Characters / Undertale - Sans

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

Sans the Skeleton

"i've gotten a ton of work done today. a skele-ton."

"maybe it's a little strange, but sometimes... it's nice to have someone call you out on being lazy. even though nothing could be further from the truth."

A lazy skeleton who hates work and loves puns. He pops up here and there, usually touting some sketchy money-making scheme or offering to take you out for lunch. He also thinks the world of his brother, Papyrus.

See the self-demonstrating page.

NOTE: As a main character (and a Walking Spoiler), this page has the potential to give away major spoilers. Proceed with caution if you have not finished the game. You wouldn't want to have a bad time, now would you?

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A-C

• Achilles in His Tent: Sans confesses that if the role of The Hero fell on his shoulders, he would have thrown in the towel. He sympathizes with the player who chooses full pacifist, as he wouldn't be able to handle the impending Morton's Fork of either killing Asgore or getting killed by said King. Yet he encourages the player to continue anyway. Go full genocide, and you'll make him leave his tent.
• all lowercase letters: Sans almost always uses entirely lowercase letters, both in his dialogue text and his handwriting (to the point that one can tell something is said in-character by him simply by the lack of capitalization). You can tell that he is being deadly serious about something whenever he stops doing it.
• All-Powerful Bystander: Deconstructed. Because of his Medium Awareness, Sans is one of the most powerful monsters in the Underground, but that same awareness is what keeps him from fighting harder to either save or leave the Underground. He knows full well that everything will end up being reset thanks to your (and Flowey's) Save Scumming, and thus no longer sees the point in trying; from his perspective, there's not much point to doing anything but standing by. He doesn't even attempt to stop you from killing his own brother because he knows that will eventually be undone as well (that being said, he also won't forgive you for doing it). He will only get involved at the end of the Genocide route, by which point he's basically the only one left who can get involved, because the protagonist was left alone to murder everything in their path.
• Alpha Strike: Sans does not mess around on the Genocide Route. His very first attack is implied to be his most powerful — a brutal gauntlet of Gravity Screw, Spikes of Doom, and Beam Spam using Wave Motion Guns that can and most likely will waste you within seconds. To top it all off, he's also the only enemy in the game that gets the first turn.
• Ambiguously Human: Ambiguously monster, in this case. You're normally given no reason to question him, considering he lives among far more bizarre creatures that are still clearly monsters, but what we learn about him at the end of a genocide route is another matter entirely. Not only does he have an awareness of the very mechanics his game is built upon, but he has the ability to override most of them as well, something no other character in the game is shown to be able to do except Photoshop Flowey, who has six human souls to his name. This allows him to be tremendously dangerous in combat despite his bare-minimum stats. During the battle, he tells you "look. i gave up trying to go back a long time ago. and getting to the surface doesn't really appeal anymore, either.", suggesting he's somehow either in the wrong place, in the wrong time, or both. And even though there's some dispute over the precise substance, something red that definitely looks like blood (it might be ketchup) spills out when he's cut open, despite the fact that monsters are Made of Magic and don't shed blood. On top of all this, he's the only character in the game who does not turn to dust on-screen when defeated with violence, which some have taken as a suspiciously specific detail to exclude.
• Ambiguous Situation: Look at that pixel pattern in the center of his chest. Is that his hoodie's zipper and a white undershirt, or is that his bare ribs and sternum? (His Smash Bros appearance suggests it's the former.)
• And Another Thing...: A humorous variant by Sans when he is about to leave during the cutscene at Grillby's:
  Sans: by the way... i was going to say something, but i forgot.
• And I Must Scream:
  • What he's essentially trying to impose on you during his boss fight. He's fully aware that he can't permanently kill you, but if he lets you get through, everything will be forever lost. So, he does the only thing he can and tries his damnedest to keep you locked in an endless loop of frustration and pain as you try and fail to kill him. What's interesting is that it works as both an in-universe and meta example, as he's deliberately designed to be as unfairly difficult as possible, leaving many a player feeling very much like they have no mouth with which to scream for hours, days, or even weeks on end. As the fight approaches its conclusion, the usage of the trope becomes more obvious, as his "special attack" turns out to be refusing to attack you and letting his turn stall indefinitely, leaving the both of you locked in an endless void where neither of you can attack the other. Unfortunately for him, he soon falls asleep.
  • Sans himself is in a very abstract, psychological example of this trope due to his awareness of you and Flowey's timeline manipulation. Since the two of you have the ability to start the game over again whenever you please, he sees his life as an endless "Groundhog Day" Loop that only you can set him free from, and in almost all circumstances his faith in you isn't exactly solid. Not even death would release him, as you have the ability to reverse that without a hitch, and as such he's left borderline-crippled with nihilistic depression.
• Annoying Younger Sibling: Inverted; as much as he loves his younger brother Papyrus, he still finds great delight in annoying the ever-loving daylights out of him.
• Anti-Frustration Features:
  • Sans' Battle Intro is quite long and you can't fast forward it, but you will reach the point where he practically speeds up and skips it. This is not a good thing, as the first time he does it is to catch you off-guard with his opening attack.
  • Sans' Damage Over Time effect can only bring your HP to 1 and cannot actually kill you. Similarly, his Interface Screw attacks do not have a Damage Over Time effect and can only bring your HP down to 1.
  • He stops attacking and gives you a chance to reconcile midway into the fight, telling you that he believes there's still a glimmer of a good person inside of you, someone who was his friend in another timeline. After his multiple, intense barrages that give you little, precious opportunity to heal, you'll appreciate this moment to heal yourself. And you'll want to, as the alternative is getting dunked on.
• The Anti-Nihilist: The most optimistic way to interpret his character. He's pretty much resigned to his life, or lack thereof, being determined by someone else's whims. He is nonetheless very jovial and friendly to pretty much everyone but those that deserve otherwise, tries to enjoy life to the best of his ability, and even laments having to kill you if you drive him to it.
  Sans: sounds strange, but before all this i was secretly hoping we could be friends.
• Armor-Piercing Attack: Yes, he can only deal 1 damage, but he also reconstructs Scratch Damage into this. When you fight him in Genocide, the fact that his attacks hit you per frame while leaving Damage Over Time makes your recently acquired Armor of Invincibility absolutely useless.
• Aside Glance:
  • During his Establishing Character Moment, he makes two bad skeleton puns, followed by winking at the camera and the requisite Rimshot. Also foreshadows that he knows about the meta nature of the game.
  • He also manages to invert this trope if you go for dinner with him at Mettaton's resort. When he self-consciously probes the protagonist about whether they think it will be worth it to continue, he awkwardly turns away from the Fourth Wall, as if unwilling to look the player in the eye.
• Attack Drones: Uses giant, floating, demonic skulls called "Gaster Blasters" in his battle. They fire wave motion beams.
• Awesomeness by Analysis: While his time manipulation isn't as strong as the player's ability to save, he's very good at filling in the blanks based on how you act and react to the events around you. There's also the fact that he can dodge your attacks just by asking himself why no one else had ever tried it.
• Bad Bedroom, Bad Life: If the player follows a specific set of actions to get the key to open it, Sans' bedroom is revealed to be a complete mess, including a self-sustaining trash tornado, a jury-rigged light fixture, and an unmade bed. Sans is pretty apathetic to what happens around him, so not bothering to keep his room tidy is in-line with his behavior.
• Badass Boast: He gives several such boasts before and during his boss fight.
  • As you get to New Home's last corridor, he receives you
    Sans: heh heh heh heh...all right. well, here's a better question. do you wanna have a bad time? 'cause if you take another step forward...you are REALLY not going to like what happens next. [...] welp. sorry, old lady. this is why i never make promises.
  • He gives this one right before your battle with him:
    Sans: it's a beautiful day outside. birds are singing, flowers are blooming. on days like these, kids like you... Should be burning in hell.
  • When you challenge him after losing his battle three times:
    Sans: hmm. that expression...that's the expression of someone who's died thrice in a row. [Beat] hey, what comes after "thrice," anyway? wanna help me find out?
• Badass Bookworm: He loves science fiction and has quantum physics books lying around in his house mixed in with his joke books, and if that's not enough evidence that he's quite the intellectual powerhouse despite what appearances suggest, the contents of his secret room certainly is. And he also happens to be, at least as far as we're shown, the most powerful monster in the Underground. ^note As noted above, it's difficult to compare his strength to Asgore's (and probably Toriel's), since Asgore is obviously holding back when you fight him.
• Badass on Paper: Inverted, checking his stats says that he has the worst stats in the game. While this is technically true, his ATK stat of 1 ignores Mercy Invincibility, his attacks are quite large and thus hard to avoid, and his HP of 1 is irrelevant — he dodges every single attack while fighting you.
• Ballistic Bone: His main form of attack. They also inflict KR, which drains the player's health over time.
• Battle Intro: As is traditional with any good Boss Battle. Problem is, he'll skip his Battle Intro to catch you off-guard if you survived his Alpha Strike previously.
• Beam Spam: He liberally uses his Gaster Blasters to fight. In his penultimate attack, he fires in excess of 100 beams in less than 10 seconds.
• Begin with a Finisher: When you fight him in a Genocide run, he opens with his strongest attack right away: a combination of Gravity Screw, Spikes of Doom, and Beam Spam with Wave-Motion Gun. He lampshades it, too.
  "huh. always wondered why people never use their strongest attack first."
• Beleaguered Bureaucrat: He's implied to become this in the Neutral ending where Papyrus becomes king by process of elimination. For the most part, all that Papyrus seems to do is make everyone spaghetti and listen to their problems, but he remarks that Sans is actually working a lot on things he's not entirely clear on, and hard at that, and that life has generally improved in the Underground.
• Beneath the Mask: Overlapping with Sad Clown, it's no surprise that Sans has a lot going on under his smiling skull. Even though he comes off as someone who truly enjoys pranking and making bad puns, Sans doesn't always speak in the goofy Comic Sans font and he hides the worst secrets of the game. His laziness can be attributed to his heavy depression due to the Eternal Recurrence he is trapped in and the friends he lost a long time ago. He no longer cares about the world and has grown huge apathy towards his environment, with the only exceptions being Papyrus and the promise he made to Toriel. He usually sneaks in his real thoughts as "jokes"; sometimes even the player wouldn't notice the hidden implications.
• Beware the Nice Ones: An extremely chill and amiable guy, Sans is liked by everyone who knows him (except Flowey, obviously) and treats you like an old friend mere seconds after you meet him. It usually takes a lot of effort to get Sans legitimately angry with you, but if you do... at best, he'll call you out for the dirty scumbag you are, and at worst, say your prayers and hope it's over quickly. Probably the only reason why he doesn't make them have a bad time then and there after killing Papyrus out of rage is because the player can still reset the timeline, thus undoing his battle/murder.
  Sans: it's a beautiful day outside. birds are singing, flowers are blooming... on days like these, kids like you... Should be burning in hell.
• Beware the Silly Ones: While Sans tells bad skeleton puns and is incredibly lazy, he can provide quite the ass-kicking when someone sets him off.
• Big Brother Instinct: Sans is older than Papyrus, and has an extremely strong instinct to keep him safe and happy. In fact, it's one of his few character traits that remains strongly present no matter what kind of run you're on: in Pacifist and most Neutral runs, he requests of you to play along with his plans to cheer Papyrus up and will express mild offense (a lot coming from him, as he's extremely even-tempered) if you decline or disagree when he asks if you think he's cool. In high-casualty Neutral runs, he will attempt to shelter Papyrus from the deaths of anybody close to him by telling him they "went on vacation", despite the fact that Papyrus is clearly an adult. Unfortunately, this instinct does very little in terms of actually protecting him from danger, because he knows full well that not only is he a Cosmic Plaything who ultimately can't do anything, but also that he's trapped in a "Groundhog Day" Loop and Papyrus will be back before long. As a result, while he's clearly utterly devastated for the rest of the game and will henceforth treat you with nothing but scorn and Tranquil Fury, he won't even try to intervene if you decide to kill him.
