Characters / Undertale - Undyne

Undertale Main Character Index
Major (The Human Child, Flowey, Sans, Papyrus, Undyne, Mettaton, Asgore Dreemurr)
Others | Monsters | Spoilers (The Fallen Human)

Undyne

"Unless you learn to face danger head-on... You won't last a SECOND against ME!"

^{Click here to see her Undying form.}"You're gonna have to try a little harder than THAT."

"SCREW IT! WHY SHOULD I TELL YOU THAT STORY WHEN YOU'RE ABOUT TO DIE!?! NGAAAAAHHHHHHHHHHHHHHHHH!"

The captain of the Royal Guard that protects the underground. Likes spears. Dislikes humans. Likes throwing spears at humans.

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A-F

• Action Girl: She loves fighting and big swords. What else could she be?
• Affectionate Parody: Everything about her seems to be one to common shonen and shojo anime tropes. She appears to be wearing Saiyan armor. She even has a transformation involving the hearts of everyone in the world uniting to defeat you on the No Mercy route.
• Aggressive Categorism: Initally, Undyne has a very skewed view of humans before the player sets her straight — to her, all humans are irredeemable, selfish jailors keeping monsterkind imprisoned. Even if you refuse to fight her and actively save Monster Kid's life right in front of her, she'll insist that you're just putting on an act and that you were using Monster Kid as a pawn to stop her from attacking you.
  Undyne (as an amnesiac Lost Soul): All humans will die!
• Always Gets Her Man: Undyne. She is a rule-abiding Determinator who is both feared and loved among the monsters. And, according to Monster Kid...
  Monster Kid: She beats up bad guys and never loses!
• Always Someone Better: She is one of the toughest monsters of the Underground for sure, and a heroine the Monster Kid admires. Yet even she had two mentors who remember her early steps — Gerson and Asgore himself, both of whom actually lived through the war with the humans.
• Ambidextrous Sprite: Her overworld sprite is mirrored and leaves a glaring mistake. When walking to the left, Undyne doesn't have her eyepatch on.
• And This Is for...: Undyne dedicates her fight to the citizens of the Underground.
• And You Thought It Was Real: Undyne believes that anime is human history. When Alphys confesses to her that it isn't, she initially laughs it off, but then begins to undergo an existential crisis. You have the option to either reassure her that it is real, or confirm that it isn't.
• Anti-Villain: She's willing to kill an innocent human child, but only because she believes it's necessary to free her people from imprisonment. If you run away from her until she tires out, then she gives up on this, presumably because killing someone who just saved her life would be dishonorable.
• The Apocalypse Brings Out the Best in People: During a No Mercy run, she prevents you from murdering the Monster Kid by jumping in front of them and taking the blow you meant for them. Then she proceeds to fight until the bitter end, making her own determination. Furthermore, as opposed to the Neutral or Pacifist routes in which Undyne's actions are done for the benefit of her people, in the No Mercy run, she's fighting you for the benefit of everyone, including humanity, the very people that imprisoned her kind.
• Arc Villainess: Of Waterfall, in the same way that Mettaton is the arc villain of Hotland. Throughout Waterfall, a fairly backstory-heavy level, you're constantly pursued by Undyne, who is intent on killing you, regardless of how you've acted up until this point. Her battle marks the point where you start making great strides towards returning to the surface. Undyne could also be viewed as this to Snowdin, since Papyrus is a total non-threat — it's wanting to please Undyne that drives him to fight you.
• Armor Is Useless: Before a patch, she had negative defense in her boss fight, despite being covered in plate armor. During her un-armored rematch after the "date", she somehow had more defense. A later revision fixed this, though her heavy armor still doesn't help if you opt to kill her.
• Asskicking Leads to Leadership: Until you encounter Undyne, the only members of the Royal Guard you've seen are five silly dogs that can be gotten past by petting. When Undyne shows up, she makes it very clear that she's the leader of the guards for a reason. Depending on your actions in the Underground, she might decide that Toriel (who's hardly a push-over herself) is too soft-hearted to become Queen after Asgore's death, and will successfully usurp the throne from her. If Toriel is dead, then Undyne gets the throne anyway by being third in line.
• Badass Teacher: Some endings have her become a gym teacher at Toriel's school after losing her job as captain of the Royal Guard.
• Bait-and-Switch: According to a monster at Grillby's, she's apparently dating someone else. Given the earlier implications and her relationship with Papyrus, it's implied at first that she's dating him, but it's later revealed that she's actually dating Alphys.
• Batman Gambit: Records a message on an Echo Flower telling the human to look behind them so she can set up a dramatic ambush. This flops when Monster Kid shows up.
• Battle Cry: "NGAHHH!!!"
• Beyond the Impossible:
  • Played for Drama on a Genocide run, in which she appears to die, but, despite being a monster rather than a human, is capable of amassing enough of her own determination to make a Heroic Second Wind and take on a One-Winged Angel form, melting as she's finally killed for real. Sure, the monsters in Alphys' lab melted too from trying to take in determination, but the important part is that Undyne has enough of her own determination, when it's established that monsters normally can't gather much of their own.
  • Played for Laughs during her battle. One of the descriptions before the player's turn can read "Undyne suplexes herself, just to prove nothing is impossible."
• BFS: Though Undyne prefers using spears in battle, she keeps a giant sword in her house. Since she believes anime is true, she thinks humans historically have wielded swords up to ten times their size.
• Black Knight: In her first appearances. Her dark armor covers her entire body, disguising her face along with her helmet having an angry scowl design. She is almost never heard speaking, giving her a very sinister and mysterious vibe. She is accompanied by an incredibly sinister theme throughout her appearances. Compared to the antagonists you've had to face so far — the motherly Toriel who only opposed you "for your own good" and the bumbling Papyrus who couldn't find it in himself to kill a fly, much less a person — she comes off as a dread-inspiring Knight of Cerebus who is deadly serious in her mission to take the protagonist's SOUL even as her reasons for doing so and her very nature remain a mystery. The game is so intent on painting her as an inexorable, unstoppable force that for your first encounters with her, you cannot face her directly at all, only run from her as she hurls waves upon waves of projectiles at you. This continues until she finally unmasks herself. It turns out her true personality is a Hot-Blooded Large Ham and the cold and dispassionate demeanor is an (ultimately unsuccessful) attempt to remain stoic, serious, and professional as befits the leader of the Royal Guard.
• Blatant Lies: "Hurt? It's nothing."
