madotsuki due to all of them being children in EarthBound-esque games with an affinity for knives, and in Frisk's case sharing identical facial expressions throughout the entirety of their respective game.
  • Die for Our Ship: Asgore is bashed a lot by the more hardcore Sans/Toriel shippers due to already having a pre-estabilished relationship and later fall out with the latter, not helping matters is that Toriel provides a lot of ammo for shippers to use in their bashing against him while making Sans sound like the ideal partner.
    • Toriel also gets the same treatment from others, often claiming that she left Asgore not for the totally sympathetic reasons she's stated to have left him for, but for petty irrational reasons brought on by a complete breakdown following the death of their son.
  • 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.
    • Toriel. While she's not a bad person, the fact that she treats the player character so kindly at the beginning of the game makes it easy to overlook her questionable treatment of Asgore. She's also an example of Base Breaker since some of Asgore's defenders take it too far and make a Ron the Death Eater out of her.
    • Depending on the mass amounts of Alternate Character Interpretation and complete utter lack of in-game evidence supporting any side, the Fallen Child may be an example of this. 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, they still weren't "the best person" according to Asriel, and they've undeniably become an evil Omnicidal Maniac by the end of the Genocide route. In spite of this, many fans portray them as simply being misunderstood in life and still not being wholly corrupt even after destroying the world and taking Frisk's SOUL.
  • 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 a character that those who work in the fast food industry can relate with. 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.
    • The Annoying Dog, who most players will find more amusing than annoying due to the ways it trolls the cast, such as apparently being one of Papyrus's death traps and sneaking into the player's inventory and then absorbing a "legendary artifact" when deployed.
    • The Tsunderplane, for being a parody of Tsunderes. Not that it likes being popular or anything!
  • 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?

    F-H 
  • 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.
    • 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. Unfortunately, this has resulted in a lot of backlash against the fandom as a whole, with its chill, friendly portions getting the same reactions as the overzealous ones from people who've seen too much of the latter.
    • 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.
      • One huge Berserk Button seems to be killing Toriel on a Neutral Run, as Dookieshed puts it. He even lampshades this with how the title "Why You Should Kill Toriel" is clickbait, and argues that Undertale is a game that needs to be experienced, at the risk of the player not doing a full Pacifist Run.
      • As an example, popular YouTuber Mangaminx ended up on the receiving end of this. It soured her opinion of the game and the hype surrounding it, though she still completed the Neutral playthrough just to spite her detractors.
    • Tell somebody who believes the Fallen Child is the true villain of Undertale that the Fallen Child was really the Narrator All Along or vice versa.
      • Or worse, tell either of them the Fallen Child was really the Anomaly and the true Player Character.
  • 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, Pokémon 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".
    • The Cave Story fandom tend to be annoyed if Undertale shows up in the "Cave Story" Tumblr tag, though many more like the idea of a Cave Story/Undertale crossover.
  • 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".
    • The Fallen Child's backstory, since it's never truly answered in game. Were they really out to unseal the barrier and free the monsters, or was it a ruse to destroy everything? Were they evil all along, or did they just have a Dark and Troubled Past? What is their relationship to Frisk: as a guide and mentor, or as a parasite?
  • Fanon:
    • The skeletons in particular invite a ton of this:
      • Sans calling Papyrus by the affectionate nickname "Pap" or "Paps".
      • As a stylistic choice, many fan artists depict Sans as literally being chubby, in-spite of his being a skeleton. Which led to the popular fanon that both he and Papyrus both have some "Ghost Flesh" or other magic organs of sorts that they can manifest at will.
      • 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 Papyrus visibly has a normal pink-colored blush during his date , and Sans probably bleeds red, assuming it's not actually ketchup).
      • 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' 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. Likewise, 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).
      • 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.
      • 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. As an aside, this is usually complimented by Papyrus keeping/wearing Sans' jacket in fanart related to the aforementioned hypothetical Genocide route where Sans is killed first instead.