• Big Brother Mentor: If the Player Character goes strictly for the Pacifist Run, Sans will fill this role for the Human Child, occasionally giving some discreet assistance in the form of dropping hints, giving emotional support, and providing levity where he thinks it's warranted.
• Big Brother Worship: Inverted. Sans makes it clear on many occasions how cool he genuinely finds Papyrus (his younger brother), for all he messes with him.
• Big Eater: Though Sans is not seen stuffing his non-existent face, Papyrus and the others mention him liking to eat a lot. Not only does he love to eat, he almost seems to go out of his way to have as unhealthy a diet as possible.
• Big Fun: A friendly, fun-loving jokester who's a bit on the rounder side, despite being a skeleton.
• Big Guy, Little Guy: The worldly, wisecracking, all-lower-case little guy to Papyrus' innocent, boisterous, all-capitals big guy. Sans is ostensibly in charge, paying for their house and all, but Papyrus is the one who keeps him on the straight and narrow.
• Big, Stupid Doodoo-Head: One of his Trust Passwords is "I'm a stupid doodoo butt". Naturally, he asks the player character to say the password to him.
• Blank White Eyes: Inverted with Sans, whose eyes go all black when he's serious.
• Blatant Lies:
  • His combat description when you CHECK him says that he's the "easiest enemy in the game." This is completely untrue, and since he leads with his strongest attack the moment the fight begins, the player will know — from painful experience — it's a lie by the time they can read it. Going far enough into the fight will have the game actually confess that reading its messages doesn't seem to be all that useful for you.
  • In a more lighthearted example, he's shown in the trailers for the Undertale DLC in Groove Coaster wearing the pixel headphones shown in the latter game's logo (which are normally shown around the "crab" alien from Space Invaders). He claims they're earmuffs that he found on the ground.
• Bloodless Carnage: Subverted: when you mortally wound him, blood pours out from his torso and mouth, despite him being a skeleton. Or maybe it's ketchup?
• A Bloody Mess: Sans, the Final Boss of the Genocide Route, bleeds when you kill him, despite the fact that it's been established that monsters are made of magic and don't bleed. It is often theorized by fans that this is actually ketchup, because he can be seen chugging a bottle of ketchup at Grillby's if you decline from using it yourself. Others theorize that he’s not even a monster, in which case the trope is Subverted.
• Bodyguarding a Badass: Subverted. He becomes Mettaton's bouncer/agent in the King Mettaton Neutral ending. While Sans is the weakest monster on paper, his combat abilities outclass anyone else's. It is left vague if Mettaton knows this.
• Bond One-Liner: His iconic one-liner when you fall right into his trap:
  Sans: geeettttttt dunked on!! if we're really friends... you won't come back.
• Bootstrapped Leitmotif: While Sans does indeed have a proper battle theme, Toby Fox's signature song MEGALOVANIA is often attributed to Sans due to it being used as his boss theme.
• Boss Corridor: The Last Corridor is completely empty until Sans suddenly appears and stops you in your tracks. Subverted in that he does not fight you, but he does assess your actions and deliver judgment, including some well-deserved Player Punches if you screwed up. Double subverted if you go on the Genocide route, where he decides to actually step into the fray and gives you one hell of a fight.
• Boss Remix: "Song That Might Play When You Fight Sans" is an uptempo version of "sans." with a bit of "Bonetrousle" at the end. Subverted in that it doesn't play at all in the game. Instead, "MEGALOVANIA" plays.
• Break the Cutie:
  • A retroactive example. It's heavily implied that Sans had a lot more passion, optimism, and sincere happiness to his name before a combination of mysterious tragedies in his past and research into the temporal anomaly his world is stuck in beat him down into the lethargic Stepford Smiler he is today. Works like a standard example as well, since the few hints about his past are all given very late in the game and are extremely hard to find; by the time you find out how broken he is, you've had plenty of time to get to know and love him, especially since he's one of the first characters you meet and is deliberately written to be likable and friendly.
  • The guy's already so jaded at this point that you'd think there'd be no way to break him further, but there is one surefire way: find the nerve to kill his little brother. Do that, and Sans will just... stop being in the game. Completely. Off-screen, he plunges into some kind of Heroic BSoD and/or Heroic Safe Mode, never talking to anybody and stalking you in silence, focusing on your general kill count. He'll only show up near the very end in full Tranquil Fury mode, and if you reach an ending where you can get him to talk to you again afterwards, he makes it very clear that you really, really hit him in the one place that still hurts.
• Breakout Character: Easily Undertale's most iconic character despite his role being smaller than more than half the main cast's on paper. His boss fight, theme, and lines in the Genocide Route led him to being so iconic that he ended up being the first Undertale character to make any sort of appearance in Super Smash Bros., and even people on The Internet who haven't played Undertale will likely recognize him.
• Brilliant, but Lazy:
  • Sans appears to be extremely knowledgeable about almost everything that goes on in the game, but spends all his time loafing around and making puns for his own amusement. It is heavily implied that Sans is some form of time traveler/scientist that has observed multiple timelines. The reason he's so lazy is that he knows the truth about how easily the world can be reset regardless of what he does. Only on the Genocide Route, where the Villain Protagonist becomes a threat not just to that timeline but all of them, does Sans actually become motivated enough to fight, and not only is he by far the hardest boss in the game, but he implies that not even what he demonstrates in that fight is his full effort.
  • Depending on the ending, Sans can get a job where he's surprisingly competent. If Mettaton becomes king, Sans becomes his personal agent. If Papyrus is crowned king, Sans does all the administrative work to spare him from being broken by politics.
• Broken Ace: In theory, Sans should have it made: he's nice, he's quick with a joke, he's got his own kind of laid-back charisma, and he makes friends so easily he's able to get chummy with the queen of all monsters without even trying or telling her his name (or vice-versa). He's got a great house, an easy job, a loving brother, and a town full of people who all know his name, with one character having some slight Ship Tease with him (that being the drunk rabbit NPC at Grillby's). He's also one of the most powerful monsters in the underground without any time wasted on explaining why, is extremely smart with a deep scientific background, and has a lot of useful odd powers like teleportation. His one and only apparent flaw, really, is his extreme laziness, and even then, it's used mostly for harmless jokes and only serves to make him more lovable than anything. But, as you've probably guessed from the abundance of spoiler tags on here, Sans is a far deeper character than he seems. Beneath everything, Sans is a deeply cynical nihilist struggling with self-loathing, existential exhaustion, and, according to many fans, crippling clinical depression. His backstory is extremely vague, but it's clearly still affecting him deeply, as it involved the death/disappearance of many people Sans was close to, including a character fairly heavily implied to have a familial tie with him. In addition, he alone knows that the world he lives in is trapped in a "Groundhog Day" Loop and, as such, finds it almost impossible to care about anyone or anything other than his brother. There's also evidence that he may be a drinker to some degree, and if you read deeper into the game's writing, you'll find that the vast majority of his relationships don't go further than friendly acquaintanceship. Sans may have the building blocks to be the underground's number-one big shot, but without the care or means to arrange them properly, all he's really amounting to is a lazy bum who's just barely able to make ends meet. Were it not for his brother, he'd likely not even have the motivation to properly take care of his basic necessities.
• Broken Smile: While his smile is not always as genuine as it looks, it becomes an outright broken one during his battle at the end of the Genocide route. It's anyone's guess exactly when it reaches its breaking point, but it clearly happens somewhere — just look at his face when he realizes he's about to die◊.
• Casual Danger Dialogue: He has a lot to say to you throughout the entirety of his boss battle, and he does so almost entirely with the exact same laid-back attitude he always does.
• The Cameo: After years of "Sans for Smash!" being a massive source of memes among both communities, it was finally revealed that he would be appearing in Super Smash Bros. Ultimate as a Mii Fighter Costume.
• Celibate Hero: Sans is too lazy for a relationship, going by Word of God. The game somewhat supports this notion, too; romance is definitely not the guy's priority. This doesn't stop him having Ship Tease with Toriel, however, and the rabbit monster at Grillby's seems to be into him.
• Chekhov's Gunman: He seems like a simple joker at first —though he can be a bit scary early on in No Mercy/Genocide runs— but as you find out later, he is the only reason you were able to make it as far as you did. In fact, he's been helping you all along; after all, what would a sentry from Snowdin be doing outside of his station, selling items that just so happen to help raise your health?
• Cheshire Cat Grin: His infamous empty-eyed expression◊, which he only makes when he wants to be cryptic or he really wants you to take him seriously.
• Combat Pragmatist: He pulls just about every dirty trick in the book, and constantly breaks the rules of the combat system to compensate for his low stats. Some examples include launching a powerful attack right at the start of the battle, dodging attacks, ignoring Mercy Invincibility, and hitting you during menu selection. In his second phase, he's also able to change attack patterns on the fly just to screw with you. On the first and every subsequent time you reload after being defeated by him, he also gains the ability to randomly interrupt his own opening speech to catch you off-guard with his first attack. He tries breaking the player character by talking, and even pulls an I Surrender, Suckers before delivering a One-Hit Kill. But all of this is justified, given the player is multiple leagues ahead of Sans in terms of stats, to the point fighting mano a mano would be suicidal for him.
• The Computer Is a Cheating Bastard: One way to look at Sans' battle is him being the only aversion to the trope. He can do literally anything you can during your battle. Both of you can dodge, both of you don't give mercy invincibility to your targets, both of you can feign offering mercy to instant-kill a tough foe, and both of you can attack the other when it isn't your turn. You just happen to do the last one better in order to finally win.
• Conditioned to Accept Horror:
  • There is nothing he can do to stop the constantly repeating time loop he's trapped in, so he eventually just numbed himself to all of it. Unless the player carries out a Genocide run, he usually just shrugs off any deaths that happen and mocks you in the end for causing them, knowing that it'll be reset anyway. The one and only thing you can do that really manages to get to him is murder his beloved brother, which sends him into a full-on Heroic Safe Mode at worst. And if you do go full Genocide, well... he may pretend to take it in stride when he dies, knowing the entire world is about to be destroyed, but the look in his eyes gives him away.
  • Taken to the next level if you kill him and reload the game to before his fight. No matter what, he has to fight you. And he still points out the savagery rather nonchalantly.
    Sans: ...that expression that you're wearing... you're really kind of a freak, huh?
• Confusion Fu: While ultimately limited, Sans has a wide variety of different attack patterns, perhaps the most compared to other bosses in the game. Then you reach the midpoint, and he starts to randomly interrupt his attack patterns into each other, and reveals even more attack patterns, to keep the surprise factor high. It may take a while before you get to see them all, so keep your guard up at all times.
• The Conscience: Sans is literally your moral compass. He's a reflection on everything you do during your journey through the Underground. He'll chastise you if you kill randomly/indiscriminately, or he'll be your best friend ever if you're benign. Kill everyone, and watch out, he'll come down upon your wickedness like a ton of bricks. Even during the final fight, he's not fighting for the Underground's sake... you've already destroyed everything he knew and loved. He's fighting for your sake as well, knowing full well that if you successfully get past him, you're eternally damned, as the Fallen Child will claim your soul.
• Cool Big Bro: For as much he enjoys messing with him, Sans' relationship with Papyrus is remarkably strong. He's laid-back, quick with a joke, and appears to be very popular with the town residents, yet he still considers his dorky brother the coolest person he knows. Played with in that, despite Sans being by far the more traditionally cool of the two, he's still the one who admires Papyrus in this way while ostensibly playing a role more similar to an Annoying Younger Sibling.
• Crouching Moron, Hidden Badass: While he's clearly intelligent, Sans is incredibly lazy and prone to making immature jokes. Definitely not the character you'd look to for material of a tough boss.
• Cynicism Catalyst: After somehow uncovering the fact that the world he lives in is trapped within a seemingly-unbreakable "Groundhog Day" Loop that neither he nor any other character is able to remember (except the one who's responsible for it, Flowey), Sans devolved into an existentially-exhausted and nihilistic shell of his former self. It's left ambiguous exactly how much of his current "carefree jokester" attitude is him healthily coping with this and how much of it is him putting up a ruse just to get through the day and avoid questions, but it's still clearly still affecting him deeply.