• The Blind Leading the Blind: She's giving Papyrus cooking lessons, and Papyrus is an awful cook. We never actually get to try any food Undyne prepares, but considering your cooking lesson with her ends with everything quite literally going up in flames... yeah.
  Undyne: [as she and the Child stand outside her burning house] Man, no wonder Papyrus sucks at cooking.
• Blood Knight: Loves fighting. In fact, it's impossible to spare her when you engage in combat — you have to run away, as she refuses to accept your mercy.
• Blue Is Heroic: Her skin is blue, and while she's not the world's nicest person, she's a genuine heroine.
• Blunder-Correcting Impulse: Parodied. During Undyne’s cooking lesson, Undyne is underwhelmed by the player’s wimpy stirring and stirs herself... by slamming the pot with her spears several times and denting it.
• Boisterous Bruiser: Undyne is a rare female example. She’s the spear-throwing boulder-suplexing spaghetti-cooking Stock Shōnen Hero who Always Gets Her Man with maximum monologuing!
• Boots of Toughness: She wears boots most of the time and is a huge badass.
• Bootstrapped Leitmotif: She has her own leitmotif, which plays during her introduction, during the chase sequences, and during her boss fight. However, the Ruins leitmotif is more associated with her, playing throughout the Waterfall zone and during her boss fight. The Ruins leitmotif even plays during her death sequence (a clipped version of "An Ending"), her transformation into Undyne the Undying ("But the Earth Refused to Die"), and the battle against Undyne the Undying ("Battle Against a True Hero"), while her leitmotif is absent from all but the last one.
• Boss-Altering Consequence: Your decision in rescuing Monster Kid after they slip off of a bridge in Waterfall will affect Undyne's health; should you rescue them, Undyne will have full HP in the boss battle. Should Monster Kid fall, Undyne will rescue them, but at the cost of some of her HP.
• Break the Haughty: She comes into battle composed and determined, fully expecting to win the battle effortlessly... but as the battle goes on, she becomes more and more frustrated at your refusal to die, eventually going from her standard smiling expression to an upset grimace. Her flavor text at this point states that she's hyperventilating, twitching her eye, and wondering if you surviving this long is a prank. And when you save her life by pouring water on her, it's implied that her pride is severely damaged by the fact she was just about to die of something as mundane as overheating and was saved by someone she was doggedly trying to kill, making her re-examine going after you. However, she'll still end up on bitter terms with you unless you've played as a complete pacifist and hung out with her.
• Butch Lesbian: Hot-Blooded, an Action Girl, refuses to do anything by halves... about the only reason she's not under Testosterone Poisoning is that she's female, and yep, she likes the ladies. Specifically, the True Pacifist ending has her wind up in an Official Couple with Alphys, and Undyne gives a sympathetic "That's rough, buddy," to Asgore upon learning that Toriel is his ex, possibly implying that she finds her attractive, too.
• Casual Danger Dialogue: Admits their feelings to the human and her new plan to free monsterkind^note Which involves a different human, while her house is still on fire and both of them haven't left yet.
• Challenge Seeker: Tell her she can't do something. Anything at all. She will do exactly what you say she can't do, or die trying! For example: when she resists befriending you, Papyrus uses Reverse Psychology on her, saying she's "not up to the challenge". After that, she insists on becoming "besties" with you.
• Climax Boss: She's the boss of a backstory-heavy level, and marks the point at which the player stops moving forward and begins advancing toward the surface. This is also the point at which the ramifications of the player's actions are made explicit: on the Neutral route, she'll call the player out for killing Papyrus or any other enemy; on the Pacifist route, she'll mock the player for being a goody two-shoes; and on the No Mercy route, she'll realize the player's out to kill the entire world, human and monster alike, transform into Undyne the Undying, and claim that the player will succumb to everyone's hopes and dreams.
• Combat Pragmatist: Undyne is largely a good-hearted and chivalrous person, but the player's various battles with her demonstrate that she isn't at all afraid to employ dirty tricks if she wants somebody dead. When she initially chases the player, she'll fire projectiles from beyond where they can retaliate, and during the boss fight proper, she'll fling a singular, fast-moving spear whenever she changes your SOUL back to white, which is liable to catch a first-time player used to her slower-moving barrage attacks off-guard. And then there's the battle in No Mercy mode, where she abandons any sense of fairness altogether and just goes full Attack! Attack! Attack!.
• Commander Contrarian: When you finally fight Undyne, she'll be angry regardless of how you played: if you killed Toriel and/or some minor monsters, Undyne will say she thought humans were compassionate, but you're just a monster. If you killed Papyrus, she'll specifically call you out on it. And if you played as a pacifist, Undyne will say she thought humans were cool, but you're just a wimp and also assume it's just an act. In truth, Undyne doesn't want to face the implications of murdering a child to save the Underground, so she just comes up with a reason for them to deserve to die anyway.
• Contralto of Strength: As the captain of the Royal Guard, she is one of the strongest of all the monsters in the underground. Fittingly, she has the lowest Voice Grunting of all the female characters.
• Cool Big Sis: According to their Undernet handles, Undyne is around 3-4 years older than Papyrus, and he seems to view her as something like a sister. She in turn is very protective of him, and it's obvious that despite their bickering, she cares about him a lot.
• Cowardly Boss: Inverted. To defeat her peacefully, you have to flee battle and then run away as far as you can before she catches up and continues attacking until she can't go any further.
• Cruel Mercy: You can do this to her by befriending her and then killing Mettaton. Alphys consequently commits suicide, and Asgore dies in the aftermath of his fight regardless of your actions. Undyne falls into the Despair Event Horizon as a result of losing two of the most important people in her life, and doesn't even have enough fire left in her to want revenge against you.
• Cultured Badass: Plays a snippet of Alphys' theme on the piano fairly well, from what little you can hear of it. Can make good tea, though the rest of her culinary skills are questionable. She also thinks she's well-versed in human history through no fault of her own, making her an incidental Pop-Cultured Badass.
• Death by Irony: Undyne refuses to let Papyrus into the Royal Guard because she believes that his general innocence and unwillingness to kill would put him in danger at worst. But Papyrus' total lack of malicious intent can make him so endearing that even capable players might refuse to harm him purely on principle. By contrast, her unrelenting effort towards killing the human can prompt less forgiving players not to even bother.
• Defiant to the End:
  • If she's slain on a No Mercy run, she'll just smile proudly (while in anguish) and proclaim that she allowed for others to escape and that King Asgore will avenge her.
