Sans

"you never gained any LOVE. 'course, that doesn't mean you're completely innocent or naive. just that you kept a certain tenderness in your heart. no matter the struggles or hardships you faced... you strived to do the right thing. you refused to hurt anyone. even when you ran away, you did it with a smile. you never gained LOVE, but you gained love. does that make sense? maybe not..."

- Sans if Frisk never killed anyone through a Neutral route

"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 getting to the surface doesn't really appeal anymore, either. 'cause even if we do... we'll just end up right back here, without any memory of it, right? to be blunt... it makes it kind of hard to give it my all. ... or is that just a poor excuse for being lazy...? hell if i know."

- Sans "The end of your journey is at hand. In a few moments, you'll meet the king. Together... You will determine the future of this world."

- Sans's introduction in the Last Corridor. Sans is a character in the indie-game Undertale, released in 2015. He is a skeleton and brother of Papyrus. He is somewhat lazy, as he doesn't really help his brother capture Frisk. He acts as a supporting character on Neutral and True Pacifist Routes and the final boss and heroic antagonist of The Genocide Route.

Appearance
Sans is a short and big-boned skeleton, wearing a hoodie or simple jacket, unzipped at all times, with a white turtleneck underneath, black shorts and sneakers according to his Steam trading card.

He is seen with a grin at all times, and hardly moves the corners of his mouth. He has white pupils, which tend to disappear when he is serious. A bright, flashing iris appears in his left eye during the battle against him, as well. This eye starts off merely glowing, but as he utilizes his time warping powers, it starts to flash vividly.

Personality
Sans is shown to be very laid-back, sleeping on the job as often as he takes breaks. He does just enough work to avoid being fired by Undyne. He enjoys making bad puns usually relating to skeletons, to the annoyance of his brother. He is kind and reassuring, but becomes eerily serious at abrupt moments (particularly when angry). He deeply cares for and admires his brother, Papyrus, and reads him bedtime stories. He likes to mess with Papyrus, as he sends him "bad puns in a goofy font" on the UnderNet. He also leaves a dirty sock in their living room for Papyrus to fuss over. Sans worries about others hurting Papyrus or taking advantage of him. In some Neutral Endings, Sans is afraid to tell Papyrus about other characters' deaths, so he lies about them going on a vacation.

He is also very observant, and his lines and actions change the most through the path and the way of playing the game. He knows the number of times he had defeated the player (in Genocide Route). He tends to say he hates to make promises, this is likely a result of having to harm a human, as he promised Toriel he would protect humans. He will deeply resent Frisk if they kill Papyrus. In the same case, he also seems to go missing, nowhere to be found until the end of the game. Despite this, he only fights Frisk on a Genocide Route, when they are a threat to the timelines. This is possibly due to his promise to Toriel to protect humans, as killing Frisk would obviously break the promise and he figures he has no choice but to do so on a Genocide Route.

His laziness is noted again during the final fight in the Genocide route - he is unsure if being somewhat aware of the timeline resetting has broken his spirit, knowing that everything he does will mean nothing... or if he's just using it as an excuse to slack off. He also seems to be quite patient, as shown by his willingness to use his "Special Attack" (which is nothing) until the end of time in order to bore the protagonist into quitting the game.

As shown during his fight on a Genocide Route, Sans isn't above using underhanded tactics in battle, as shown when he tries to manipulate Frisk into sparing him, then killing them if they yield. Granted, Sans knew that Frisk would very likely end all timelines if they get past him, so this may be out of desperation to stop them, or out of spite for their murder of Papyrus and other monsters.

Somehow, Sans knows that he's really in a video game, as if the player gets an "Impossible Ending" by somehow meeting none of the requirements for completing Undertale or by hacking the game, Sans will explain in his phone call that the call is an error-handling message and will suggest contacting the developer so he can fix it or add another ending. Sans then says "chances are, though... you're just a dirty hacker, aren't you? yeah, get outta here." before hanging up.

Trivia

 * Sans's boss fight is widely considered by fans to be the hardest boss fight in the game, despite only having 1 HP, 1 AT and 1DF and the game literally describing him as "The easiest enemy. Can only deal 1 damage."
 * Sans is the only enemy in the game who starts his turn before Frisk does in battle, which can catch unwary players off-guard. He is also the only enemy who dies off-screen after his battle, which can lead people to question whether or not he really died.
 * According to Papyrus, Sans has a pet rock that he forgets to feed.
 * If Papyrus is killed, Sans won't appear at all until The Last Corridor. In the same case, a flavor text in Shyren's battle mentions that a hooded figure is watching the battle from afar. Given that this text only appears if Papyrus is dead, the hooded figure is most likely Sans.
 * Sans's dialogue is shown in all lowercases, not even capitalizing sentence starters and names, with a few exceptions. This probably represents his laziness.
 * His name is also a pun on the Comic Sans font, as that is used whenever he speaks.
 * Sans occasionally speaks in the Sans Serif font, usually when trying to get a serious point across. When this happens, Sans's text includes capital letters, appears on screen slower and with no sound. Notable instances of thus include Sans's line "You'd be dead where you stand." and the start of his judgement.
 * If Frisk spares Papyrus instead of killing him, Sans will appear in Hotlands, selling hot dogs. The second hot dog bought will be a hot cat instead. If Frisk tries to buy one when their inventory is full, Sans will start stacking hot dogs on their head, free of charge. He will do this up to 30 times.
 * Sans is implied to have a scientific background, as he has a book about quantum physics, a lab behind his and Papyrus's house, is implied to know Alphys, owns a telescope and mentions researching into the timespace continuum. As well as that, he uses Gaster Blasters in combat, which may be related to W. D. Gaster, the former royal scientist.
 * The snails of Blook Family Snail Farm remind Papyrus of Sans, as according to him, they, like him, are round, slow and "constantly emitting slime".
 * Unlike Papyrus, Sans is never seen moving or opening his mouth, even when drinking ketchup.
 * Sans's battle theme, "MEGALOVANIA", did not originate from Undertale. Toby Fox originally composed it for his hallowen hack of Earthbound. He later used another version for the webcomic "Homestuck", before using yet another different version for Undertale.
 * According to Papyrus, Sans is somehow able to man two sentry stations at once and, as such, slacks off twice as much.
 * Papyrus mentions in a phone call that Sans's illegal hot dog stand in Hotlands is actually one of his sentry stations, but he sells hot dogs instead of doing his job.