D-G

• Damage Over Time: In addition to massive direct damage when you touch them, his attacks also leave a slow damage over time effect called Karmic Retribution. The more you get hit, the longer this effect lasts.
• Dark and Troubled Past: We don't know anything specific whatsoever, but whatever Sans went through prior to the events of the game clearly wasn't pretty. All we know is that it involved a group of unidentified people Sans was close to but will never see again, him trying for what is implied to be a very long time to "go back" somewhere before eventually just giving up, a broken machine that according to Word of God was never fixed no matter how hard "[either] of them" tried, and the ever-mysterious W.D. Gaster. It's extremely likely this is a contributing factor to Papyrus' status as Sans' Living Emotional Crutch; of all the things he ever cared about, he's the only one he managed to avoid losing.
• Deadly Hug: While the game implies that Sans "hugs" the child to death in the English release should you accept his offer, he outright states this in the Japanese localisation instead...
  "Huh? You died? That's weird. All I did was try to hug you..." (translated from Japanese)
• Deadpan Snarker: When his humor isn't puns, pranks, or some combination of the two, it's this, and many of his lines are dripping with it. Especially in response to Papyrus.
  Papyrus: YOU'RE SO LAZY!! YOU WERE NAPPING ALL NIGHT!!
  Sans: i think that's called... sleeping.
• Death by Irony:
  • Sans spends the entire fight playing on your expectations and trying to catch you off-guard. How do you defeat him? By catching him off-guard with a surprise attack while he taunts you after pulling yet another Nonchalant Dodge. Especially notable is that you're not supposed to be able to attack twice in one turn. He bends and breaks combat rules, then gets done in by the same approach.
  • He can inflict this on you. Earlier, Papyrus spared you and offered a hug, only to be killed while his guard was down. Midway through the fight with Sans, he tries to spare you. If you accept his mercy, he also offers a hug, then kills you while you're off-guard.
• Death of a Thousand Cuts: Basically his combat strategy. Individually, his attacks are the weakest in the game. But they also avoid triggering your Mercy Invincibility, and he can attack a lot.
• Deconstruction:
  • Undertale already deconstructs the Save-Load mechanism in the first place, but here we get to see its effect on a personal level. Knowing your hard work can be undone without any warning easily breeds apathy, and that's the reason why Sans is such a Lazy Bum.
  • Another happens at the end of Genocide, where he turns the conventions of Turn-Based RPGs on their head. He starts the fight with his strongest attack, powerful enough to waste you within moments. He plays into the expectation that every boss battle has that Battle Intro — only to skip said intro to sucker-punch you. You may have 99 ATK, but there's no Dodge stat in this game, meaning there's nothing that says he can't just sidestep your attacks. And he knows that Scratch Damage will hurt, even against 99 DEF, when done a lot, with added poison effect. Then he starts to outright cheat by not letting you have your turn in peace, attacking you while it's your turn. And this is all before his space/time bending abilities, tricking you into Smash Cuts after Smash Cuts. Needless to say, this makes for a very difficult battle.
• Dem Bones: He's a talking skeleton who loves to eat and sleep.
• Depending on the Artist: In official materials, the colour of Sans' slippers varies from white (in-game sprites), blue (official tarot card), or pink (official plushie and Super Smash Bros. Ultimate).
• Despair Event Horizon: He's goofy and a Pungeon Master, but underneath that big grin, Sans... Sans is not well. Living with the knowledge that at any second, your life can be irrevocably snatched away from you, for every minute of the day tends to do that to a guy.
• Determined Defeatist:
  • When he decides to step in and take action at the end of Genocide run, he's pretty much resigned to his tragic fate. But knowing you're about to destroy the world... he just can't afford to not care anymore. He gives one hell of a final fight with the hopes of getting you to Rage Quit or reset to a more merciful game route, but at the end of the day, you have the Save Scumming ability that he has no counter against.
  • This mentality even reflects on his special attack. He admits he'll never be able to beat you, so he refuses to let you have your turn and traps you in the battle screen as a last resort. How long is he prepared to wait for you to get bored? Forever.
    Sans: heh heh heh... ya get it? i know i can't beat you. one of your turns... you're just gonna kill me. so, uh. i've decided... it's not gonna BE your turn. ever. i'm just gonna keep having MY turn until you give up. even if it means we have to stand here until the end of time. capiche?
• Did You Just Punch Out Cthulhu?: Through sheer power of badassery, annoyance, and persuasion, Sans is the only character who even remotely stands a chance of stopping you from destroying the world.^note Granted, any character could technically hold that title if they happen to be the character who made you give up and Reset, but Sans stands out in that he's the only one fighting against you, the player, rather than your character.
• Didn't See That Coming: Sans is savvy enough to keep his guard up at all times, even when he's offering reconciliation or sleeping... But he didn't expect you to be able to attack two times in a single turn. The look on his face says it all.
• Dissonant Serenity: No matter how people react around him, Sans has the same big grin on. This is implied to be an act, since he keeps this up even when it's clear he's in anguish or furious. The only time he's even close to frowning is when you finally land a hit on him. But, he keeps calm and casually talks about getting some food once he's mortally wounded.
• Don't Make Me Destroy You: Frequently warns you not to keep pushing him. You really won't like what'll happen next if you succeed. He's not bluffing.
• Double Meaning: His pre-battle speech, specifically the bit where he said how you are "REALLY not gonna like what happens next", can easily be interpreted as how difficult the fight is going to be. However, reading his lines further, specifically "Seeing what comes next" and "Don't say I didn't warn you..." when you mortally wound him, also opens the interpretation that 'what happens next' is an Apocalypse How caused by the Fallen Child followed by a Heel–Face Door-Slam where you must give up your soul to restore the world if you wish to return.
• The Dreaded: Anybody who successfully invokes Sans's wrath learns to fear him. Flowey states in a post-Neutral conversation that Sans is the only thing that has kept him from reaching Asgore on his Genocide runs, and he's had to reset multiple times to avoid him; the only time Flowey gets the better of him is in the True Pacifist run by catching him and everyone else off-guard. The Fallen Child, despite expressing nothing but contempt and confidence during the rest of Genocide Run, will express concern only when they have to fight Sans.
• Droste Image: He keeps a joke book in his and Papyrus' house which hides a Quantum Physics textbook, which hides another joke book, which hides another Quantum Physics textbook. Pretty good metaphor for Sans himself, actually.
• Dying Alone: By the time poor Sans gives his last breath at the end of your battle with him, every monster close to him has either died or ran away. He doesn't even share his last moment with the player and walks out of the battle screen, calls out for his dead brother as his last words, and dies a Sound-Only Death.
• The Dying Walk: If you kill him at the end of a Genocide Route, Sans will walk off the battle scene and suffer a Sound-Only Death.
• Earn Your Happy Ending: If you make a True Pacifist run, stop there without making a True Reset, or copying and then rewriting your saved games, Sans will finally have his.
• Entertainingly Wrong:
  • When Papyrus tells Sans that a talking flower has appeared to him several times giving him advice, Sans concludes that someone must be using an echo flower to trick Papyrus. The player can probably already guess what's really going on. Conversing with Flowey multiple times after the Neutral ending suggests that he's been purposefully avoiding Sans, which is why Sans doesn't know about Flowey at all.
  • Downplayed when he suggests that the player is responsible for all the resetting of the timelines that he's noticed. It's true that the player is responsible for all the resetting after Frisk falls into the realm of the monsters. But there was plenty of resetting before that point, too. Flowey is known to have done plenty of resetting before you and Frisk showed up, and a piece of dialogue with Toriel on a second (non-True Reset) playthrough of the game suggests that the human children from before also had this capability.
• Establishing Character Moment: Stalking the protagonist in the woods outside Snowdin, only to reveal himself with the line "don't you know how to greet a new pal?" His hand-whoopie cushion establishes him as a good-natured prankster who doesn't work hard at all, but his mysterious introduction subtly foreshadows that there's more to him than meets the eye.
• Establishing Character Music: Sans plays a prank on you and is accompanied by music that is smooth and chill, establishing his laid back and fun personality.
• Exact Words:
  • Sans never actually lies to you during his boss battle, even though the implication of his words and his actions don't line up. When asking you to Spare him, he says it would "make his job a lot easier," which is technically true in that his job is to ensure your death, and once you do Spare him, he responds that he "won't let this go to waste," which he doesn't, because it's an opportunity to kill you quickly.
  • If you CHECK him in battle, he is described as "Can only deal 1 damage." This is true, but he dodges all your attacks, and his rapid-fire attacks override Mercy Invincibility and leave behind residual damage, meaning every frame that heart touches his bone attacks will drop your HP point-by-point very rapidly.
  • Most infamously, when he uses his "special attack", he says "here goes nothing." What happens next is... literally nothing. As in Sans refuses to end his turn, since he knows that your next move will be his last. So he simply waits, hoping that you'll get so bored that you'll give up and quit. Sadly for Sans, he gets A Taste Of His Own Medicine...
  • A lighter instance can be seen in a post-it note argument Papyrus has with him.
    Papyrus: SANS! PLEASE PICK UP YOUR SOCK!
    Sans: ok.
    Papyrus: DON'T PUT IT BACK DOWN! MOVE IT!
    Sans: ok.
    Papyrus: YOU MOVED IT TWO INCHES! MOVE IT TO YOUR ROOM!
    Sans: ok.
    Papyrus: AND DON'T BRING IT BACK!
    Sans: ok.
    Papyrus: IT'S STILL HERE!
    Sans: didn't you just say not to bring it back to my room?
    Papyrus: FORGET IT!
  • "Song That Might Play When You Fight Sans" is a Boss Remix of "sans." The operative word is "might"; the song isn't even in the game's files and Sans' boss music is "MEGALOVANIA".
• Exposed to the Elements: Not nearly as much as his brother, but he's still wearing shorts and slippers in the snow.
• Expository Pronoun: The official Japanese localization plays with this. On most occasions, he uses the first-person pronoun oira, a somewhat regional pronoun which is often reserved for country bumpkins or otherwise laidback and somewhat goofy characters. During his battle in the Genocide run, he switches to the more traditionally masculine and rough ore, hinting that his previous usage of oira was part of his Sad Clown facade. In a bit of foreshadowing, he can also be seen using ore in his Lost Souls encounter during the final boss of the Pacifist route, as the lines he says are taken directly from the Genocide route encounter, as well as in the Neutral route should you kill Papyrus (during the "you dirty brother killer" bit).
• Failure Is the Only Option:
  • Sums up just about most of his life. During his boss battle, we learn that he firmly believes the world is going to be reset no matter what happens, despite the fact that he still continues to root for the protagonist anyway during the Pacifist Route (considering the vastly different circumstances of both routes, however, it's quite possible that he legitimately Took a Level in Idealism during the latter while being pushed further past the Despair Event Horizon in the former). He also makes vague remarks about "never being able to go back", which is most likely related to the unfixable machine hidden in his basement and to his pictures with people the player character can't recognize.
  • Taken up to its logical conclusion with his Genocide run fight, where he's aware that he's in a Hopeless Boss Fight.
    Can't keep dodging forever. Keep attacking.
• Fatal Flaw:
  • He is really lazy, even if the cause of that is debatable. While it's played for laughs every time it comes up for the most part, he winds up exhausted should the player make it all the way to the end of his final barrage in Genocide. From there, he starts to doze off...
  • When Sans unleashes his special attack, which turns out to be prolonging his turn to get you to quit, the only way to beat him is to drag the box you're in over to the 'FIGHT' option. The only hint you're given to do this? Any time you get too close to the lower left-hand corner, Sans' eye flashes blue, and you are thrown back to the center. His precaution is ultimately what causes his death.
• The Fatalist: His humorous façade hides a deep and bitter cynicism. He stopped caring a long time ago and prefers to observe, rather than act. However, he eventually gets motivated to take a stand near the end of the Genocide run, despite acknowledging that due to your ability to reset, it's impossible for him to really win. The most he can hope for is to get you to Rage Quit.