      • It's very popular to depict Sans's hoodie with fluffy or furry lining on the hood, despite official artwork and merchandise showing that's not the case.
      • 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 using any kind of attack (likely for dramatic/symbolic effect) or feeling particularly strong negative emotions, which somewhat conflicts with the in-game implication that his eyes turn black to indicate this. 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 flashing blue iris and nothing more, but many fans draw it being flame or neon-like, often trailing or flaring out of his eye.
      • 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. In the same vein, for all the different ways fans have chosen to design Gaster for their fanart, it's also 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.
      • 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.
      • Papyrus's Voice Grunting noises are nondescript compared to Sans's, but he is frequently interpreted as sounding like Skeletor. Similarly, Sans's voice has been interpreted as sounding identical to Cr1TiKaL so often that he pointed it out in his video playing the game, saying that he had gotten a ton of messages and emails telling him about how well his voice would fit. 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, and Sans having a low, drawling voice.
      • A popular joke/plot point in fan material is Sans having a ridiculously high tab at Grillby's that he never pays off, so much so that "pay your [optional expletive] tab" is practically Grillby's fanon catchphrase. Although we do see him put a meal on his tab at one point in the game, we're not given any definite evidence that he does this regularly, and the notion that he does actually conflicts with the bird monster's statement that Sans is one of Grillby's best customers (Of course, said bird monster later admits that he's making everything he says up, so that doesn't really mean much).
      • 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." However new dialogue from the 1.001 update supports this a touch: After abandoning a Genocide route at Papyrus, in his room is the box of unused bones. When examined:
      Papyrus: HAD I ONLY USED MY SPECIAL ATTACK, YOU SURELY WOULD HAVE BEEN BLASTED TO... WAIT, WHERE'S MY SPECIAL ATTACK?
    • As a popular 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. A possible explanation for why this is popular is simply that ambiguity is such an integral part of the protagonist (to the point that calling Frisk "he" or "she" is enough to start a flame war) that even assigning a certain voice to them just doesn't seem right.
    • There have been quite a few works which portray the six other human souls as children just like the protagonist and the Fallen Child, even though the game doesn't provide any concrete indication as to any of their ages. There is evidence this could be correct, however; Toriel's house does have toys and kids' shoes of various sizes, which supports the theory, and Toby also referred to several children falling into the monsters' realm in an interview before the game came out.
    • 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. Likewise, the Mad Dummy's ghost is assumed to be yellow or orange like them, and the Ruins Dummy is assumed to be a green ghost with a missing eye.
    • Similarily, we never do learn Napstablook's cousin's name before he became Mettaton. A large majority has settled on their name being "Happstablook" due to Papyrus dubbing them as such. Note (spoilers) 
    • In the fandom, The Fallen Child is almost universally drawn with red eyes, this might be because of the Soulless Pacifist Ending, where they pull an Eye Awaken with glowing red eyes. This is done regardless of whether Chara is being portrayed as evil or not.
    • Similarly, in contrast to the protagonist's gender which is often left completely open-ended, fandom does tend a little toward interpreting Chara/the Fallen as female. A few factors in this are the rosy cheeks on Chara's sprite (which can be gender-neutral but are often associated with femininity) and 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"). 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.
    • The First Child's real name being Chara. Officially they have no canon name for the reasons given above, but the game has a special response if you use it and the in-game files refer to the character as such, causing many to consider it as good as canon at this point.
    • In some lighter hearted stories with The Fallen child/Chara and Frisk (either be it share mind or somehow separate), They're generally shown as sociopathic in words alone, being harmless.
    • 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.
    • The six previous humans have varying thoughts on what they should act like, but the Dark Blue/Indigo SOUL is generally portrayed as being extremely violent, verging on No Mercy run levels of murderous due to their lost possessions, which are said to be covered in dust, which people frequently take to specifically mean the corpses of one or more monsters.
    • Fan depictions of Mettaton often portray him as someone who Really Gets Around and hits on Anything That Moves. While he's a vain Mr. Fanservice who banks a lot of his popularity on his good looks, the game never makes any indication of his sexual or romantic inclinations, even in Getting Crap Past the Radar form.
