Sans

The older brother of Papyrus, Sans first appears in Snowdin Forest, outside the town of Snowdin. Sans is a major character in Undertale, and depending on the protagonist's actions, is either a supporting character or a heroic antagonist.

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.

Abilities
He has obtained the knowledge of multiple timelines and the SAVE function, presumably through his scientific research, as can be found in Sans's room (possibly with the aid of W. D. Gaster). However, it appears he does not actually retain knowledge when the player loads a SAVE, and cannot use the SAVE function himself. It may be that his awareness of the situation and his innate perception allows him to recognize when the player is behaving unusually.

He likewise is shown to be able to move exceedingly quickly and/or seemingly teleport with his frequent use of "shortcuts" to get places. During battle, he can shoot what seems a kind of beam using multiple "Gaster blasters" (the name of his weapon's sprite in the files), a device which resembles a goat or dragon skull. The gasterblasters also bear similarities to the DT Extraction Machine found in Alphys' lab, Gyftrot's head, and Asriel's first form's head.

Personality
Sans is shown to be very laid-back, sleeping on the job as often as he takes breaks. He enjoys making bad puns 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 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 hates to make promises, although he promised Toriel he would protect the player.

His laziness is noted 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.

Although he does not outwardly show it, Sans exhibits depression, evidenced by his lack of motivation and feelings of hopelessness to the point where he is willing to give up. His depression stems from his knowledge of multiple timelines and the SAVE ability, knowing that even if monsters escape the Underground, the timeline will be reset and the monsters will just end up back in the Underground without retaining their memories. Sans is also fully aware that his own life is already forfeit.

Neutral Route
The protagonist first meets Sans after exiting the Ruins into Snowdin Forest, where the protagonist is pursued by Sans, who at first appears as a silhouette. After introducing himself, he explains that despite being a sentry, he has no interest in capturing humans, although his brother, Papyrus, does. He then hides the protagonist from his brother behind a conveniently shaped lamp, while he and Papyrus have a heated discussion of Sans's laziness, and the importance of puzzles. Sans appears multiple times throughout Snowdin Forest, usually alongside Papyrus, observing and commentating on the puzzles but not actively engaging in any japery, except for the word search.

Sans is absent in Snowdin Town, although he appears abruptly during the protagonist's date with Papyrus, sticking out of his room with a trombone. If the player kills Papyrus, Sans is absent throughout the rest of the game, until he resurfaces to give his judgement.

After entering Waterfall, Sans is found manning another sentry post, and asks if the protagonist wants to hang out with them. If the protagonist agrees, Sans takes them to Grillby's by using a "shortcut". Sans is greeted warmly by the patrons of the diner, and proceeds to treat the protagonist to a burger or some fries. After some ketchup nonsense, a spotlight falls on Sans, and Sans reveals to the protagonist that Papyrus has mentioned a flower that has been talking to him, and mistakenly believes that someone is pranking him with an Echo Flower, when Papyrus is actually talking with Flowey.

Sans is later encountered again in the Waterfall, and pranks the protagonist with a red-eye telescope.

The protagonist passes him sleeping at one of the sentry posts in Hotland when fleeing from Undyne. Once Undyne notices him sleeping, she becomes quite angry, causing her to stop to berate Sans. Sans is also encountered later on in Hotland selling hot dogs.

The protagonist later encounters Sans outside MTT Resort, where he offers to grab a bite at the MTT Restaurant. If the protagonist agrees, Sans will escort them there via one of his shortcuts. While in the restaurant, Sans reveals that he had befriended a woman that had a similar affinity for puns and resided on the other side of the entrance to the Ruins. Sans then told the protagonist that the woman made a request: to watch over and protect any human who came through the door, to which Sans promised to obey. Sans states that if the promise had not been kept, the protagonist would be "dead where they stand." Sans then says that he's joking, and changes the subject to how he's doing a good job at protecting the protagonist, though if the protagonist has gotten a game over, he questions whether or not he's wrong. He then tells the protagonist to take care of themselves, because there's someone who really cares about them, before exiting the room.