• Fat and Skinny: The fat to Papyrus' skinny. He's surprisingly stocky for a skeleton, considering he doesn't have any flesh to speak of. Because he's not fat, he's big-boned.
• Fat Slob: "Fat" might be pushing it a little, but he's definitely not a hard-working or particularly organized fellow.
• Final Boss: Of the Genocide route. After him, you move on to Asgore, who's automatically one-shotted, so he's the last true boss you'll fight.
• Foil:
  • Compliments nearly all of Papyrus' personality traits, the only exceptions being their kindness (a trait most monsters have anyway), their care for one another (although they show it in completely different ways), and the fact that neither of them are to be trifled with.
  • He's also one to Flowey. Both are Fourth Wall Observers who present themselves as being harmless and goofy, but hide a much more cynical personality beneath that veneer. But while Sans is a loyal friend and loving brother who only acts against you if you go out of your way to be as evil and monstrous as possible, Flowey is using you the entire game and attempts to murder you in cold blood several times no matter how kindly you treat him.
• Foolish Sibling, Responsible Sibling: He manages to be both with Papyrus; in terms of behavior and lifestyle, he's the foolish, but in terms of intelligence and maturity, he's the responsible.
• Foreshadowing:
  • Despite Sans downplaying Papyrus's abilities, Papyrus's definitely no pushover. Sans has pretty high standards when it comes to power.
  • When you first enter Snowdin, there's a large branch in the path that's "too heavy to move", implying that it's fairly large. Once Sans starts following you through the forest, you hear a loud cracking sound. If you backtrack a bit, you'll notice that the branch has been snapped in half. Considering Sans and the Human Child are about the same size and a skeleton is probably much lighter than a human, Sans must have crushed it with magic... and demonstates this ability to you first-hand during his fight, where he flings your soul violently against the walls, ceiling, and floor.
  • During the epilogue of the Golden Ending, Papyrus got his wish of driving a car on the highway. Which then got outsped by Sans on a tricycle. Was Papyrus just driving really slowly? (But then again, he's on the highway.) ...Or does Sans have hidden talents?
  • Pester Flowey enough by completing multiple Neutral Runs, and he will eventually tell you to stay away from 'Smiley Trashbag' for causing him 'more than his fair share of resets'. Then you try to walk in Flowey's footsteps, and boy are you in for a surprise.
• Forgiveness Requires Death: If you accept his offer of friendship in Genocide Run...
  "If we're really friends.... you won't come back."
• Fourth-Wall Observer: Sans is a deconstruction of this trope. Rather than being aware that he exists within a video game outright, he is instead aware that the world he lives in — the world that he still perceives as the only reality that exists — has to fall under all the rules that a video game does, and every implication therein. Because of this, he, among other things, knows that some god-like being is controlling the time-space continuum around him, loading and resetting events and memories as it sees fit, and has the exact mental state one would be expected to have after such a revelation.
• The Fourth Wall Will Not Protect You: He's really showing it in his boss fight on a Genocide run. A lot of his boss fight mechanics are designed to screw you, the player playing the game, instead of the in-universe character. Of particular notes are him skipping his Battle Intro straight into an Alpha Strike, throwing attacks at your cursor browsing the menu, and his ultimate attack of doing nothing so you never get your turn to end the battle. He also refers to you, the player, as 'The Anomaly', and outright tells you his Hidden Depths and calling out your actions across the game.
• Fragile Speedster: Sans is a One-Hit-Point Wonder, but is skilled at dodging and makes very rapid attacks with his magic. Good luck getting to the end of the battle.
• Friendly Enemy: This is how he truly perceives the protagonist throughout most of the game. He deliberately acts friendly to try and keep them happy on a Pacifist playthrough, and while it's likely sincere to some degree, he still clearly remains wary of their actions. Deep down, he wishes to have a genuine friendship with them, however; he admits as much during his boss battle. By the end of the Pacifist route, while he does unambiguously care about them, it's still somewhat up to debate how much he truly trusts them due to his paranoia.
• Friendly Skeleton: In most runs, he's an amiable skeleton who likes to make puns.
• Frothy Mugs of Water: Or rather, a nice bottle of ketchup. Sans slammed the bottle like water. Fans even like to imagine him getting drunk off it.
• Frozen Face: Downplayed in that he's able to emote with his eyes, but doesn't seem capable of changing the shape of his mouth, giving him a permanent skeletal grin. Or so we're led to believe. Look very closely during two specific moments (when Flowey grabs him near the end of the Pacifist route and when he gets fatally wounded near the end of the Genocide route), and you'll see the corners of his mouth shift very slightly, turning his grin into an expression of shock and pain. It's still up to interpretation, however, if this is the full extent of his ability to do so, or he's actually capable of emoting normally and simply chooses not to. It's debatable whether or not this can be considered canon, but in the Undertale Kickstarter Reward Tier video, there's a brief moment where Sans takes out a drink and drinks it, and when he does, his mouth actually closes.
• Full-Contact Magic: In his boss fight, he slams your soul into the walls and ceiling of the action box with a wave of his hand.
• Funetik Aksent: His "-ing" words usually end in an "-in'", he says "'em" instead of "them", he occasionally strings words together like "forgeddaboudit" and "howzabout it", and throws in a "capice" for emphasis at one point. Since the game has no voice acting, fans typically interpret this as some variety of New York accent.
• The Gadfly:
  • Messing with people for kicks is his favorite hobby besides relaxing, usually taking the form of harmless pranks or deliberately-bad jokes.
  • During the Playable Epilogue, if you call Toriel (who answers you through text), she lends the phone to Sans because it's too small for her hands: he uses it to mock her motherly demeanor. She then takes the phone back to mock his Big Brother Instinct.
• Game Face: Sans is always grinning, constantly looking around to make aside glances while he talks, giving him a cheerful, animated feel. When his eyes disappear and he just stares straight at you with empty, pitch-black eye sockets, you know things have just gotten real. And when his left eye starts glowing... start praying, for all the good it will do. On the flip side, his eyelights also disappear at one point in a Pacifist run, when he tells you his secret secret codeword: "I'm the legendary fartmaster.", implying confiding such is Serious Business.
• Genocide Backfire: His fight at the end of the appropiately named Genocide Run can be interpreted as this. He is the toughest boss of the entire game and has brought many players to just give up, but he will actually never fight the player in any other timeline but the one where you literally kill everyone in the Underground.
• Glass Cannon: He's capable of rapidly shredding down your considerable HP, yet he himself only has 1 HP. To make up for this, he dodges each and every one of your attacks until the very end.
• Glowing Eyelights of Undeath: His general expressions have those. They actually help making him look friendly and approachable (probably because of the artstyle). They will disappear when he wants to sound threatening.
• Glowing Eyes of Doom: During his Alpha Strike and the latter half of his fight, his right eye begins rapidly pulsing between cyan and yellow (the colours of Patience and Justice) when he unleashes his gravity and teleportation powers.
• Goddamned Boss: Invoked and played with. Sans is most definitely the most formidable fighter in the game, but he also knows that the player only has to defeat him once in order to annihilate everything, and they can keep battling him indefinitely until they finally break through him. With this in mind, his strategy to defeat you is to frustrate you into submission by violating every rule in the book (stealing the first attack, attacking while you are in the menu, attacking while pretending to offer mercy, refusing to take his turn) in the hopes of getting you to finally quit.
• Godzilla Threshold: You. The one thing that can make Sans get off his ass and actually try is the threat of a player character about to kill all monsters and poison every timeline in the game.
• Good Counterpart: To Flowey the flower, the other Fourth-Wall Observer of the franchise. Saying that both of them present themselves through alliterative names and have two Voice Gruntings to make the difference between their silly and serious moments would only be a stretch on both their surfaces.
  • Both introduce you to the game's mechanics in different ways where they alternate between their silly and serious side: Flowey shows a childish side before stabbing you and makes you believe that Violence is the Only Option. On the other hand, Sans poses himself as a threat for the Human Child when they exit the Ruins, then reveals his more silly side to show them than even the most aggressive monster can become a friend.
  • Both are smiling, trolling, nihilistic, and have been broken to discover they are characters in a game and that some entity can rewind time in it and undo everything they did. However, while Flowey actually lived as a video game player, rewinded time to see everything himself, actually remembers everything you did in detail and won't hold himself to taunt you with it, Sans' knowledge on the question is a mystery, but he became fatalist because of it and can only interpret your resets through secret codes and by looking at the Human Child's facial expressions.
  • Both of them are pranksters, have a talent for imitation, and watch for the protagonist in the shadows. Flowey avoids contact, plays cruel pranks on you consisting of giving you a Hope Spot, makes imitations in the same vein (for example, the infamous Echo Flower in Waterfall where he mimics Toriel's voice before taunting you), and takes action once you've fought Asgore yourself to take the Human Souls for his own benefit. On the other hand, Sans frequently chats with them, plays childish pranks on them^note for example, the telescope prank in Waterfall (though it could be more serious than it seems to be), offers them a dinner where he shows his talent by imitating Toriel, judges them before they meet the King, and only takes action if they go too far.
  • Amusingly, they both have different opinions on Let's Players and their audience. In the Genocide run, Flowey calls the audience of Let's Players cowards and sickos for wanting to watch the Genocide route without doing it themselves. In the Neutral and Pacifist routes, specifically on the Xbox edition inside Papyrus' sink, Sans, ever the lazybones, asks why you wouldn't just watch a video of the "exclusive [Xbox] game" instead of playing it yourself.
• Good Is Not Soft: Although Sans is a genuine Nice Guy and almost always acts it, he was apparently completely prepared to murder you the moment you stepped out of the Ruins if not for a promise. While his implied motivation for doing so — ridding the world of the creature that he believes is the cause of the time anomaly his world is stuck in — was clearly good (and, in many runs, completely justified), it still speaks volumes that he was willing to stone-cold murder a child in order to achieve it. And, if you kill enough people to make him intervene and fight you, he treats you absolutely ruthlessly; with the exact same smug-as-hell attitude he always has, he insults you, mocks you, torments you with deliberately-unfair attacks, cheats, lies, rubs his own power in your face — whatever it takes to discourage you enough that you stop playing, since he knows that's the only way he can possibly save everybody at this point.
• Go Out with a Smile: In the Genocide run, despite probably being in immense pain from being slashed across the torso, he still manages to keep smiling even as he dies.
• Grin of Audacity: That grin is all he has in the way of expression, but his attitude throughout most of his boss battle, when it's not Tranquil Fury, absolutely screams this trope.
• "Groundhog Day" Loop:
  • Sans compares you starting and restarting the timeline via saves as this in the Genocide path as you're fighting him. Part of his laziness is because he knows how futile everything would be if it all resets again.
  • It's implied that he forced Flowey into this as well, as the latter remarks how much he hates Sans for the number of resets Sans caused him.
• Groundhog Peggy Sue: Sans is an odd case; he's aware of being stuck in a time loop and is very alert to when a reset happens, but he carries over little to no memories of previous timelines to the present day. During a Pacifist run, all he does is play the role of a funny Pungeon Master so you could have a satisfying experience with the game and stop the resets for good once you reach the Golden Ending. If you decide for a Genocide run, however...
• Guest Fighter: Of a sort. He's available as a Mii outfit in Super Smash Bros. Ultimate. He's unique in that buying the costume also unlocks a song as well, the song being a remix of Megalovania.
• Guile Hero: On both the Genocide and Pacifist routes. On the Pacifist route, Sans plays the role of a funny jokester to make sure you're satisfied with your gameplay and you'll be done with the game forever, letting everyone live happily on the surface. On the Genocide route, he states that he believes that there's some good left inside you and he asks you to spare him so he can get an opportunity to cheap-shot you, which works on a few people.