    • Burgerpants, to many fans, seems to usually be depicted with a slight Southern US/Texas drawl, and usually smoking a cigarette, sometimes to the point of chain-smoking (despite only one of his expressions featuring a cigarette).
    • Despite her very modest sprites in-game, most artwork and fictional descriptions of Toriel depict her as having a very full, voluptuous figure, typically sporting very large breasts and wide hips. While this is no doubt a result of the common sexualization of fictional characters, it's also likely the result of Toriel's motherly nature, as full-figured or voluptuous figures are commonly associated with matronly archetypes.
    • It's not uncommon to see fans depicting the flame girl in a Sailor Fuku (who is sometimes simply called "Fuku") as either Grillby's daughter or at the very least related to him somehow.
    • Virtually everything about the nature of the Fallen Child's purpose in-game.
      • One of the most popular theories, by far is that the Fallen Child is the one who begins moving Frisk during the cutscenes in the Genocide Route.
      • Another big idea that's risen recently is that The Fallen Child is the narrator in all routes, not just Genocide.
    • Pretty much any full-body depiction of the shopkeepers, since their sprites are only seen from the waist up.
    • A pretty popular "after the Golden Ending credits" theory is the idea that Frisk came back and retrieved Flowey in the underground, with the determination of including him in their new family in the hope of eventually redeeming him. Cue Flowey learning (though always perplexed and unwilling) from all the main characters.
  • Fan-Preferred Couple:
    • Outside of Canon pairings, Mettaton/Napstablook enjoyed popularity for a while, although its popularity dropped significantly after Mettaton/Papyrus became big and Napstablook and Mettaton being cousins became a more well-known fact.
    • 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.
    • Chara x Sans is also very popular, coming on the heels of the final Genocide route battle and the preexisting popularity of Self-Insert Fic shipping with Sans, which Chara's metatextual nature plays well into.
  • 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 Jonathan 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. There's a lot of crossovers in their ranks, usually under the name "Mothertale."
    • 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 ZUN's musical style on a few songs (most notably 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.
    • Fans of Spec Ops: The Line have been warm to Undertale, as both games heavily deconstruct the idea of the player killing everything in sight to horrific extents.
    • As result of various shoutouts and similar messages, fans of Yandere Simulator and Undertale get along quite well. It also helps that YandereDev himself seems to be a fan of Undertale.
  • "Funny Aneurysm" Moment:
    • For Black Comedy, Flowey tricking the player into catching all the "friendliness pellets" and making it clear that "In this world it's kill or be killed". In a Genocide Run, his "best friend" the Fallen kills him so that he faces Cessation of Existence.
    • Flowey makes a surprised squeak when Toriel bats him away in the game's beginning. She also calls him a terrible creature for tormenting a child, that is, you. Then you learn that Flowey is the soulless incarnation of her son Asriel, which means Toriel hurt her own child and didn't know.
    • Dr. Alphys as an Establishing Character Moment appears to be an Adorkable scientist who accidentally created a "killer robot" that loves a good show. Then you learn that the "killer robot" was a charade she made up to as to insert herself into your story, and she performs one Epic Fail in putting on a show to "save" you. Furthermore, you may learn that she was suicidal and does kill herself in several endings. In a True Pacifist Run, you learn that part of her baggage comes from creating the Amalgamates while attempting to save monsters.
  • Gateway Series: To Bullet Hell games.
  • Germans Love David Hasselhoff: Undertale is very popular among Korean and Japanese gamers. The Korean translation of the demo, the only current official translation in a language other than English, was made in response to fan request, with a fan translation of the full game in Korean coming out soon after its release. An official Japanese translation is also under way due to fan demand, with some big names among the ranks including Yoko Taro, Swery 65, and Hato Moa.
  • 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.