  • If she's slain on a Neutral run, she'll continue fighting even when her HP bar is depleted. But as you keep beating her down, her body breaks down and her attacks slow to a crawl, all of her bullets will only cause one HP of damage, and the music changes to a very somber piano piece. She keeps refusing to die, but her body eventually can't take it anymore, and melts before turning to dust.
  • Just before the final battle of a True Pacifist run, after everyone's captured by Flowey's vines, she's visibly struggling against hers.
• Despair Event Horizon: In the neutral ending where only Mettaton is killed, Alphys is implied to have commited suicide. This causes Undyne to fall into a deep depression, blaming herself for her love's disappearance.
• Determinator: Undyne is extremely tenacious, regardless of how you approach her. Particularly if you choose to Fight her or take the No Mercy route. In both cases, upon taking fatal damage, she maintains her form through sheer willpower and continues to fight. In the former case, becoming invincible to damage but slowly weakening until she disappears. In the latter case, she reforms herself into Undyne the Undying, and becomes one of the most difficult combat encounters in the game. It's because she's so Hot-Blooded that she's making her own determination, which is supposed to be impossible for monsters.
• Does Not Like Spam: She doesn't like cold food, so Alphys fixed up her fridge so that it heats up food instead.
  Undyne: Hot Fridge...the world's greatest invention!
• Do Not Go Gentle: Just before the start of the fight in the Genocide route, she literally manages to stay alive with the power of Determination to become Undyne the Undying. And her Final Speech just before she dies has her declare that 'this world will live on' because her death spurred Alphys to evacuate everyone and get Asgore to absorb the SOULs.
• Dramatic Irony: Her Final Speech during the Genocide Route allows her to Go Out with a Smile because she believes that Alphys has sealed off Hotland, Asgore has absorbed the six human souls, and that the player character is going to fail. Turns out, Muffet allows the player to continue forward, Asgore hasn't absorbed the souls yet (and he's a Cutscene Boss who goes down in literally one hit), and the player succeeds in their goal.
• Ear Fins: Being a fish monster, she has a fin on either side of her head that can be seen when she removes her helmet.
• Elemental Weapon: The spears that she summons appear to be made of water.
• Establishing Character Moment: The cutscene right before her boss fight in a Neutral/Pacifist run. Up until now, her actions and dialogue had made her seem like the standard Black Knight archetype: serious, cold, and direct in her efforts to stop the player. Then she suddenly tears off her helmet and goes on a really hammy rant about why the player's existence is a detriment to all monsterkind and how she'll do anything to protect them, all while her real Leitmotif — a heroic and somewhat goofy-sounding theme — plays in the background.
• Establishing Character Music: Twice.
  • Her first appearance in non-Genocide runs has no music at first, but then the foreboding "Undyne" plays after she notices the player's movement. Both the initial lack of music and tone of the song establish her as a much more legitimate threat than any previous enemy.
  • Her battle music in non-Genocide runs is bombastic and loud, fitting her real personality as a ridiculous Large Ham.
• Everyone Has Standards:
  • Apparently, she won't fight someone who's on the phone. During the middle of the fight in the Neutral/Pacifist run when Papyrus calls you, she won't attack while you're talking to him. This isn't a matter of Talking Is a Free Action — if you speak with Undyne in the same room after befriending her, she'll say she had to wait because you had a phone call.
  • She also insists on making sure the fight is a fair one — if you fail to block her first attack, she'll remind you how the mechanic works, and if you keep failing, she'll complain that she gave you a spear to block with just to make things fair and become quite frustrated.
  • She also believes in Sacred Hospitality, so as much as she loathes you at the time, she won't attack you while you're at her house (with the exception of the "rematch" that gets triggered if you are on good terms with her).
  • As shown in the Genocide Route, while she hates humans with a passion, she knows better than to think their extinction is anywhere near okay, and fights the protagonist on their behalf as well as that of the Monsters' in said route.
• Eye Beams: Upon transforming into Undyne the Undying, she gains the ability to shoot energy out of the eye that had been covered by her eye patch. However, it's just for show, and is never actually used to attack you.
• Eye Colour Change: When she taps into her Determination, her sclera turn black and her pupils white. It only happens after she's mortally wounded in a Neutral or No Mercy run.
• Eyepatch of Power: Most people will barely notice that she wears one. And it's not for nothing either, as she is missing that eye. Subverted in a No Mercy run, where she loses it as Undyne the Undying.
• Face Death with Dignity: When Undyne dies in the Genocide route, she gives a Final Speech about how 'this world will live on' because her Heroic Sacrifice brought Alphys time to prepare for an evacuation and tell Asgore to absorb the six human SOULs, which, as far as they know, would make him the most powerful monster in the underground.
• Fantastic Racism: She has a rather odd love-hate relationship with humans. While it's clear she thinks very little of them, she also watches a lot of anime, which she believes is "human history", with Alphys, and it's clear that it's an influence in her demeanor. She also mellows her demeanor toward humans on a Pacifist run, and on No Mercy, she realizes that you're just as much a threat to humans as you are to monsters and fights for them as well.
• Fiery Redhead: She's a Hot-Blooded Large Ham, and her overworld sprite reveals that she's got a big red ponytail under that helmet.
• Final Speech: Makes one if she dies. They're different depending on whether you kill her in a Neutral Run or a Genocide Run.
  Undyne (Neutral Run): Ngahhh... You were stronger... than I thought... So then... this is where... it ends... No... NO! I won't die! Alphys... Asgore... Papyrus... Everyone is counting on me to protect them! NNNNGAH! Human! In the name of everyone's hopes and dreams... I WILL DEFEAT YOU! Come on, is that all you've got!? ...pathetic. You're going to have to try harder than that! S-see how strong we are when we believe in ourselves? H...heh... Had enough yet? ...I won't... Give up... Ha... ha... Alphys... This is what I was afraid of... This is why I never told you... No... No! Not yet! I won't die! NGAHHHHHHHH!!! I WON'T DIE! I WON'T DIE! I WON'T DIE! I WON'T-

  Undyne (Genocide Run): Damn it... so even THAT power... it wasn't enough? ...Heh... heheheh... If you think I'm gonna give up hope, you're wrong. 'Cause I've ... Got my friends behind me. Alphys told me that she would watch me fight you... And if anything went wrong, she would... evacuate everyone. By now she's called ASGORE and told him to absorb the 6 human SOULs. And with that power... this world will live on...!