H-M

• Hand Behind Head: He does this whenever he's feeling awkward or self-conscious, such as your 'dates' with him at Grillby's or at MTT resort.
• Have a Nice Death: He'll mock you every time you die and come back against him, but his biggest one is the now-memetic line:
  Sans: geeettttttt dunked on!!
• Heavy Sleeper: Sans can be seen sleeping on the job while you're being chased by Undyne, and despite the fact that Undyne visibly stops to demand help from him, he doesn't budge an inch. This might be an act, as even when he falls asleep during his boss fight, he still dodges your swing without missing a beat. What finally does him in is the fact that you attacked twice in a turn.
• Hero Antagonist: In the Genocide run, he's the last fight before Asgore and the only character besides Undyne to give you any trouble. The flavor text, music, and general atmosphere of the fight all emphasize how Sans is the last line of defense between you and destruction of his world, and how the player deserves everything that's being thrown at them.
• Heroic BSoD: If you kill Papyrus, regardless of how few others you kill, Sans completely disappears from the game until his judgement. He cares a lot about his brother.
• Hero of Another Story: It's plain as day once you've learned what he knows that before Frisk came around, Sans was the only one working behind the scenes trying to save the Underground from Flowey. He's been keeping tabs on the activity of the timeline, has been working diligently on some kind of machine in the basement of his home, and he keeps tabs on virtually the entire Underground with a little help from his teleporting shortcuts. To top it off, when you face him in battle, he can attack you at all times and avoid all attacks, almost as if he was the player character and you were the boss monster. If you complete a few neutral runs in a row, Flowey warns you to steer clear of Sans, as he had barred Flowey's path multiple times before, and caused him to reset. Successfully.
• Heroic Safe Mode: One could say this is his default mode during all runs. In Neutral and Pacifist, he acts friendly towards the player to keep his world (especially his brother) safe and stalks them around to keep tabs on their kill count. If you kill his brother or start a Genocide run, this mode is taken up to eleven; he stops trying to befriend you and only focuses on trailing your actions, until the Judgement Hall where he either gives his judgement with very clear Tranquil Fury or outright fights you to save the world.
• Heterosexual Life-Partners: Although he and Papyrus are brothers, their relationship also easily qualifies as this. They live with each other, are completely dedicated to maintaining each other's safety and well-being, are the best of friends despite their constantly-clashing differences, have no other known family, and, at least at the beginning of the game, next to no other friends either. This is perhaps best exemplified in a run where you kill Papyrus, as it causes Sans to almost completely vanish from the game in grief.
• Hidden Depths: The sheer amount of spoiler tags on this page should be giving something away. Sans tends to keep a low profile, but displays awareness of his alternate selves, the multiple timelines and resets, praises you if you didn't die up to the restaurant scene, counts the number of times he kills you, possesses incredible time-space abilities similar to yours, and owns items such as the group shot you get by clearing a Pacifist run and choosing not to stay with Toriel. Just what is his deal?
• Hoist by His Own Petard: After an entire fight of throwing Interface Screw after Interface Screw at you, you beat his 'special attack' — i.e. him refusing to make his next move so you can never take your turn — by breaking the interface yourself, allowing you to press the FIGHT button in spite of it not being your turn. When even that doesn't work, The Fallen Child says "fuck it" and hijacks control from you to finish the job.
• Honor Before Reason: He only fights you when it's clear you are about to destroy the entire world and not at any other point, even after you kill Papyrus in cold blood. This is because he promised Toriel to protect and guide you, and he is a man of his word.
• Hopeless Boss Fight: His battle in the gENOCIDE run is this. Not for you, but for him. While he can (and will) kill you very quickly and messily, he knows it won't do any good since you can just reload your save and try again, and it's only a matter of time before you finally defeat him. The best he can hope for is to try to get you to Rage Quit from the difficulty.
• Horrifying the Horror:
  • Flowey dreads the moment Sans gets serious, and warns you, seemingly sincerely for once, that you should stay away from him for your own safety and to not let him find out anything about you. With how much bloodshed and chaos Flowey causes without batting an eye, his wariness speaks volumes about what Sans is capable of.
  • If you kill him and then reset the battle twice, he will provide a unique piece of dialogue at the opening of each new fight which suggests that he might be feeling a bit fearful himself. Of course, that's just one way of interpreting it — given his lack of voice acting and very unreadable demeanor, they could just as easily mean he's impressed, surprised, or just outright disgusted.
    ... that expression that you're wearing... ... well, i won't grace it with a description.
    