    • A common "glitch" in games created with Game Maker which is also present in Undertale: walking up against a wall and holding both the up and down arrow keys will cause the Human Child to walk in both directions at once, e.g. walk up for one frame, down for one frame and then repeat, creating a flickering affect as they stand in place against the wall. Similarly, holding down an arrow key and keeping it depressed while switching to another window will trick the game into thinking you're still holding the key down, making you walk in place. Despite looking rather silly, both these techniques are considered beneficial, can be combined together so you can do something else while waiting for monsters to show up, and are commonly used in the No Mercy run due to the encounter rate being determined not by time, but by the amount of steps you've taken.
  • 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.
  • 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.
    • A cross-game moment, but Bravely Second, which came out in Japan just a few months before this one, also uses Breaking the Fourth Wall to call out the player for messing with the game's characters for their own sick amusement, and has a Final Boss that addresses the player, messes with your save files and can attack while you're on the menu screen. However in that game, it undergoes a Decon-Recon Switch almost immidiately.
  • 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 at all 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.
    • Within the fandom itself, the track "Megalovania", used for the No Mercy Final Boss fight. Some fans have formed the opinion that while it's a great track, its sheer popularity dwarfs that of most other tracks in the game that don't get as much as love as they feel they deserve.

    I-Z 
  • 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.
      • 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, being by far the game's most popular and well-known character, has had some people turn on him because of this trope, due to his seeming ubiquity and being a source of Hype Backlash. It doesn't help that you need to see or play the Genocide Run, which the game and a good section of the fanbase dissuades you from doing, to understand his popularity.
  • 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.
    • It's actually incredibly easy to miss the fact that the first scene in which the player character walks through the woods after fighting Toriel is meant to be creepy and evoke a sense of being stalked if you know that Sans is actually a nice guy.
  • Jerkass Woobie: Once one learns exactly why he is such a jerk in the first place, it's easy to feel sympathy for Flowey.
  • Just Here for Godzilla:
    • A prominent subset of the Undertale fandom is centered on Sans and the many AU versions of him, with little interest in the rest of Undertale. It's gotten to the point that many Sans fans view themselves as separate from those of the actual game or its other characters.
    • More specifically, a lot of people play the Genocide Route just to fight Sans, which is notorious for being the hardest fight of the whole game. Whether they reset after the fight is over and go for a Neutral ending or go all-out with the Genocide ending varies from player-to-player.
  • 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 Loser: The protagonist and the Fallen Child, often shown in fan videos getting dunked on by Genocide!Sans. A compilation of these sorts of edited videos can be found here.
  • Memetic Mutation: Lots, with this game's writing. Now has its own page.
  • Memetic Troll: Chara gets this reputation for their influence in the backstory, as well as the permanent effects they have on the game after a Genocide run is complete. While Flowey and Sans are canonical trolls, Chara undergoes Alternate Character Interpretation as a smartass that casually does mean things (usually to the Human Child or the Dreemurs) just because they can.
  • Misaimed Fandom: Overlaps with Do Not Do This Cool Thing. 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.
    • On the other end of the spectrum, there are MANY fans who openly object to people not playing the True Pacifist route for their initial blind playthrough, which defeats the purpose of playing the game blind and making your own choices without knowing of the consequences.
  • 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 either restarting the war between monsters and humans or, in a lighter approach, saving all monsters.
    • 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 after killing every other monster in Hotland 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.
  • Most Annoying Sound:
    • Get used to the sound of your soul shattering into pieces followed by "Determination", especially if you're on a No Mercy run and find yourself against the two bosses that pose an actual threat.
    • The constant notification sound you get from Alphys's social media posts when you're trying to navigate Hotland can get quite grating, but then again, that's the whole point.
  • Narm: 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.
  • 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.
  • 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. On top of that, it looks like a giant sperm, which may have been accidental.
  • 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. His Mettaton NEO form is also fairly popular due to how Badass it looks, yet the sole scene it appears in is a boss battle where he dies in one hit and doesn't attack at all.