• Fish People: Looks like a fish when she takes her armor off. Her online handle is "StrongFish91".
• Flat "What": Undyne gives one during her rematch against you when you pretend to attack her. Said attack leads to the screen flashing white... before revealing that you only dealt 1 damage.
• Friend to All Children: She apparently knows the children that live in Snowdin Woods on a personal basis. In the Genocide Run, she even saves one of them from you by Taking the Bullet.

G-P

• Gaydar: Amusingly enough, subverted. Despite being a lesbian herself, she is completely clueless to the obvious UST between two of her male subordinates. Even if they hook up (with the player's help), she still seems to think they have a close, platonic friendship.
• "Get Back Here!" Boss: Inverted by Undyne — if you want to overcome her non-lethally, you'll be the one doing the fleeing, while Undyne shouts at you to hold still so she can kill you.
• Getting Hot in Here: Something similar happens to Undyne in the Pacifist or Neutral Route. You run away from her and lead her to Hotland, which has the same effect, but instead of taking her armor off, she collapses from dehydration. Giving her a glass of water revives her and gets her to leave you alone.
• Glass Cannon: Statistically, anyways. She has a very powerful attack score of seven, but her defense score is zero, despite all of that heavy armor she's wearing. She has enough health to make up for it, though. Averted during a No Mercy route, as she's one of the few things that can take more than one hit from you.
• Go Out with a Smile: On the No Mercy route, she dies happy believing that her friends will stop you from killing everyone after her death.
• Half the Woman She Used to Be: When she takes the hit for Monster Kid in the Genocide route, she's sliced in half diagonally, and the top half starts to slide down after Monster Kid leaves.
• Handicapped Badass: She's missing her left eye, wearing an eye patch over it. Doesn't seem to slow her down much.
• Helmets Are Hardly Heroic: She's wearing a full helm in her first appearance, but ditches it right before she is fought and never wears it again.
• Hero Antagonist: She's the strongest monster in the Underground, according to them. As Captain of the Royal Guard, she's something akin to a superhero in the eyes of monsterkind, and fights to protect "everyone's hopes and dreams." She also has no qualms about killing you, so you still have to find a way past her.
• Heroic BSoD: In most Neutral endings where she's alive, Undyne either swears vengeance on you or forgives you, depending on your friendship with her. However, if your Neutral run ends with Asgore and Mettaton dead, Papyrus explains that Alphys vanished. As a result of losing both Asgore and Alphys, Undyne completely gives up at life, spending most of her time either working at Sans' hotdog stand or laying on his couch.
• Heroic RRoD:
  • In a Neutral run, she eventually faints after reaching Hotland. The player can choose to dump water over her.
  • In the No Mercy run, she has so much determination and willpower to strike you down in order to save her homeland that energy radiates out of her eye socket where her eyepatch was, and then her body begins to melt before disintegrating (a side-effect of monsters with determination, as Alphys is all too aware of).
• Heroic Sacrifice: During the No Mercy Run, she sees you about to kill the Monster Kid and unhesitantly jumps in front of them to shield them from the fatal blow. She gets better, though. If you kill her when she's Undyne the Undying but don't complete your genocide, then her battle with you saved monsterkind from extinction.
• Heroic Second Wind: If you kill her during the Neutral route, she shows signs of pulling this, but it turns out to be a mere Hope Spot. During the No Mercy route, however, after taking a killing blow directed at Monster Kid, she manages to come back even stronger than before. In both situations, she finally dies when she begins to melt, meaning she had SO MUCH willpower, she was creating her own Determination.
• Hero-Worshipper: She seemed to be this in her youth for Gerson, the old turtle archeologist (who was once a hero himself). That relationship wasn't much unlike the current relationship between her and the Monster Kid, as she was also described as being clumsy back then.
• Hidden Depths:
  • The version of Undyne most people see is a Hot-Blooded anime-loving Blood Knight, but she has an insightful, gentle, and caring side. She admires Asgore for his pacifism, doesn't want to let Papyrus in the Royal Guard despite his fighting skills because of his naiveté and kindness, and is in love with Alphys for her passion. She won't hurt an innocent... except the protagonist on a Pacifist run, which is a source of great conflict to her. The strange behavior on the date with her just highlights how much of a dilemma she's been facing with the apparent salvation of monsterkind involving killing a kid.
  • She has a piano at her house and can be heard playing it quite well when you meet Papyrus outside. She also created the piano puzzle in Waterfall that leads to the legendary artifact because she wanted an excuse to put a piano there.
• Honor Before Reason: Despite wanting your SOUL, Undyne, during the fight with you, gives you a spear and allows you to defend yourself, rather than simply capturing you and bringing you to the King. She's emulating all the anime she's watched. In the No Mercy run, you can still defend yourself, but she's the one at a disadvantage this time.
• Hot-Blooded: Does absurd things like suplex boulders in the middle of a fight, just because she can. Even her cooking is hot-blooded, making spaghetti sauce by punching the vegetables, and stirring the pot by stabbing it with her spear.
• I'm Melting!: Undyne's body may be full of DETERMINATION, but it can only carry her on for so long after she's supposed to die, resulting in her melting in both routes she dies in. In the Neutral route, she keeps fighting until she literally can't do so no more, while in Genocide, she manages to bring out one Final Speech about how her sacrifice wasn't in vain because Alphys watched the fight and has now started evacuating the monsters and calling Asgore to absorb the six human SOULs.
• Inspirational Martyr:
  • If Toriel is alive at the end of a Neutral run, she succeeds Asgore, becomes Queen of the Monsters, and ends the war against humanity. However, killing Undyne (even if you didn't kill anybody else) will outrage the other monsters enough to overthrow Toriel and plunge the underground into anarchy.
  • If you kill Undyne during a No Mercy Run, it inspires Alphys to evacuate as many monsters as possible to the True Lab in an attempt to save them.
• Insult to Rocks: Even with her disdain for humans, when she fights you in No Mercy, one of the lines she uses is "Human... no. Whatever you are...", which seems to have a taste of this to it; she regards you as being even more contemptuous than humans in general, and that's saying a lot. Justified, as she realizes that you're not just a danger to monsterkind but also to humanity as a whole and thus you need to be put down for good.
• Interface Screw: She turns your SOUL green, preventing it from moving; you have to use a spear to block her incoming attacks. You also won't be able to run away from her (which is required to defeat her non-lethally) until your SOUL turns red again.