    ... that expression that you're wearing... ... you're really kind of a freak, huh?
• HP to One: KARMA will never reduce your health to 0. Similarly, you can't be killed by getting hit by the bones in the menu while you're taking your turn, and his penultimate attack when he's bashing you against the walls won't drop you below 1.
• Humans Are the Real Monsters: Some dialogue with him implies a distrust of humanity that runs deep, in contrast with his brother, who would at least give a human caught wandering the Underground a fighting chance. Sans reveals he's less lenient in that regard when he confides to the player late in most Neutral runs or a Pacifist run that he would've killed you the moment he saw you if Toriel hadn't made him promise not to harm any humans he came across.
• Hyper-Awareness: Sans may be the 'laziest' monster you'll meet in the Underground, but he's highly attentive to detail, down to subtle facial expressions. He also makes it a point to know and speak to everyone, getting a read on their personalities. He knows through a combination of science, guesswork, and observation that a time-space anomaly has been ongoing for some time. It's also implied that he's a competent tracker, should he stay awake long enough to do his job as a sentry.
• Hyper-Competent Sidekick: Zig-zagged. Papyrus seems to view him as a sidekick (and Sans never denies the role), but again, from Papyrus's point of view, Sans would be anything but competent. In the Genocide route, however...
• Iconic Outfit: His hoodie/sport shorts/slippers combo.
• I Fell for Hours: Weaponized for his penultimate attack, combined with Gravity Screw and Spikes of Doom. And it's not even the deadliest part of said attack. At first, the attack looks admittedly quite funny, with Sans pulling various poses as the player's soul flies past... until it takes a decisive swerve when the last sprite of Sans seen in this segment of the attack features him with dark eye sockets.
• I Gave My Word: Sans hates making promises, but if you can get him to make one, you bet your life he'll keep it. Deconstructed however, as you can ruthlessly slaughter just about anybody you want in the underground, up to and including his own brother, and despite the pain it clearly causes him, he still won't break the promise he made to Toriel to keep you safe. Only once you're at the end of a Genocide route and he knows for sure that you're going to destroy everybody and everything if he lets you live does he finally set this commitment aside to try and stop you, and even then, the last thing he does before attacking you is apologize to her for it. This decision is somewhat justified, however, since he's aware of your ability to reset the game, so he knows that you're more than likely going to start over shortly after you finish and everything will be back to normal. It's only on a full Genocide route that everything gets destroyed permanently with every future time-reset variation irrevocably tainted, hence him breaking the promise.
• "I Know You're in There Somewhere" Fight: Sans' comments at the midway point of his boss fight has him call out to "someone who wanted to do the right thing", followed by a plea for them to let it end and reset. It's not made clear if he can remember a previous Pacifist run, can sense Frisk's kind heart beneath the Omnicidal Maniac the Fallen/the Player has become, or if he's simply bluffing, but it's clear he knows this isn't how it's supposed to be, and he's trying to get Frisk to realize that too. Unfortunately, Frisk is either completely gone or still can't break free from your control, so the effectiveness of this plea comes down to it reaching the player, not Frisk.
• I Let Gwen Stacy Die: In several neutral endings, it's pretty obvious he's feeling this after you killed many, including people close to him, and he didn't intervene because of his promise to Toriel.
• Inexplicably Awesome: Despite the fact that he's never trained a day in his life and has (presumably) been around for a tiny fraction of monster history, he is far and away the most powerful creature in the Underground, and easily among the smartest. This, combined with his awareness of the game's mechanics and constantly-manipulated timeline, almost make him feel like he's on another level compared to his peers, so much so that it's not unusual to find fans who are skeptical if he's even a monster at all. Incomplete fragments of his backstory can be found scattered around the game, but we're clearly not getting anything definite anytime soon.
• Interface Screw: Many of Sans' attacks abruptly change from one to the next, and after the halfway point, he'll send bones through the menu during your turns to hit your SOUL cursor. He's able to change the direction of gravity during the Blue Heart phases as well.
• Intergenerational Friendship: With Toriel, as both of them really enjoy bad jokes. Thanks to that, Sans didn't kill off Frisk the first time they met.
• Ironic Echo:
  • Papyrus insists, in a Genocide playthrough, that anyone can be a good person if they try. Before fighting Sans, he asks you if you think "anyone can be a good person if they just try", then laughs before you can give an answer. He then asks if you wanna have a bad time, which is itself a callback to the warning he gave you prior to the fight with Papyrus.
  • Also, if the player attempts a Genocide run, Sans echos something that Asgore says on Neutral and Pacifist runs when they met the protagonist, but of course with a different meaning.
    Sans: it's a beautiful day outside. birds are singing, flowers are blooming. on days like these, kids like you... Should be burning in hell.
• Irony: Sans is a skeleton who is excellent at reading facial expressions. Hmmmm....
• I Surrender, Suckers: Do you enjoy sparing people only to kill them when their guard is down? Well, if you think Sans is being sincere when he offers a truce to a mass murderer like you... prepare to get dunked on. Why would he let you go so easily if you're going to end everything?
• It Never Gets Any Easier: It's strongly implied that the reason Sans acts so bizarrely nonchalant on most Neutral runs, chatting it up and being friendly with you despite the fact that you could be killing people left and right, is because he knows he exists within a "Groundhog Day" Loop. Once you're done, you're almost certainly going to reset the game and send everything back to the way it was before, so he doesn't waste time mourning or fearing for his people's future. However, there is still one thing you can do in a Neutral run that will never fail to cut him to the core, no matter how many times you do it: kill his brother.
• I Warned You:
  • Shortly before fighting Papyrus in the Genocide run, Sans will take you aside and give you a very clear warning: if you continue down the path you're on, you're gonna have a bad time. Should you ignore his warning and continue killing, it will be the last thing Sans tells you before he vanishes from the game entirely. Until you make it to the Judgement Hall, where, after giving you one last chance to turn back, makes good on his warning and initiates battle with you. You'll probably find yourself wishing you had listened.
  • After you cut him down, before he stumbles offscreen and dies, he gives a defeated "don't say i didn't warn you." He's very aware of what's about to happen if you continue through the next two rooms and finish the playthrough, but it's no longer in his hands.
• I Will Fight No More Forever:
  • With the little info on his backstory, the only clear part is that Sans wasn't always a "lazybones". For a long time, he did his best to "fix" the broken machine in his basement; possibly to bring back people he lost and by extension, to get to the surface. Unfortunately, the machine can not be fixed according to Word of God and Sans has no power to stop the resets. By the time you meet him, he's already resigned to his fate and doesn't care about going to the surface or seeing his lost loved ones anymore.
  • This is the main point of his "Special Attack", where he decides to not end his turn, intending to bore you into quitting the game.
• Laser-Guided Karma: While fighting him, your life bar is under a "KR" effect that slowly decreases your HP like poison every time Sans hits you. Given a few battle narration lines in the script mentioning "KARMA", many speculate that KR stands for Karmic Retribution. He also attempts to kill you by feigning a truce, much like you easily could have done for Toriel and many ordinary monsters at this point.
  "You felt your sins crawling on your back." (0-10 KR)
  "You felt your sins weighing on your neck." (10-20 KR)
  "KARMA coursing through your veins." (20-30 KR)
  "Doomed to death of KARMA!" (30-40 KR)
• Last Stand: His boss fight. Sans knows that your power to reset means that he can't beat you. But after seeing all the death left in your wake, knowing that you'll go on to destroy the entire world, and that there's no one else who can fight you, he decides he can't afford to not care anymore. Sadly, despite his best efforts to get you to give up, his laziness ends up costing him his life.
• Lazy Bum: Zigzagged, despite being one of his most obvious character traits. While in most runs, he's so apathetic as to be barely mobile, he puts more effort into stopping you than anyone but Undyne on a Genocide run. It's also implied that he busted his ass in the past to try and solve his world's problems through time and space before he realized it was simply impossible, resulting in his current depressed, hopeless state. That being said, he admits that he might just be using depression as an excuse for his faults, and his lack of energy ends up leading into his downfall when he falls asleep during his "ultimate attack", allowing the player to press the FIGHT button to deliver the final blow.
• Leaning on the Fourth Wall: More than a bit Medium Aware. He uses this to full effect when denying you your turn in the battle against him, thus denying you the ability to actually progress through the game. Also extended to how he treats you reloading your SAVE file, which he seems to observe from an in-universe perspective. While Flowey is actively aware of how you handle the timeline and abuses savestates to skip around, Sans seems to treat your different loads as alternate timelines and simply skips forward in battle without any visible justification. The main method of figuring out you've reloaded? Not through hard evidence, but simply recognizing how you react to different things. "That look on your face...it means something."
• Leitmotif: The appropriately titled "sans." is a laid-back, quirky theme that suits him well. The song that plays when you have dinner with him and "Song That Might Play When You Fight Sans" are remixes of this song.
• Lethal Joke Character: Sans is described as the easiest enemy who can do only a single point of damage per hit. As it turns out, what this really means is 1 ATK per frame. And boy, does he take advantage of this to the extreme. Not only does he attack at ridiculous rates and in ridiculous amounts, but also, the 1 ATK means his attacks pierce Mercy Invincibility to deal constant damage and leaving a poison effect. And they're some of the most intense in the entire game, because of the speed and range of his attacks (in some cases, there's literally just one tiny spot in the box where you can hide). He leaves his 1 defense point moot by constantly dodging one hit kill swings.
• Let's Get Dangerous!: In a Genocide run, he comes to realize he can't stand by anymore, and resolves to finally do something to stop you.
  Sans: all i know is...seeing what comes next...i can't afford not to care anymore.
• Losing Horns: When the Annoying Dog eats Papyrus' stash of bones and gets away, Sans peeks out of his room and plays a few mocking notes on a trombone.
• Mad Eye: During his boss fight, his left eye changes from black with a small white iris to a black sclera, blue iris and pin-point black pupil, which is remarkably scary given how his right eye doesn't match.
• Mad Libs Catchphrase: On a Genocide run, he's frequently mentioning to "have a bad time".
  • Before fighting Papyrus, he warns you not to keep your merciless streak up:
    "if you keep going the way you are now... you're gonna have a bad time."
  • He repeats the line again at the Judgement Hall as a Badass Boast:
    "do you wanna have a bad time? 'cause if you take another step forward... you are REALLY not going to like what happens next.
  • The line is used a third time in his fight by the battle's flavor text.
    "You feel like you're going to have a bad time."
• Magic from Technology: While he does have powers similar to Papyrus, Sans doesn't have the power to SAVE or Ripple-Effect-Proof Memory. He shows that he can make up for it with a scientific background and, if you can figure out how to access it, what is heavily implied to be a broken time machine in the locked room behind his house.
• Manchild: Not nearly to the extent of his brother, but he definitely has shades. On top of having a very goofy sense of humor, his work ethic, cleaning habits, and general lifestyle are all reminiscent of a lazy, irresponsible teenager. Unlike Papyrus, however, this has a lot less to do with his actual maturity level and lot more to do with him having very little motivation in life to do much of anything due to a lot of unresolved emotional baggage.
• Man of a Thousand Voices. In-Universe example. He does a pretty good Toriel impression, insofar as you can tell from the sound of text scrolling. According to Toriel, he also does a great Papyrus impression (Toriel is able to recognize Papyrus via his voice).
• Marathon Boss: Yes, he only has 1 HP. But you're forced to use 22 turns just to get a chance at hitting him, not including turns spent to heal.
• Meaningful Name:
  • He has a very comical personality, and his speech is written in Comic Sans. He's also very lazy, and Comic Sans is often seen as a lazy choice. The font itself is infamous for being misused in horribly inappropriate situations, and Sans himself doesn't quite fit the tone of his typeface once you know more about him, and some of the more serious moments he has are still written in said font. Subverted when he's being really serious, where it changes to a more conventional, "serious" sans-serif font (8bitoperator).
  • "Sans" on its own means "without" or "lacking". Which fits his hopeless view on the world; poor Sans is lacking in a lot of things.
  • Played With for his battle theme, "Song That Might Play When You Fight Sans". It doesn't play. "MEGALOVANIA" is the song that does play when you fight him, but Sans isn't a megalomaniac. By that point, the psychotic power-hungry maniac is YOU. Would be an Aversion of the trope if not for the fact that it says it might play when you fight Sans, not that it will.
• Medium Awareness: Sometimes breaks the fourth wall as part of a gag. He knows much more than he lets on, however. For instance, he's aware that a God-like being he has no hopes of meaningfully communicating is manipulating and reseting reality at will. This is the reason why he's so apathetic and lazy — the only alternative he can see is living in constant paranoia that the world will reset at any moment. He's also aware of how battle structure works and tries defeating you by prolonging his turn, denying you your turn.
• Mellow Fellow: He's calm and laid-back, in contrast to his excitable brother.
• Metaphorically True: His stat readout lists him at 1 attack and 1 HP. And while that is technically true, he not only refuses to let your attacks land, but his attacks continually deal 1 HP for every instant their hitboxes make contact with you and apply a poison effect that ticks down at — you guessed it — 1 damage per frame.
• Mid-Battle Tea Break: Halfway into the battle, he will stop attacking, pleads for you to remember him as your friend from a different timeline and to stop the entire thing there and then. You can use this opportunity to heal yourself, and can even invoke the trope literally if you still have the Sea Tea. And you'll want to, as the alternative is getting dunked on.
• Mind over Matter: His ability to slam your character's SOUL against the inside of the battle box via hand motions seems to suggest he can do this.
• Minored In Ass Kicking: Sans would very much prefer to sit around and make jokes all day, only stepping into your adventure once in a while to share food or friendly advice with you, but if pushed far enough, Sans becomes the single most dangerous monster in the underground.
• Mundane Utility: Uses both his time manipulation and his incredible guesswork to set up a prank that transcends time. And according to Papyrus, he does things like this frequently. He also teleports himself and friends anywhere and everywhere for any reason, the most common reason being "a cheap joke".
• My Friends... and Zoidberg: A schedule of events can be found at the MTT Resort advertising "performers, comedians, and Sans" implying they don't consider him to be either.
• My Rules Are Not Your Rules: Sans can break the rules of the entire game by ignoring your Mercy Invincibility, randomly jumping from attack to attack, dodging (which is something other characters can't do), and killing you when you accept his offer to spare him.
  • Can actually be seen as an inversion of this. Almost everything he does is really following your rules to the letter. He gets your ability to attack first. He can dodge during his turn, just like you're able to do. If you offer to spare him, he'll instant-kill you, just like you can do to the other bosses. He can even take as long as he wants during his turn, just like you. The only real exception to this would be attacking you during your turn, but you win the fight by attacking him during his turn, so you're capable of the same thing after all.
• Mysterious Backer: Pops up here and there to provide you help in the form of advice, emotional support, affordable healing items, and ambiguous threats.
• Mysterious Past: The game has deliberately-placed hints about his prior life littered throughout it, all of them vague but poignant in implication. There's a reason he and his relative/friend/co-worker/something W.D Gaster have spawned more speculative fan material than any other characters.
• Mysterious Stranger: A very unconventional example, in that he treats you like an old friend from practically the moment you meet and outwardly acts very friendly and laid-back. Because of this, it's not easy to notice just how little we find out about him as the story progresses or pick up on the subtle hints towards just how much he's hiding from you, but every once in a while, you get an unexpected reminder.
• Mysterious Watcher: He keeps a sharp eye on you throughout your entire journey, as you indirectly find out in the Judgement Hall. Not counting subtext, there is very little foreshadowing that he's doing this, with the exception of a single noteworthy moment that the player is very unlikely to run into unknowingly.^note If Papyrus is dead, hum to Shyren twice during your encounter with her, and you will receive the following battle text: "A hooded figure watches the commotion from afar."