    • The only time you ever encounter the Fallen Child is after beating the Genocide Route, but man, are you in for something: pure Nightmare Fuel combined with calling you out for finishing the Genocide Route, topped off with them destroying the game and asking you for your soul to play again. If you thought you were the most dangerous thing in the Genocide route, think again!
  • 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?
    • From Sans's point of view, your ability to reset timelines is a major source of this. He may have gotten the happy ending that you worked so hard for...but that can all be taken away in an instant the moment you hit "True Reset". And a lot of players go on from that point to try out the No Mercy path out of curiosity.
  • 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.
      • And if you at this point decide you can't go through with No Mercy anymore, Sans appears to offer you mercy, and you can use the Spare command on him. He seems to sympathize with your decision to throw away everything you've worked so hard on, and offers to console you...before killing you in an Unblockable Attack and telling you to get dunked on. It's too late for forgiveness.
    • Finished the No Mercy run and decide to go for a Pacifist run afterwards? Nope! Chara shows up at the last second of the ending to remind you that with Frisk's soul, they can possess the poor kid and undo everything Frisk worked hard for. Congrats, your desire for 100% Completion has ruined the game forever.
  • 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.
  • Rescued from the Scrappy Heap: The months since the game's release have seen an increase in the use of the Comic Sans and Papyrus fonts, which before were two of the most hated fonts of the lot.
  • Ron the Death Eater:
    • Asgore is a frequent victim of this. While he did vow to take revenge on humankind for the death of his son and killed six humans to do it, he's clearly shown to regret doing the latter, and is so resistant to the former that he's so reluctant to act on the latter that he's willing to leave the monsters trapped Underground forever to stave off a war. However fanworks frequently portray him as abusive towards Toriel and driving her off with his bloodthirstiness, and AU versions often have him Take A Level In Jerkass. The chances of this happening skyrockets in works shipping Toriel with Sans, due to Die for Our Ship.
    • On the opposite spectrum, those who defend Asgore's actions make Toriel into this by making her looking like a crazy and emotionally abusive woman that can't let go and will give Asgore grief at every chance she can get all because Asgore's actions (or rather inaction) with the humans and keeping his people trapped in the underground. This is also despite the Golden Ending where Asgore wants to make amends and it shows him trimming hedges at Toriel's school, implying that the two of them might be working things out.
    • Alphys is definitely a flawed character, but portions of the fandom frequently exaggerate her negative traits by turning her into a full-blown manipulative Jerk Ass responsible for Gaster's death and abusive towards Mettaton and Sans whom it's implied she collaborates on research with. Even though she's shown to have a very strong conscience and dislike for actually hurting anyone.
    • As mentioned under Draco in Leather Pants, the Fallen Child may or may not be this. Their backstory is vague, and it's unknown whether they really wanted to free all monsters or destroy humanity, and to complicate matters further, there's a widely-growing chunk of the fanbase who believe the Fallen Child is really the narrator, and that they only become corrupt in the Genocide Route. If that really is the case, then all of the Axe Crazy Omnicidal Maniac depictions of the Fallen Child fall under this.
  • 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.
  • 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 is (usually) goofy and harmless, and while his laid-back attitude can be charming, fans sometimes portray him as more suave than he actually is. In addition, although the guy does indeed have a darker side to him, fans who are into that sort of thing often exaggerate it to a more standard case of Troubled, but Cute, sometimes giving him sharp teeth or other similar features. Many of the various AU versions of Sans exist for the sole purpose of this trope, or if they didn't start as Self-Fanservice to begin with, have otherwise been Flanderized from their original concepts in order to make Sans more appealing to fans' given tastes.
    • To work with the fact that Sans and Papyrus are both skeletons, fanartists usually give them the ability to magically conjure colorful appendages and other organs for sexual pleasure, deriving from Sans' 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 tends to attract fanart that downplays 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.
    • Muffet is cute either way, but in-game she has some clearly non-human features: besides the obvious, she has a long thin neck, no nose, and stick limbs. A good deal of fan artists ignore this and draw her as a purple-skinned, but otherwise normal-looking girl with fangs, three extra eyes, and six arms.