• Irony: She's a Hot-Blooded Screaming Warrior who also plays the piano… an instrument whose name means "soft" or "quiet" in musical notation.
• Irrevocable Order: If you've befriended Undyne before venturing into Hotland and phone her in a specific spot, she tells you that she would've gladly phoned RG 01 and 02 to call off the kill-on-sight order she set on you before… If Alphys hadn't shown her an episode of Mew Mew Kissy Cutie, which convinced her that humans have mind-control abilities and set up a clause with the two guards to disregard any orders to not kill any humans because that would've been proof that they've already controlled her mind.
• Jerk with a Heart of Gold:
  • Very much so. She's extremely aggressive, but also completely dedicated to justice for all of monster-kind. This trope can be seen in full effect whenever she shows "affection" toward the people she likes.
  • She will lambast a Pacifist player for being a wimp and a coward. However, if you spared her during the fight and visit her house later, she quickly breaks out of that standoffish facade.
• Knight of Cerebus: Subverted. She's the first boss character in the game who wishes to kill you, marking a turning point for the tone of the game, and gets quite a lot of build-up as someone the human child shouldn't mess with. After you meet her, however, she drops the silent, stoic facade, revealing she's just as prone to the bizarre antics as a lot of the rest of the cast.
• Kill All Humans: Should she ever become leader of the monsters, she perseveres in Asgore's goal to wage war on humanity.
• Knight in Shining Armor: Ironically, despite fitting all the criteria for a Black Knight, she has the personalty and moral code of this trope. While she is out to kill the Player Character, it is purely to free her people, and even then she's not exactly happy about it. Furthermore, in the Genocide Route, she'll be the Heroine that stands in your way. She's even acknowledged as such in-game.
• The Lad-ette: She shows traits of being one, anyway. Loud, rude, loves fighting with a passion… And then there's her tendency to involve projectile vomiting in her puzzle ideas.
• Large Ham: Does everything (aside from crafting puzzles), from making friends to cooking, with an extreme degree of ardor that would make Ira Gamagoori proud.
  Undyne: CHERISH HARDER!
• Leitmotif: At first, "Undyne." After she unmasks herself, it's replaced by "NGAHHH!!"/"Spear of Justice." As the Undying, she gets "Battle Against a True Hero" as her battle theme.
• Lethal Chef: Suffice to say, one cooking lesson from her will highlight where Papyrus gets his "skills" from.
  Undyne: (After setting fire to her kitchen) Ah. Man, no wonder Papyrus sucks at cooking.
• Let's Fight Like Gentlemen: She wants to defeat you fairly, so she gives you a spear at the beginning of the battle to block her attacks. If you repeatedly fail to block her first attack, she gets extremely pissed off over you being such a worthless opponent, and unleashes an extremely brutal flurry of attacks.
• Like Brother and Sister: Her relationship with Papyrus falls squarely into this.
• Living on Borrowed Time: In the No Mercy run, she's able to gain a Heroic Second Wind because she's producing her own Determination. Even if she wins the fight (i.e. she kills you and you decide not to try again), that Determination will still kill her.
• Logical Weakness: A water-dweller wearing full plate armor that heats up easily? Of course she doesn't do too hot in a volcano area…
• Lonely Piano Piece:
  • In a Neutral run, a clipped version of "An Ending" plays once you have depleted Undyne's health, but before she dies. This hammers in the fact that she is going to die, no matter how hard she tries to pull herself back together with Determination.
  • On a No Mercy run, the triumphant "Battle Against a True Hero" plays. It has a piano segment that sounds like a feeling of false hope.
• Mama Bear: If you have killed Papyrus on a Neutral run, Undyne becomes incredibly cold and straightforward. No heroic declarations, not even the hamminess she has on No Mercy, nothing but quiet rage. And, with no shouting or theatrics whatsoever, she swears that she will kill you.
• Masculine Girl, Feminine Boy: The Masculine Girl to Papyrus' Feminine Boy.
• Meaningful Name: An undine is a female water spirit. Undyne is a Fish Person that can conjure spears made from water. Also comes up in the Genocide Route — Undyne the Undying. An undine's goal is to obtain an immortal soul, and Undyne's goal is to obtain the player's SOUL.
• Monster Knight: Quite literally. She is a monster, and a knight, and she has the sense of honor and justice that comes with the description.
• Motivational Lie: In a Genocide Run, after Taking the Bullet for the Monster Kid, she tells said child that she's unharmed. In reality, the wound is fatal, Undyne knows it's fatal, and she's only saying otherwise so the Monster Kid will run away to safety instead of worrying about her. This might also be so the Monster Kid doesn't blame themself for her death.
• Muscles Are Meaningless: She's surprisingly skinny underneath all that armor, and she suplexes boulders!^note Because she can!
• Non-Mammalian Hair: She has red hair, despite being a Fish Person.
• Non-Mammal Mammaries: Possibly. When she is out of her armor, she has what might be a visible, albeit small, bust. Granted, it could also be muscle definition^note  Did you know I can bench-press seven children?! ;the game's simple pixel art style makes it a little hard to tell. In the epilogue, she is seen wearing a two-piece swimsuit, adding to the possibility.
• No-Sell: She's so determined to capture you that even the debug command that makes monsters instantly spareable doesn't work on her.
• No Social Skills: The two of you hanging out starts with her breaking her table in half with her spear when you try to select a beverage, and ends with the two of you accidentally burning her house down while trying to make spaghetti.
• Not in Front of the Kid: One phone call with her has her cutting herself off before she says the word "ass" to you and substituting the word "booty" instead.
• Not So Above It All:
  • Although she seems like a very straight-laced, dutiful knight fitting her position as head of the Royal Guard, once you befriend her, it turns out she's just as big of an enthusiastic goofball as Papyrus himself. The monologue she subjected you to just before her fight was something she'd been practicing for just the occasion, and the reason she cut it off partway through was because she forgot the rest of it.
  • Possibly shown later when you question her about the soda in her house. She claims to not like it because it's "gross" and "it rots your fighting spirit", but she responds with an embarrassed silence when you ask her why she even has it if she doesn't like it. It's either an example of this or, considering the look on her face, it's also possible that she keeps it for Alphys.
• Official Couple: Become a couple with Alphys in the true ending route.
• One Free Hit: On a pacifist route, as the dinner at her house falls apart, she decides to opt for a rematch with Frisk and allows Frisk to get the first shot. However, Frisk's blow is so weak and does such minimal damage that Undyne decides she can't go through with fighting.