N-S

• Neutral No Longer: Though he will call you out for any murders you commit (especially Papyrus), he won't try to fight you unless you've committed to killing everyone in the game.
• Nice Guy: Occasional creepy and depressed moments aside, he's very friendly and always treats the player like an old buddy, even after they just met. Makes sense after all, he's well aware that this most likely is not the first time he's met the player; him befriending them is his way to make a potentially genocidal player think twice about going through with it. He has his limits, however.
• Nightmare Face: A subtle example — if he's threatening you or being very serious with you, the shine in his eye sockets vanishes, which makes him look less like a friendly cartoon skull and more like, well, a talking dead person.
• Nonchalant Dodge: Sans can sidestep your attacks, and wonders why no one else tries it... But then it's subverted to disturbing effect when the Fallen Child takes over and slices him in two despite his attempts to dodge.
• No-Nonsense Nemesis: Granted, it takes a lot to push Sans into dropping the nonsense and becoming your nemesis, and even that is only because he made a promise to Toriel, but once you've crossed the final line of no return and there are no other options left, he'll cut out the puns and... well, the Combat Pragmatist section up there can tell you all about it... "You feel like you're going to have a bad time" indeed.
• No-Sell: Sans' battle is designed so that none of your equipment will give you an edge — not even the Real Knife or the Locket — as he dodges every one of your immensely powerful attacks and pierces your defense via Death of a Thousand Cuts. You can still get healing boosts from the Burnt Pan (Stained Apron cannot be obtained in the Genocide Route due to the barrier preventing you from reaching it) or gain a slight few Mercy Invincibility frames from the Torn Notebook and Cloudy Glasses, but it doesn't count for much.
• Non Sequitur, *Thud*: A rare Played for Drama example. In the Genocide route, should you defeat him, he'll say "welp, i'm going to grillby's. papyrus, do you want anything?" At this point in the Genocide route, Snowdin — the town Grillby's is in, had been reduced to a ghost town, and Papyrus has long since died thanks to you.
• Not So Stoic:
  • While he does have a subtle emotional range throughout the entire game, the one and only time where he truly and genuinely loses his cool is near the end of his boss fight. Should you survive his penultimate attack, he, in a fit of what is clearly rage and/or desperation, turns your SOUL blue and begins slamming it around the battle box like a child flinging around a toy, a stark contrast from the quick-but-controlled way he'd been fighting the entire battle.
  • During the final battle of the Pacifist route, not only is Sans not using his usual speaking font while a Lost Soul, but he's also the only character whose dialogue text is shaking. Combining this with with the fact that it's the only time in the Pacifist route where he fully opens up and reveals just how much pain he's been hiding, it's very possible that this represents Sans showing genuine weakness in his voice for the first and only time in the game, maybe even to the point of being in tears.
• Obfuscating Stupidity: Sans appears to be a regular goofball — unless you take a No Mercy/Genocide route, where he threatens you. And in Genocide, he challenges the player to an extremely difficult boss battle. In addition, the contents of his workshop and his Offscreen Teleportations imply he's a quantum physicist with a working time machine, who has a working knowledge and connection with the space-time continuum in-game.
• Odd Friendship: With Alphys. At the end of the True Pacifist run, they apparently know enough about each other to play a game of "Jinx", with her beating Sans to the punchline since she knew what kind of joke he was gonna make. This is despite never having much onscreen interaction throughout the game, and Papyrus expresses surprise that they've met before. Alphys stumbles to make an excuse for why, and Sans brushes it off by saying everyone knows him.
  • It becomes less odd-sounding when you discover a couple commonalities between them; namely, Sans' scientific background, his potential connection to the previous royal scientist, and how both Alphys and Sans have had run-ins with Flowey in the past. invoked Toby Fox has also implied that she might have been trying to help him fix the machine in his basement.
  • The XBox port expands a bit on this at the casino. He recalls having asked her about game recommendations for Toriel in the past, and he was apparently around when she was collecting scrap to build Mettaton.
• Offscreen Moment of Awesome: Sans vs. Flowey. We never get to see it, but Sans apparently caused Flowey 'more than his fair share of resets' in the past, so much so that he will outright tell you that avoiding Sans is the best advice he's got.
• Offscreen Teleportation:
  • Sans can warp around when out of sight and often shows up way ahead of you through the areas for no explained reason. The abilities he displays during his battle suggest he has some control over time and space like you and Flowey do.
  • Subverted for once in the Genocide Route. He tells you that you will have a bad time if you continue, then the screen blinks, and he's gone. It makes for a very creepy moment.
• Oh, Crap!: His expression just before he is sliced in half.
• Older Than They Look: His age is extremely vague, but he's clearly an adult. Despite this, his overworld sprite is almost eye-to-eye with the player character's, and his presumably more-proportionate battle sprite is still squat enough to give the impression that he's a child.
• One-Hit Kill: Partway through the fight, Sans will offer to spare you. This is a trap; if you accept this offer, Sans will fill up the combat box with bones arranged in a jail-cell pattern that can't be dodged, taking off all your health and resulting in a game over.
• One-Hit-Point Wonder: Deconstructed. Checking Sans' stats reveals that he only has one HP and his attacks only deal one damage, but he is still the hardest boss in the Genocide Route, thanks to him dodging all your attacks and his attacks ignoring Mercy Invincibility. ^note In practice, he must be attacked 22 times, as the battle only advances when he dodges your attack. However, because you never actually hit him until the very end of the fight, your ATK stat doesn't matter.
• O.O.C. Is Serious Business:
  • Sans is almost always cracking wise or lazing about, seemingly without a care in the world. Yet he has his moments of gravity, and when it comes time to deliver them, his pupils fade into darkness, his Voice Grunting disappears (often taking the background music with it), and he loses his all lowercase letters speaking style. He even changes fonts (he usually speaks in Comic Sans, but switches to the sans-serif 8bitoperator for his serious lines). Also, if it's your fault that Sans finally decides to stop being lazy, well... you're gonna have a bad time.
  • Also, he avoids hard work, even if you kill all the leaders… but his love for his brother is stronger than his sloth. If Papyrus becomes king, Sans will work really hard for him.
• Optional Boss: Zig-zagged. Sans is required to finish a Genocide run, but attempting one is not required (nor is it recommended) to get True Pacifist or even Neutral. Just getting to Sans requires killing every monster in the Underground, which is easier said than done due to the increasingly sporadic random encounters and Undyne the Undying. By contrast, Omega Flowey and Asriel are much more straightforward with how they're fought, and both have to be beaten to achieve True Pacifist.
• Orphaned Setup: Due to coding error, some of his lines from Genocide in the earlier versions of the game didn't have the intended punchline, as below (the missing parts are in bold).
  Sans: that's the expression of someone who's died five times in a row. convenient, huh? that's one for each finger. but soon... you'll need a cool mutant hand to count all of your deaths.
  that's the expression of someone who's died six times in a row. that's the number of fingers on a mutant hand. you'll need to find a mutant hand with even more fingers.
  that's the expression of someone who's died seven times in a row. hey, that's good. seven's supposed to be a lucky number. who knows, maybe you'll hit the jackpot... and that number will multiply tenfold.
  that's the expression of someone who's died eight times in a row. that's the number of fingers on a spider. wait, don't spiders have legs?
• Overly Long Gag:
  • At his hot dog (and hot cat) stand, if your inventory is full, he'll start putting hot dogs on your head instead. He'll keep stacking them until you have 30, at which point the stack of hot dogs is going off the screen.
  • He'll count how many times you've died to him on the Genocide run, with a snarky comment for each one of the first eleven attempts. Only after 12 deaths in a row will he stop coming up with unique lines.
• Painting the Medium: To start, Sans speaks in comic sans. Secondly, he spends most of the game talking in all lowercase letters... but at random he'll leave his I's capitalized, and he will sometimes use apostrophes correctly but sometimes won't. Yes, Sans is so lazy he can't even use all lowercase letters consistently. Or to put it another way, Sans speaks like he's typing but is too lazy to type properly; now remember that Toby Fox had to type all of Sans' dialogue.
• Pay Evil unto Evil: During his fight in a Genocide run. By this point, you've basically committed mass-slaughter and killed Papyrus, who wasn't fighting and wanted to help you. Sans puts you through absolute hell, without hesitating to cheat and play unfairly, even faking a spare.
• Perfectly Cromulent Word: At one point, he asks if you want to help him find out what goes after "thrice".
  Sans: Quice? Frice? Welp, won't have to use it again anyways.
• Perpetual Smiler: Sans always has a mysterious smug, sneaky air about him, at least when he's not exuding a Tranquil Fury attitude instead. Even when his eyes are giving away a different emotion, his giant grin never moves, most likely because he's a skeleton. The closest his grin ever changes is when your blow finally connects, and even then it's just barely.
• Pintsized Powerhouse: He's about as tall as the Child, and is undeniably one of the strongest bosses in the game.
• Platform Hell: Most of his attacks are done while your heart is Blue; they're fast, tricky to dodge, and deal a crapload of damage.
• Player Nudge: During his "ultimate attack", attempting to move the SOUL to the left will make him teleport it to the center of the bullet board. This is a hint to defeat him: the player needs to wait until he falls asleep, then use the SOUL to push the bullet board to the lower left corner where it can then press the FIGHT button to attack during Sans' own turn.
• The Prankster: Not only does Sans love bad jokes and puns, he also likes to pull harmless pranks on the protagonist. Whoopee cushions in the hand and on the chair, loosening the cap on a ketchup bottle and putting paint in a telescope's eyepiece stand out.
• Pre Ass Kicking Oneliner: "it's a beautiful day outside. birds are singing, flowers are blooming... on days like these, kids like you... Should be burning in hell." This is even played with to mess with you even further — he'll cut the speech off mid-sentence and immediately jump into his opening routine entirely at random.
• Press Start to Game Over: His first attack is not only extremely powerful, consisting of fast patterns that can take you from full to zero health before the end, but he precedes your turn, making it very likely that he will kill you before you even get to the battle menu!
• Pungeon Master: Sans enjoys harassing Papyrus with bad skeleton-related puns.
• Rage Quit: Invoked Trope. The whole point of his boss fight is to make you do this. If being a batshit-hard boss doesn't do the trick, then he hopes that simply extending his turn infinitely by doing nothing will cause you to give up in frustration.
• Reasoning with God: You, the player, are effectively the god of the world of Undertale and are treated as such in-universe. In battle, Sans reaches out to you personally and pleads with you to call off your current playthrough, reminding you that you were once friends.
• "The Reason You Suck" Speech: Delivers one to the player in the second stage of his battle, questioning why they want to destroy his world so badly. Given that this happens after his attempts to stop the Player Character through trickery and just plain making the battle difficult have failed, it doubles as an attempt to Break Them by Talking.
  Sans: "i always thought the anomaly was doing this cause they were unhappy. and when they got what they wanted, they would stop all this... but that's ridiculous, right? yeah, you're the type of person who won't EVER be happy."
• Red Oni, Blue Oni:
  • The blue to Papyrus' red. While Papyrus is energetic and passionate, proclaiming to anyone who will listen about how he will not rest until he has captured a human and joined the royal guard, Sans spends all his time slacking off and doesn't give a damn about his job or duties. Visually, Papyrus dresses in red and speaks in ALL CAPS while Sans wears blue and never uses upper case letters at all.
  • In the endgame, he's also the Blue to Toriel's Red, the other Pungeon Master with an aloof personality. When they first meet in person, Toriel is very expressive and excited while Sans remains calm and unreactive even when he's clearly pleased to finally meet her. Her being charmingly out of touch with monster society after spending so long isolated in the Ruins is accentuated by her dichotomy with Sans, who's more knowledgeable about the Underground's customs and is extremely popular despite having no close friendships with anyone but his brother.
• Relative Button: While he will rightfully reprimand you for any evil that you do, the only way to truly get on Sans' bad side is killing his brother Papyrus. And oh boy, is it a bad side to get on.
• Retired Badass: Setting aside the question of just how in the world he got to be such a badass, the game also strongly implies he used to be a Science Hero of sorts. He even has a badge of some sort lying around in his secret room, suggesting his role was official to some degree.
• Right for the Wrong Reasons: He's correct in that you, as the Anomaly, are causing timelines to reset, given that the player's likely gotten the Neutral and Pacifist endings before going down the Genocide route (never mind Save Scumming). However, Sans has no idea about Flowey's actions prior to the player's intrusion into events, meaning he's accusing you of all the prior resets that induced his apathy in the first place as well.
• Ripple-Effect-Proof Memory: Although most characters in the game have a severely downplayed example of this, only allowing them to retain subconscious, deja-vu-esque memories from previous timelines that serve no practical purpose, Sans took it a step further by apparently training himself to read people on a borderline-Orwellian level. Just by studying your facial expression, he can make a number of accurate guesses as to what you've been up to in your previous playthroughs, including if you ever had the nerve to kill his brother. He never reveals the full extent of this ability unless you make it to the final battle with him, where he's able to, among other things, keep track of the exact number of times you've died up to your eleventh attempt just by glancing at your face.
• Route Boss: Sans is only fightable on the Genocide route, serving as its Final Boss.
• Sad Clown:
  • Alluded to when facing him as a Lost Soul, and explicitly stated when fighting him on a Genocide run. His knowledge of the player's and Flowey's interference with the timeline (through saving and resetting), and the belief that things will inevitably be reset again in the future, has left him exhausted and existentially depressed. It's implied that this also has something to do with the strange machine Sans is working on...
  • Sans also displays a degree of self-loathing, most notably in the ending where you kill all of the important characters and at least one other monster, leaving the Underground utterly bereft of leaders. Sans talks about why he didn't take up the throne and explains that it isn't his style, and that he prefers to take things easy... before explaining that that was a joke and the whole mess they're stuck in is exactly what happens when "people like [him]" take it easy.
• Scratch Damage: His attacks only deal one point of damage per hit. This means it practically ignores your Defense, and considering it hits per frame while leaving a Damage Over Time component, this is a lot more dangerous than it sounds.
• Sealed Evil in a Duel: Attempted. His 'Special Attack' is simply to start his turn... and never end it. He outright says that he intends to just sit there until you get bored and turn off the game. Unfortunately, he falls asleep after a while, allowing you to finally kill him.
• Sensitive Guy and Manly Man: The manly man to Papyrus' sensitive guy, being far less emotional and more traditionally masculine.
• Series Mascot:
  • While Flowey has been pushed to being one, Sans has a lot of icon power on The Internet, to the point where people who have only glanced at Undertale but never played it can probably recognize what game he's from when they see him elsewhere. Tends to happen when you're one of the most liked characters and a freakishly difficult and memorable boss.
  • He and his brother are regulars in the console port announcements.
  • In the trailers for Groove Coaster's Undertale DLC, he appears to address the viewer and promote the DLC.
    Sans: "what? haven't you seen a guy with three jobs before?"
• "Shaggy Dog" Story: Sans firmly believes his life is a big one due to resets and his mysterious failures to "go back", and excuses his laziness to his awareness of this fact. He is not completely wrong, although it is possible for you to finally put an end to it either by abstaining from a True Reset after completing a Pacifist run or by shooting his dog by playing a Genocide run.
• Sheathe Your Sword: What his special attack turns out to be — he just never ends his turn, meaning you never get yours, unless you find a way to cheat the system.
• Shoot the Shaggy Dog: What his life story amounts to if you complete the Genocide route. On top of his past failures and the life he endured, the last effort he puts in order to stop the anomaly ends tragically and The Fallen Child destroys the world. Even if you restore the world afterwards, the Golden Ending is always doomed to end with the Fallen Child taking over and killing everyone, including Sans.
• Sibling Team: Sans works with his brother Papyrus to capture any humans that come into Snowdin. Well, Sans doesn't work, per se, but he's always around to crack a joke while Papyrus fusses about.
• Sibling Yin-Yang: As mentioned previously, although they do have some important similarities, he and Papyrus contrast each other in almost every way.
• Signature Move:
  • His opening attack, which is a string of extremely powerful attacks and, more importantly, goes before your first turn.
  • His "ultimate attack": He simply stands there and does nothing, in hopes of boring you into giving up on your omnicidal ways.
• The Slacker: Sans takes laziness to an art form. At one point he converts a guard booth into a makeshift shop to hawk food to passersby. As an impressed Papyrus notes, he's not doing his job by doing another job.
• Sleepyhead: He tires easily according to Papyrus. While the claim is slightly suspect due to Papyrus being such a boundless well of energy he thinks seven hours of sleep is too much for one night, it would be very in-character and we do see him sleeping on the job on one occasion. It's unclear how much of it comes from him just being lazy that way, how much of it is from him secretly working his tailbone off behind the scenes, and how much of it stems from his depression, but it's likely some combination of the three.
• Smash Cut: The way Sans' teleport seems to work is by smashing in a few black cutaway frames and then cutting back to reality. He liberally begins using it during the second phase of the fight against him in order to smash cut you out of and into various attack patterns.
• Smug Smiler: One of the better tropes one could use to describe his default attitude. While it's usually a lighthearted example to emphasize his status as The Gadfly and a Deadpan Snarker, it takes a turn for the deadly should you manage to drive him to the point of having to fight you.
• The Social Expert: Most would consider him to have the most normal social etiquette of any character in the game, and he is very popular with the other residents of Snowdin. Pretty much everybody knows him by name and his mailbox is always jam-packed. Taken to the extreme when it's revealed in the Genocide route that he can read people so well he can practically read their minds, all the way down to extremely nuanced detail.
• So Proud of You: To Frisk during the judgment if you don't kill anyone. To a lesser extent, he also does it if you abandon a Genocide Run by sparing Papyrus when you encounter him:
  Sans: hey, i really respect what you did back there. thanks.
• Sound-Only Death: Sans is the only character you can kill who doesn't die onscreen. After you deliver the final blow, he resigns to his fate, then shuffles offscreen. Seconds later, you hear the "monster turns to dust" sound.
• So Unfunny, It's Funny: Both in-universe and out. Even Papyrus can't help but crack a smile at his terrible puns.
• Sour Supporter: Part of what's Beneath the Mask, Sans is very good at hiding this side of him throughout the Pacifist route. He sort of implies his disbelief towards the protagonist's desire to get back home during his dinner date, but it's not until the last minute that we understand the extend of his cynicism. And this is if you didn't kill his brother; if you dare to kill Papyrus, he disappears from the game entirely, and if you didn't invoke a Genocide run, the only support you'd ever get from him would be not being brutally murdered by him.
• The Spook: Apparently he and his brother just sort of showed up in Snowdin one day. With Papyrus, it's less apparent, since he wears his heart on his shoulder, but Sans keeps most people at an arm's length, and it's never made explicitly clear just how much he knows about (or is involved in) any given plot point, though it's at least a fair bet to say "quite a lot".
• Squishy Wizard: While all monsters are this to some degree, Sans takes this trope up to eleven:
  • He is a statistical one (1 attack, 1 defense, and 1 HP) but uses convoluted magical attacks with no Mercy Invincibility to compensate.
  • He's also this trope offensively: his final move before his special attack is a brutal beatdown which only takes 1 HP per hit until your character only has 1 HP left. After that, his moves only smash you on the walls without any effect.
• Status Effects: KARMA has a damage-over-time effect, similar to a poison/bleeding effect. The more you've been hit, the more damage you take from it.
• Stealth Pun:
  • He's not fat, he's just big-boned.
  • After the Annoying Dog steals Papyrus' bone, Sans sticks his head out of his room and plays Losing Horns on a trombone.
• Stepford Smiler:
  • During normal gameplay, there are many small hints that by themselves wouldn't mean much, but taken altogether, paint a bleak picture of his everyday life; he overeats unhealthy comfort food, sleeps far too much, puts bare minimum effort into his work, has very few close friends despite appearing popular, and may or may not be a regular drinker. None of this is surprising on the surface — it's just the sort of thing a lazy slob like him would do — but when you contextualize it with the little information we're given on his backstory and the fact that he's living with the knowledge that his life is a "Groundhog Day" Loop where nothing really matters and he'll never get the chance to move forward, and it goes from being amusing to a pretty obvious implication of depression. Exactly how much this affects him varies depending on how the player affects the world, how hopeful things seem, and personal interpretation on the player's part. Even on the best possible path, he has enough awareness of just how easily things can be turned back that it's very likely he'll be fighting an uphill battle to become his old self once he's living on the surface.
  • This is also his stance if you spare Toriel but kill Papyrus: he tells you that he doesn't have the heart to tell her that you killed his brother because she still has love for you. But still warns you that you are not welcome in the monster world anymore.
• The Stoic: Downplayed in that his casual, comically-smug attitude doesn't change very much no matter what mood he's trying to get across. His emotional range in general is very subdued, even on the rare occasions where he's clearly feeling otherwise.
• Stopped Caring: He doesn't seem to care about anything aside from bad jokes and his brother. He's figured out that someone or something has been resetting and manipulating the timeline, and has given up trying to stop them... aside from befriending everyone he meets in the hope that one of them is the source and he can finally make them contented enough to stop.
• Stronger Sibling: Ambiguous. On paper, Papyrus has significantly higher stats than Sans, but then again, on paper, everyone has higher stats than Sans, "the easiest enemy". But Sans has an enormous arsenal of attacks, including Wave Motion Guns, access to Karmic Retribution, the ability to dodge, and the ability to Interface Screw the player to hell and back, making him potentially the more skilled sibling if not the stronger one. However, it is left ambiguous whether Papyrus lacks Sans's capabilities, or simply the combat pragmatism to use them in a fight. If the player stops a previously No Mercy/Genocide run on Papyrus, his dialogue implies that he may have access to Gaster Blasters, yet he never displays them in a battle.
• Superboss: Sans' fight is completely optional on most runs, and getting to him takes a lot of time and patience required to kill everyone in the Underground. However, once you meet the criteria for his fight, he's easily the hardest boss in the entire game with his attacks doing one damage per frame, said frames causing Damage Over Time, and an assortment of quick attack patterns that include Gaster Blasters, his brother's Blue Attack, and messing with your screen for one heck of a bad time.
• Survivor Guilt: There's vague but plausible breadcrumbs of evidence within the game that, when strung together, suggest that he's the only survivor of some great catastrophe that either killed or permanently separated him from the people featured in his scrapbook, contributing to the cynicism and self-loathing he lives with today.
• Symbolic Blood: When he's killed at the end of a Genocide Run, a red liquid that looks like blood starts dripping from his wound. However, it's been clearly established that monsters don't bleed when they die, and simply turn to dust. Because of this, it's unlikely that the substance is actually blood (unless Sans isn't really a monster) — it might be ketchup.