    • The protagonist and the Fallen Child, despite explicitly being stated to be children, are often drawn in fanart to look like they're in their teens.
  • 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.
    • Mostly averted with Undyne and Papyrus. Undyne has one line to Papyrus about "extra private one-on-one training" which raised a few eyebrows, but her relationship with Alphys usually takes heavy precedence and her later interactions with Papyrus don't imply anything further.
  • Signature Scene: The fight with Sans in the final corridor at the end of a No Mercy run has become the most iconic moment in the game, for its emotional weight, awesome music, and unrelenting difficulty.
  • Suspiciously Similar Song:
  • That One Attack:
    • For first-time players, the Dogi's axe attack. It's the first attack you see that doesn't have an obvious way to dodge it; you have to move underneath the axes as they lift, but it looks dangerous at a first glance, and the attack comes out quickly enough that they might hit you anyway. Since the axes have a huge hitbox, getting hit once means you'll probably get hit again. Combine this with the Dogi's rather complex Spare method, and it's very likely a player will experience their first death by being hit with an axe. Also, if you decide to kill Dogamy, then Dogaressa will switch to only using this attack, and it lasts much longer and goes faster, so death is quick unless you can kill her fast.
    • 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.
    • The True Final Boss's Shocker Breaker. Even if you can't die during the battle, this attack consists of huge bolts of lighting quickly being struck down upon the player and you are given very little time to dodge it.
  • 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.
    • While most of Mettaton's "fights" are typically fun little minigames that are very hard to actually fail, his colored tile maze puzzle requires you to remember a rather long-winded set of rules which Mettaton himself admits were from "like a hundred rooms ago" and for which the presenter of the puzzle himself gets confused if you ask him to repeat said rules. Even if you do somehow remember all the rules, the puzzle is long enough that you'll only barely have enough time to complete it if you do everything right. Ultimately it's a moot point since just like with the others there's no punishment for failing, though it is noteworthy that this is the only one of Mettaton's puzzles that you're outright expected to fail.
  • 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. Even Toriel and Papyrus got memorable Player Punch death scenes; Mettaton NEO just makes a silly comment about you not wanting to join his "fan club" and then explodes.
    • 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.
    • Chara/Frisk especially in the No Mercy ending. The former can come off as Yandere if you choose to destroy the world:
      Chara: You are a great partner. We'll be together forever, won't we?
      • With the recent rise of the idea that Chara is the narrator, new Chara/Frisk depictions show Chara as Frisk's guide and mentor. It also commonly depicts Chara encouraging Frisk through the scary and sad times, while Frisk helps Chara come to terms with what they've done in the past.
  • True Art Is Angsty: Surprisingly, inverted by many fans! One of the biggest complaints is that you can't permanently restore Asriel for a maximum happy ending. Of course, defenders will argue this as to why Asriel shouldn't be saved.
    • Played straight with Sans. His mysterious Dark and Troubled Past and the ever famous his boss battle at the end of Genocide Route is a huge Fanfic Fuel amongst fans for showing his Stoic Woobie side.
    • Previously played straight with the Fallen Child, but recent shifts in the fandom are beginning to invert this as well. Despite their vague backstory, the first and previously most popular depiction of the Fallen Child was an Axe Crazy Omnicidal Maniac who only wanted to destroy humanity, using Frisk as their puppet, but now an increasing majority of fans depict them as The Atoner and Frisk's Mission Control, who really did want to free all monster, no matter what.
  • 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.
  • Viewer Gender Confusion:
    • Many a player and let's player mistook Alphys for a male at first, not noticing her Tertiary Sexual Characteristics (her big eyelashes). It doesn't help that she's hardly referred to with female pronouns until she actually appears in person.
    • Many people tend to mistake Mettaton for a female due to his EX form having curvy legs, pink stilettos, long hair and being a bishonen.
  • 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".
  • The Woobie: Has its own page.

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