• One-Note Cook: Her overexuberance leads to disaster when she cooks, but her tea is specifically noted to be good, though too hot to drink properly when you first get it.
• One-Winged Angel: A heroic (but still antagonistic) example. As Undyne the Undying. Her attacks get more complex, deal far more damage, and move much faster. She also switches the player's SOUL color between red and green more often, and the player is prevented from escaping the fight.
• O.O.C. Is Serious Business:
  • Undyne becomes uncharacteristically stoic if you kill Papyrus but aren't on a No Mercy route. Once you reach the area where you must fight her, there are no cheesy speeches, no flashy intros, nothing but sheer, silent, hatred towards you.
    Undyne: What did you do to him? What did you DO TO HIM?
  • Does this towards Alphys in a near Genocide route^note which involves aborting a Genocide route in Hotland where she called Alphys in crazed desperation, which spurred her into action, upon which Alphys evacuated the rest of the Underground to safety.
  • If you only kill Mettaton, Undyne lose all motivation to do anything due to Alphys's disappearance. Papyrus suggests that she try to get revenge on you, only for her to turn it down, saying that it "won't bring anybody back".
• Painfully Slow Projectile: Zig-Zagged.
  • Some of her attacks involve leading with very slow, easily-blocked spears before she throws a barrage of faster ones. Her opening attack in the Genocide route behaves this way to show that she means business.
  • One of her attacks involves not just a few, but an entire barrage of very slow spears converging on the player's SOUL, which makes it difficult to decide which direction to block next.
• Perpetual Smiler: Once the helmet's off, she almost never frowns. Due to her large teeth, this can border on a Slasher Smile at times, but it is also strangely endearing. Reminding her of how much she loves to smile is one of the ways you can Act when facing her as a Lost Soul.
• Platonic Life-Partners: She's huge friends with Papyrus, but they're clearly only friends, and it's enforced by the fact that Undyne's a lesbian.
• Playful Cat Smile: After befriending her on a Pacifist run, she usually wears a mischievous cat smile whenever Alphys comes up in a conversation.
• Playing with Fire: Examining her stovetop during the Child's visit to her house will prompt Undyne to remark that despite the wonders of technology, flame magic-cooked food beats the stove when it comes down to it, so she might have access to such magic herself, or possibly when Asgore makes a visit. Regardless, she pulls this trope literally, cranking the heat up on the stove to needless levels and promptly burns her house down when it explodes in her face.
• Power Incontinence: In her normal boss battle, once you deplete her HP, she lingers around, requiring a few more attacks to fully defeat her. While this happens, her body begins wavering in and out of focus. Her will to stay alive is strong enough to generate Determination, but without the realization that you are a threat to the entire world giving her the final push, she simply can't muster enough of it to stave off death. The result is only a partial transformation (her visible eye's colors invert) and she begins to melt before disintegrating into dust.
• The Power of Friendship: Professes this to cheesy anime levels.
• Punny Name: Undines are mythological elemental creatures of water. Undyne is a fish looking monster who lives in a house that also looks like a fish. It's also a play on "Undying", which comes into play when you fight her on a Genocide route.

R-Y

• Rebel Leader: If Toriel is alive at the end of a Neutral run, she becomes Queen and ends the war against humanity. However, if you killed Papyrus and/or ten monsters, Undyne spearheads a rebellion to overthrow Toriel and reverse her reforms. Downplayed in that that these conditions being met will spark a rebellion against Toriel even if Undyne is dead ^note Undyne's death on its own is also a condition for rebellion.
• Red Oni, Blue Oni:
  • Undyne initially seems to be a cold-blooded mercenary, making her the Blue oni to Papyrus'^noteWhen Undyne and Papyrus appear together after you befriend the former, this dynamic is more ambiguous. and Monster Kid's friendly Red onis. However, after her loud outburst and after she discards her helmet, Undyne easily is one of the most Red onis in the game!
  • When you date her in a Pacifist run and she tells you her story with Asgore, Undyne reveals to be the Red oni while the King is the Blue: he trained her personally after she attacked him with all her strength as a child while he dodged her attacks, then apologized. This contrast can also be seen if you kill them on a Neutral run: as a Red oni, Undyne has a brief Heroic Second Wind and gives everything she has before melting while Asgore, as a Blue, who already restrains himself during the battle, begs you to kill him and to take his soul to leave the Underground.
  • Undyne's also the Red oni to Alphys' Blue oni: she is a Hot-Blooded and hammy Action Girl while Alphys is a shy and Endearingly Dorky scientist. They are a couple.
• Right for the Wrong Reasons: When you fight her after having killed people, she'll call you a cold-blooded murderer and immediately reject your protests of self-defense by saying "You didn't kill them because you had to. You did it because it was easy". She is somewhat correct, as the player can reload saves as much as they want and thus arguably has no excuse but to strive for the perfect nonviolent outcome. However, how exactly she came to this conclusion without knowing about the fourth wall is anyone's guess.
• Retired Badass: Barring the two endings where she becomes ruler by succeeding Asgore or overthrowing Toriel, the others have her losing her job as Captain of the Royal Guard. After this, she becomes either a Gym Teacher at Toriel's School, Alphys' Lab Assistant, or Sans gives her a job at his hotdog stand after losing Alphys kills her motivation. Depending on player choices.
• Sacred Hospitality: Sure, she's antagonistic with you until her house gets burned down, but you're not going to get up for anything as long as you're a guest in HER house… even if it means throwing a spear at the table you're sitting on to prove her point.
• Serious Business:
  • Exploited when befriended. When Papyrus tries to set up the player and her as friends, she refuses. Papyrus plays it up as a challenge, she does it for the need to win.
  • Anime, which she mistakes for examples of actual human history. When you finish Alphys' friendship-making sequence, she's conflicted over whether anime is real or not. You can either tell her that "anime is real" and she gets pumped over the idea of becoming a Hot-Blooded shounen anime protagonist, or you can tell her "anime is NOT real" and crush her dreams.
• Senseless Sacrifice: If you kill her during a No Mercy Run, her dying words are that she bought enough time for the rest of the Monsters to evacuate to safety. If you continue down this path, then she prevented nothing and you ultimately kill all the monsters anyway.
• She Who Fights Monsters: She's so determined to get another soul for Asgore that she's willing to kill a child to get one, making her just as bad — if not worse (No one can say for certain if during the war, humans killed monster children) — than the people who locked her people underground. She is only better than the player in the Genocide Route.