T-Z

• The Tape Knew You Would Say That: If you refuse his false offer of mercy after having previously fallen for it, he not only immediately guesses what must've happened just from the pissed-off look on your face, but references what the other timeline's version of him said to you during the game over screen despite affirming that he is a completely separate person from that version of Sans.
• That One Boss: In-universe; according to Flowey, whose sociopathy and ability to SAVE drove him to start treating the Underground like an RPG, Sans caused him "more than his fair share of resets." He's likely to do the same to you in a Genocide run.
• This Is Gonna Suck: If you survive his opening barrage in the boss fight, he immediately opens his eyelights again and says "here we go".
  "You feel like you're going to have a bad time."
• This Looks Like a Job for Aquaman: His stats are the worst of any monster in the game, including a literal child, and he can only deal 1 damage to the player with each attack. Howevever, his special ability being called KR seems to imply that he is specifically effective against the player at the end of a Genocide Run because of all the LOVE collected before fighting him.
• Threshold Guardian: He blocks your path in the Final Corridor, and will judge you for all you have done across the game. If you pass with flying colors, he will basically say 'nice work'. If you've been engaging in Video Game Cruelty Potential, especially if you kill Papyrus, he has some Player Punch to deliver. If you've been violent without going into outright Kick the Dog territory, he basically leaves you to judge yourself unless you press the issue. And if you've been going for the Genocide route, he gets off his ass to stop you personally.
• Time Police: Some of his dialog when fighting him implies that he operates as something akin to this (or did once, in any case), as he received reports from someone about changes to the timeline, implying that he's part of formal organization that has been tracking the manipulation of time.
• Took a Level in Cynic: After Flowey's constant resets, Sans lost the passion he once had. He also has a broken machine in his basement that according to Word of God can not be repaired, no matter the amount of hard work Sans put in to fix it. One can only imagine the pain he went through to get to the point of not caring about anything anymore.
• Took a Level in Idealism: Thankfully, the poor guy seems to be in a healthier state of mind by the end of the Pacifist route, or at least, on the road to recovery. He expresses genuine faith in the protagonist during the final battle, something that he almost definitely doesn't do lightly given what he's been through, and if you check his secret room during the Playable Epilogue, the scrapbook containing photographs of all the mysterious people he lost now has a photo in it of him surrounded by all of his new friends.
• Took a Shortcut: A Running Gag of his. Despite staying behind each time you encounter him (or even walking in the opposite direction that you're going), he somehow always ends up ahead of you later. He can take you with him as well on two occasions (where he even quotes this trope verbatim). His boss battle reveals that he does this by way of teleporting, an ability that fans have dubbed "shortcuts".
• Trademark Favorite Food: He seems to like ketchup a lot — we see him down a whole bottle of it at Grillby's, and he keeps a container of it at his sentry station (along with other condiments like mustard and relish). It's possible that the Symbolic Blood leaking from his body after you kill him is also ketchup, since it has the exact same color.
• Tragic Dream: We don't know exactly why, but he really, really wants that machine in his secret room fixed. Sadly, Word of God has stated that this will never happen, and he's long given up even trying anymore.
• Tranquil Fury: Sans, of course, is a very chill skeleton... and the most chilling thing about him is his no-nonsense font and lightless sockets if you manage to piss him off. Exaggerated during the final battle with him, where he's still grinning with one hand in his pocket as he pulverizes you with the other one, one of his eyes literally flashing with rage.
• Trash of the Titans: Within Sans's room is a mini tornado. Examining it reveals the following:
  Narration: It appears to be a self-sustaining tornado made of trash.
• Trickster Mentor: Thoughout a Pacifist Run, Sans will discreetly give the Human Child occasional hints on how to progress through the underground.
• Troll: Screwing with people is pretty much his M.O, and since it's normally in good nature, one of the more subtle ways he lets you know that you've pissed him off is when he starts taking a page from this trope. One of the best (and arguably most hilarious) instances of this is during his boss fight, should you accept his offer of mercy (and get hit with the subsequent — and completely unavoidable — One-Hit Kill attack). The "Game Over" music will be replaced with a sped-up version of the goofy "Dog Song" track as Sans mocks you. Reload the game and talk to him again and he'll mention how incredibly pissed off you look.
  Sans: geeettttttt dunked on!!
• Trust Password: If you reload a save after listening to his judgment during a Pacifist run, he will give you a set of words, if you reset and talk to him again, will utterly fail to convince him that you're a time traveller. He will, however, give you a second password that, when reset and told to him again, causes him to give you a key to his room. The passwords are, respectively, "I'm a stupid doodoo butt" and "I'm the legendary fartmaster."
• Turns Red: While other bosses get gradually more difficult, Sans does his difficulty spike all at once. Once you reject his offer of mercy, he'll step up his game. This is the part where he outright starts cheating, unleashing combos that require you to do different things and attacking you while it's your turn.
• Unblockable Attack:
  • If you fall for his false offer of mercy, he kills you with a cage of bones that fills up the Bullet Board and is impossible to dodge.
  • The final attack in his ultimate gauntlet of attacks has him slamming your SOUL into the borders of the Bullet Board again and again, depleting your HP rapidly. Fortunately, it cannot actually kill you.
• Unstoppable Rage: He unleashes hell on you for all the innocent lives you've taken, and if you went and killed Papyrus as well, he's already let you know that you have exhausted his goodwill.
• Unexpected Gameplay Change: Don't like platform games? Too bad. Sans does!
• Unexplained Accent: As mentioned above, he seems to have some variety of New Yorkian accent. Pretty weird coming from a guy who's spent his entire life underground among monsters who mostly lack readable accents, especially since his own brother doesn't even have one.
• The Unfought: You never fight Sans on a Neutral run, especially if you have aborted the Genocide route. Downplayed and implied in True Pacifist as a lost soul. He is forced to "fight" the player in this state, but while it is hard to tell ingame, only Papyrus's bone attacks appear onscreen, indicating that Sans doesn't even bother to attack you.
• Vehicle-Based Characterization: During the credits, Sans rides a tricycle faster than Papyrus drives a car. This little detail shows his Crouching Moron, Hidden Badass personality.
• Video Game Cruelty Punishment: He will judge you near the end of your journey, and he has some choice words should you engage in Video Game Cruelty Potential. And if you've been going for the Genocide Route, he bars your path as the single toughest boss in the game.
• Vitriolic Best Buds: He deliberately annoys his brother Papyrus almost every second they're on screen together, who constantly nags and complains about him back, but it's always in good nature and they're nothing if not completely dedicated to each other. Sans even mentions in an optional piece of dialogue that he appreciates Papyrus' nagging, since it's the only thing that actually motivates him to do anything productive.
• Voice Grunting: A goofy, low-pitched voice clip, which is actually sampled from Patrick Star. When he's being super serious about something, his Voice Grunting disappears entirely, causing his text to be delivered with total silence.
• Walking Shirtless Scene: Maybe; both his overworld and battle sprites are designed in an ambiguous enough fashion that it's possible to see him as wearing nothing underneath his jacket. Specifically, it depends on whether you interpret the pattern on his front being a white shirt/sweater and the individual teeth of his zipper, or as a sternum and a set of ribs. It's worth noting, however, that these sketches from Temmie Chang (who did not design Sans) are the only piece of semi-official artwork that use this interpretation, and most official art and merch has a t-shirt underneath
• Walking Spoiler: There's quite a bit more to Sans than just the joke-loving skeleton you meet early on. Those avoiding spoilers take heed: even glancing at fan art of Sans will almost certainly give away something about his character. And, y'know, there are many Spoiler marks here for a reason. To sum up, Sans is one of the two Fourth Wall Observers, the Final Boss of the Genocide Route plus how he fights, and the fact he warns you if you win that it's the Point of No Return.
• Wave-Motion Gun: His "Gaster Blasters", which are skull-like cannons that fire off massive beams in a straight line with little to no time spent to let you get a sense for their angle. Sans is capable of summoning multiple of them at once of varying sizes and patterns to catch you out. They're easily one of his hardest to dodge attacks, and given the entire fight is ridiculously hard, that says something.
• Weak, but Skilled: Technically speaking, he really is the weakest enemy in the game, having only 1 HP and only capable of doing 1 damage. He makes up for it through clever abuse of game mechanics. He only has 1 HP, but he dodges all of your attacks instead of just taking them like every other monster in the game. His 1-damage attacks don't trigger the Mercy Invincibility, so he does 1 damage per frame. That's not even getting into his SPARE trick, or his "special attack". Indeed, the only reason you can even kill him is by breaking the rules yourself — dragging your SOUL over to the FIGHT button during his turn.
• Wham Line: Sans has several lines that are much more serious than what he usually says. These are often accompanied by the background music cutting out and Sans' talksprite changing to one with no eye lights.
  • "You'd be dead where you stand." — referencing that if Toriel hadn't asked him to leave you be, he would've instantly killed you the moment he saw you.
  • "LOVE, too, is an acronym. It stands for Level of Violence." — Near the end of the game, Sans reveals that the EXP and LV stats refer to something much different than in other games. EXP stands for "execution points" and quantifies the pain you've inflicted on monsters, while LV/LOVE stands for "Level of Violence" and measures how easy it is to kill them. The one hint of this being the case happens early in the game and is easy to miss, so Sans spelling it out at the end can easily come out of left field.
  • "if you keep going the way you are now... you're gonna have a bad time." — Said on the Genocide run, in place of what was previously innocuous advice about the upcoming battle with Papyrus. The screen then quickly cuts to black and back, and Sans is gone when the view of Snowdin returns. This is the only time in the game Sans is seen teleporting on-screen; every other time, he goes offscreen and ends up somewhere he shouldn't be able to reach from the direction he went.
  • "here, i'll give you some advice about fighting my brother. don't. capiche?" — Said on a No Mercy route, also in place of what was previously innocuous advice about the upcoming battle with Papyrus. Like with the previous example, it's an attempt to keep you from only killing so much.
  • "what? you think i'm just gonna stand there and take it?" — Upon dodging the one-hit kill strike that felled so many bosses beforehand.
• When He Smiles: Definitely not the trope you'd expect a Perpetual Smiler to have, but there's apparently a distinct enough difference between the smile Sans normally wears and the smile he makes when he's genuinely happy that it's even recognizable in a photograph. Depending on the context in which you do so, finding the photo album in Sans' room and having the description of its contents followed by a simple "He looks happy." is either one of the saddest or one the most uplifting text-only moments in the entire game.
• Why Won't You Die?: Near the end of his boss fight, he tells you that if you don't want to see his 'Special Attack', now would be a good time to die. And when you manage to weather his final barrage of bones and lasers, he goes ballistic, his eyes glowing as he slams your SOUL repeatedly to the arena borders. That look on his face is the wordless version of this trope.
• Wide Eyes and Shrunken Irises: The lights in his eyes become very small pinpricks of white after you mortally wound him.
• Willfully Weak: He has his reasons for not attacking the player, the main being laziness and that he's restrained. The Genocide run shows his true strength and power after you killed Papyrus and the all other monsters.
• World's Best Warrior: There is not a soul in the Underground more deadly than the short, goofy pun-lover. Sure, he might be the physically weakest monster there, but he'll essentially break reality to take you down.
• Would Hurt a Child: Claims he would've killed you the moment he first saw you were it not for his promise to Toriel, and on a Genocide run he will do his very damnedest to break the promise.
• Wowing Cthulhu: Very few things even remotely faze the protagonist during the Genocide route, let alone impress or frighten them. Sans is one of those things. He's also the one and only character Flowey, out of fear, respects on any level aside from the Fallen Child.
• Wrong Genre Savvy: He muses briefly why opponents never start with their strongest attack as he does in his boss fight. As the rest of the fight later shows, when his strongest move fails, he has to spend the rest of the time doing what he can to turn the tide of the battle other ways to compensate.
• Yank the Dog's Chain: In a Genocide run, he reveals that his apathy stems from a fear of this trope, because he knows that any happy ending can be undone by the player resetting.
  Sans: you can't understand how this feels. knowing that one day, without any warning... it's all going to be reset. look. i gave up trying to go back a long time ago. and going to the surface doesn't really appeal anymore either. 'cause even if we do... we'll just end up back here without any memory of it, right? to be blunt... it makes it kinda hard to give it my all.
• You Bastard!: If the player kills Papyrus on a Neutral run, they will get called out by Sans as a "DIRTY BROTHER KILLER."
• You Can't Go Home Again: During his dinner date, he mentions that he could relate to the player's desire to go back home. It turns out he isn't (only?) talking about the surface; it's implied he and possibly his brother are displaced from the time and space they once belonged to, and before the beginning of the game, he worked hard to go back to the place he considered home; this is even implied in the name of the slower version of his leitmotif, "It's Raining Somewhere Else". Alas, nothing came out of it, and he gave up and slowly grew into the nihilistic personality he displays in the game. Even though he's good at keeping his feelings to himself, during his appearance as a Lost Soul, he can't hide the extent of the despair he's in:
  Sans: just give up. i did. why even try? you'll never see them again.
• You Have Researched Breathing: He's the only character in the entire game who even makes an attempt to dodge your attacks. Sure, by that point in the game, you're evil incarnate and could kill anything you want with a single blow, but you're still a small child wielding a kitchen knife (or whatever the Child has equipped) — jumping out of the way shouldn't be rocket science. Sans even smugly points this out, which makes sense given his awareness of other mechanics in the game.
• Zen Survivor: Sans knows a lot that he's not letting on, borne from hard experience, but knowing that telling you won't do any good, he masks this and acts to guide you in oblique ways while seeming apathetic to everything.

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"well, that's all. take care of yourself, kid. 'cause someone really cares about you."