• Shoot the Shaggy Dog:
  • She was only trying to keep the world safe… from YOU.
  • Furthermore, on a No Mercy run, she's confident that Asgore will avenge her. He puts up even less of a fight than Undyne Mettaton NEO when you face him later on.
• Screaming Warrior: "NGAHHH!!!"
• Signature Scent: "Smells like sushi" and "smells like angry fish" will occasionally pop up in her battle.
• Sir Swears-a-Lot: The level of language in Undertale is pretty conservative to begin with, but Undyne is the most crassly-spoken among the cast.
• Statuesque Stunner: She's among the tallest of the characters, only after Asgore, Mettaton, and Toriel.
• Stealth Pun: She's a kind of fish-monster who fights using spears. A spearfish.
• The Stoic: When she stalks you she is completely silent, and never says a word. Eventually, she gets sick of your shit and starts behaving like an overly hammy anime heroine. The latter is what sticks for the rest of the game.
• Straight Gay: Aside from the occasional Ship Tease, the only way to get a clear hint of her lesbianism is if you try calling her from the garbage dump, where she talks about how it's "a good place to pick up chicks", with a clear air of personal experience. And the only way to confirm it is by playing the Pacifist route, where she eventually recruits you to help win Alphys' heart.
• Tactical Suicide Boss: Well, not exactly suicide. During the battle, she can smack your SOUL with her spear to switch it from green to red and vice versa. She intends for this to allow her to use a second attack pattern, but she forgets something; the red SOUL has the ability to run from battle.
• Take a Third Option: If you successfully befriend her, she finds a loophole in her Moral Dilemma; spare you and wait for a genuinely awful human being that actually deserves it to fall into the Underground and kill them for the final SOUL instead.
• Taking the Bullet: On a No Mercy run, Undyne takes the bullet for the Monster Kid when the player attempts to kill the Kid. This allows her to turn into Undyne the Undying moments later.
• Taking Up the Mantle: Some of the endings of the Neutral Route have her succeeding King Asgore as ruler of the Underground and continuing his war to exterminate humanity. If you kill Papyrus or ten monsters, then she will usurp the throne from Toriel and banish her back to the Ruins to do this. If you kill Toriel, then Undyne directly succeeds Asgore.
• Talking Down the Suicidal: Her first meeting with Dr. Alphys was at the bottomless abyss in Waterfall's garbage dump, where Alphys was staring into the depths, looking "contemplative." She asked Alphys where she thought the abyss led to. Cue several hours of Alphys running through every theory she had almost without stopping, leaving Undyne captivated. In the end, Undyne managed to talk Alphys out of committing suicide without having to say much at all.
• Tank-Top Tomboy: When she's out of her armor, she's usually in a black tank top.
• A Taste of Defeat: If Monster Kid is to be believed, Undyne has never lost a fight before and the Player Character is the first to have defeated her. She doesn’t take this well.
• Technically a Smile: She tends to show these when her plans derail or something unexpected happens. During the first third of your time hanging out with her, she shows one constantly which makes her, as you know what she is capable of, extremely scary as you are constantly afraid that she will impale you with a spear.
• Teleportation: She seems to have some sort of teleportation ability — notice how she just vanishes into the darkness with a "magical" sound effect after you first see her. She doesn't use it nearly as liberally as Sans, though.
• Terminal Transformation: In her no-mercy fight, her Heroic Willpower becomes so great that she starts to generate Determination and transforms into Undyne the Undying. The feat is Beyond the Impossible for a monster... and monster bodies can't contain Determination, so you need only defend yourself as her new body melts away.
• This Is Unforgivable!: Won't befriend you if you've killed any monsters beforehand (and explicitly mentions if Papyrus doesn't show up to his meeting with her…), and killing anyone after befriending her will cause her to hate you far more in the ending phone call than she would if you had just killed people.
• Throwing Out the Script: Just before her boss fight, Undyne prepares a dramatic speech about monsterkind's history. She stops before going into detail because she's going to kill the human anyway, so why bother? She replaces it with a "Reason You Suck" Speech condemning you for every monster you've killed, if you have. Even on a Pacifist Run, she blames you for not giving yourself up so Asgore can use your SOUL to break the barrier. If you befriend her and call her from the area the fight with her began, she'll eventually reveal that this is because she forgot the speech she'd memorized.
• Tin Tyrant: In her first appearances, her body is fully covered in platemail, so her face doesn't show. She even makes a heavy clanging sound when she walks. She discards the helmet when she battles you.
• To Be Lawful or Good:
  • Being a chivalrous fighter, Undyne has some doubt about the righteousness of killing a child to save monsterkind. She doesn't own up to this unless she's befriended, however, and will come up with varying "reasons" that necessitate your death. Depending on your behavior up to this point, her reasons for attacking you range from understandable (depending on how you've been violent up to that point, with her being the outright heroine in a No Mercy run), to a note of It's Personal (if you've killed Papyrus on a Neutral run), to grasping at straws (if you've been entirely pacifistic).
  • She comes up with one while befriending her, as despite how much she wants to free monsterkind, she can't bear to kill her friend to do this. She then figures out she'll just wait for a human who deserves to die and use their SOUL to break the barrier.
• Toilet Humor: Loves this. Particularly notable is her puzzle idea of "timing pukes."
• Tomboy: She partakes in male-oriented activities and has all the masculine gender roles you can think of.
• Tomboy and Girly Girl: Undyne is a tough warrior who loves throwing spears at humans. Her crush Alphys is a reclusive nerd who likes the girly dating sim Mew Mew Kissy Cutie.
• Tomboyish Ponytail: Sports a long red ponytail underneath that heavy helmet of hers.
• Tomboyish Voice: Her deep Voice Grunting implies that she has this.
• Took a Level in Badass: Not that she wasn't badass before, mind you, but in a No Mercy run, she takes several levels when she becomes Undyne the Undying. And we do mean "several".
• Tranquil Fury:
  • If Papyrus is killed on a Neutral run, she slips into this.
    Undyne: ...Forget it. Look. Papyrus didn't come to his meeting today. Say what you want about him. He's weird, he's naive, he's self-absorbed... But Papyrus has NEVER missed a meeting. And no matter what time you call him on the phone... Night, day, afternoon, morning... He ALWAYS answers within the first two rings. ...But now he's gone. And his brother isn't around, either. ...What did you do to him? What did you DO TO HIM? Papyrus, who I have trained every day... Even though I KNOW he's too goofy to ever hurt anyone... ...Go ahead. Prepare however you want. But when you step forward... I will KILL you.
  • She falls into this during the Genocide version of her fight. After her Hot-Blooded speech about how she can feel everyone's hearts beating as one, she becomes completely quiet for as long as the battle goes on, focusing entirely on killing the Player Character.
• Troll: Has a tendency to do this, especially on Papyrus.
  Papyrus: OH MY GOD??? NO? WHY??? I THOUGHT YOU HATE MAKING PUZZLES!!
  Undyne: Yeah, but I love driving you crazy!
• A True Hero: On the Genocide Route, the game plays up Undyne’s heroism and shows that she is a better hero than you ever were, through Undyne taking a blow to save Monster Kid and her monologue while transforming into her Undying form. The songs that play with her monologue and fight are even called “But the Earth Refused to Die” and “Battle Against a True Hero,” respectively. Hint: you’re not the True Hero. Also, if Undyne’s The Hero, what does that make you?
• Tsundere: She acts extremely aggressive in order to cover up her softer side. The "dere" side is mostly reserved for Alphys. She also shows some of her softer side with Papyrus, though platonically. She's actually called a tsundere by Alphys in the branch of Neutral Pacifist where you dated Alphys but didn't lock yourself into True Pacifist yet.
• TV Never Lies: Watching "human history" with Alphys has left her convinced that anime is real.
• Twinkle in the Eye: Undyne the Undying gains one after her transformation, with a ray of light almost like a lazer shining out of the eye that is normally covered by her eyepatch.
• Uncertain Doom: Spare her in her Neutral-Pacifist route encounter but fail to give her some water from the cooler and the game will imply she has "fallen down" (i.e. she becomes gravely ill and possibly on death's door), making it impossible to befriend her.
• Video Game Caring Potential: After you run away from her battle, she will follow you into Hotland, but collapse due to the heat. You can revive her by getting a cup of water from the nearby cooler and pouring it onto her.
• Video Game Cruelty Potential: ...Or, instead, you can pour out all the water onto the ground in front of her until the cooler is empty.
• Virtuous Character Copy: She basically received all of Dedan's traits that Papyrus was originally supposed to have — being the brutish local Sir Swears-a-Lot who wants to beat up the protagonist. But even she is severely toned down compared to Dedan; she never abuses her subordinates, instead showing genuine care for them (especially Papyrus), and has a sense of justice which drives her to fight the protagonist who she thinks is a danger to her people. At worst, she is a Jerk with a Heart of Gold.
• Voice Grunting: Not a particularly unique sound, but she has the second deepest 'voice' in the game. Gains a subtle reverb effect as Undyne the Undying.
• Wake-Up Call Boss:
  • She's the first enemy you see that's actively trying to kill you, and it shows! She's much more difficult to deal with than Toriel and Papyrus. The method of sparing her is quite unorthodox as well. That is to say, you can't spare her. You run away.
  • She's also this if you're attempting a No Mercy run. Players expecting her to get one-shot like every other enemy in a No Mercy run are going to be in for one hell of a surprise.
• Was It All a Lie?: In the True Pacifist playthrough. She laughs off Alphys' confession that anime is fiction... but as soon as Alphys is out of earshot, she begins to undergo an existential crisis.
• Water Is Womanly: Inverted. She has water powers and is named after the Undine, the elemental of water traditionally portrayed as a beautiful water nymph. Instead, she's a monstrous, aggressive fish Knight who is very Hot-Blooded.
• Well-Intentioned Extremist: She believes in Asgore's ideals more fanatically than the king himself. She also is willing to murder children, even the player in the Pacifist route. She realizes she is wrong in the Pacifist route, both in True and Flawed. Averted in Genocide, where she fights better than ever because she is fighting genuine evil.
• Who's Laughing Now?: Her story of being humiliated by others for losing a fight against Asgore can be inferred as this, as said fight led to her promotion as captain of the Royal Guard.
• Why Can't I Hate You?: During a Pacifist run, you give her water after she collapses in Hotland while trying to fight you, and she walks away, not having the heart to kill someone who saved her life. She still doesn't like you and only tries to befriend you as a challenge from Papyrus, but she eventually comes to admit that you're not just a wimpy loser… you're one with a big heart.
• Why Won't You Die?: As the battle drags on, she gets progressively more angry at the fact that you're able to survive her attacks, eventually reaching a point where she abandons her main attack pattern and just starts hurling spears around. This is the best time to start running from her, as she'll never turn your SOUL back to green afterwards.
  Undyne: NGAHHH!!! DIE ALREADY, YOU LITTLE BRAT! I WILL NEVER TAKE MERCY FROM THE LIKES OF YOU! YOU! WILL! NEVER! SPARE! ME!
• Wrong Genre Savvy:
  • After Alphys tells her that anime and comic books are "human history," she's become convinced that the world works that way. As a result, she acts much like a typical Shōnen anime protagonist.
  • In a No Mercy run, she's more or less correct. She really is a genuine hero, powered by The Power of Friendship and sheer determination, fighting to prevent the world's destruction. The only thing she's wrong about is that the hero always succeeds… That said, the battle is so brutal that it makes some players give up on the run altogether, so her cause is not so hopeless after all.
• You Are a Credit to Your Race: She has a grudging respect for "human culture". It's implied she eventually feels this way about a Pacifist protagonist, as when she's saved from being a Lost Soul, she mentions some humans are alright.
• You Are What You Hate: Aside from being influenced by human anime, she possesses a number of attributes that are described in-game as belonging to humans, such as a strong will to kill and implicitly, Determination.
• You Bastard!:
  • On a Pacifist or Neutral-Pacifist run, she'll call you out for 'standing in the way of everyone's hopes and dreams,' and wasting your time on 'befriending people' because the monsters only need one more human soul to break the Barrier and declare war on humanity.
  • On a Neutral run, her opening speech calls you out for particular monsters that you've slain. Especially Papyrus.
  • If you abandon a Pacifist run by killing someone after befriending her, Papyrus notes in the epilogue that Undyne took your betrayal of her belief in you very harshly, and that if you ever returned to the Underground, nothing would be able to stop her from killing you. The only exception is if the "one enemy" is Metatton; since it causes Alphys to commit suicide, it leaves her in a full-blown Heroic BSoD. Papyrus even suggested to her trying to find you so she could get revenge, but Undyne tells him it's no use because it won't bring Alphys back.

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