Judgement
While passing through the final corridor, the protagonist encounters Sans, who then proceeds to judge the player for every point of EXP earned throughout the game. Sans reveals that EXP is an acronym meaning "execution points" and that LOVE stands for "level of violence." His judgement is determined by the protagonist's actions throughout the game.
 * If the protagonist gained no EXP, Sans will begin to talk normally, telling the protagonist that they never gained any LOVE, and that they have always striven to do the right thing. He goes on to say how he appreciates the protagonist for having kindness in their heart, telling them that the Monsters' fate is in their hands; they can either let Asgore take the protagonist's soul for their freedom, or take Asgore's soul for the protagonist's escape. He tells the protagonist that he believes their determination will help them do the right thing. He then leaves.
 * If the protagonist resets from a save point after hearing his speech, Sans will take note that you've already heard this speech before and will give you a secret codeword. The protagonist must repeat this about two more times until Sans gives you the key to his room.
 * If the protagonist gained some EXP, Sans has a few different things he may say.
 * If the protagonist spared Papyrus but killed other monsters, Sans tells the protagonist that it will be left up to the protagonist to decide what will happen. In certain conditions, Sans judges the protagonist based on how high their LV is.
 * If the protagonist is at LV 2, Sans says that it's pretty sad that the protagonist messed up the slightest amount, and that they probably didn't know what they were doing, but then says that he's kidding, stating: "who gets to LV 2 on accident? get outta here."
 * If the protagonist is at LV 3, Sans gives you a C+, and tells you that you can do better.
 * If the protagonist is above LV 3, Sans says that they probably killed some people on purpose, though maybe some of it was in self defense, and that he's not sure, as he wasn't watching.
 * If the protagonist is above LV 9, Sans says that that doesn't mean that the player is 50% good, and questions "what can I say that will change the mind of a being like you...?"
 * If the protagonist is above LV 14, Sans will say that the protagonist is a "pretty bad person, " but that they could be worse, and that they "pretty much suck at being evil"
 * If the protagonist killed Papyrus, Sans goes on to tell the player that he suspects they have some sort of special power, and asks if they think it should be their responsibility to do the right thing. If the protagonist says "Yes," he then asks why they killed his brother. If the protagonist says "No," he says that he won't judge the protagonist for their viewpoint, but calls them a "dirty brother killer." Either way, he will leave after reminding the protagonist of Papyrus's death at their hands.

Endings
After the fight with Photoshop Flowey, Sans will call the protagonist, and inform them of the events that transpired after their departure. The content of this phone call varies depending on the protagonist's actions, and a list of the different endings which Sans can describe is found on the Neutral Route page.

True Pacifist Route
Sans appears during the cutscene that occurs after the protagonist initiates the battle with Asgore. Toriel recognizes his voice, and they meet face-to-face for the first time. Toriel recognizes Papyrus as his brother, saying that Sans has told her a lot about him. Sans, along with the protagonist's other friends, are bound by Flowey, yet still protect the protagonist against Flowey's attacks. However, Sans is absorbed by Flowey, along with the souls of all the other monsters.

During the battle against Asriel, Sans appears as one of the lost souls.

After defeating Asriel, Sans stands alongside the other main characters. If you talk to him, he says a certain remark depending on what, if anything, the protagonist said to Toriel at the beginning of the game. Sans also uses Toriel's phone to text for her a few times. After the protagonist is ready to leave, Sans exits the Underground and heads to the Surface along with the other monsters.

Papyrus runs off to introduce himself to the humans, and Sans says someone's got to keep him away from trouble, and runs off to ensure that it's not him, leaving Undyne to do it. In the credits, Sans is seeing riding a tricycle on a highway, while his brother races along in a car.

Genocide Route
Sans stalks and introduces himself to the protagonist as usual, and although he recognizes the protagonist as not being human, he tells them to keep pretending to be human as Papyrus had always wanted to see one. He goes along with Papyrus' japery, although he does not appear in the same places he does on regular routes. After the protagonist crosses the bridge, Sans warns them not to fight their brother, or else they'll "have a bad time." He then suddenly disappears, and is absent until the final corridor.

In the final corridor, Sans asks the protagonist whether or not a person can change. Not expecting an answer, he continues and asks if they "wanna have a bad time? ", warning them to not take another step forward. After the protagonist continues forward, Sans then engages a fight against the protagonist, and apologizes to Toriel (who is absent) for breaking his promise to protect the protagonist.

After some short monologue, Sans begins a furious onslaught of attacks against the protagonist, while contemplating why he has not done much to prevent the protagonist's actions. After he finishes his speech, Sans offers to spare the protagonist, telling them that accepting will make his job a lot easier.

If the player spares him, he tells them that he won't let the protagonist's work go to waste. He then kills them with an unavoidable attack, and asks them in the game over screen not to return if they really are a friend. If the player does come back, Sans' second attempt to convince the player to spare him is a little half-hearted, suggesting that he knows the player has already fallen for it and is unlikely to do so a second time. When the player attacks him, he says that means they never really were friends, and asks the player not to tell the other Sans-es.

If the protagonist does not spare Sans, then the fight continues, with Sans saying that it was "worth a try". Sans continues to assault the protagonist with further attacks, and the textbox proclaims that "The REAL battle begins." After the protagonist dodges all of his attacks, Sans repetitively slams the protagonist's soul against the walls of the battle box, though the protagonist survives this. Sans then uses his "special attack", which does literally nothing (and thus never ends Sans's turn), as Sans hopes that if nothing happens, the protagonist will quit out of boredom or simply remain trapped forever. However, Sans eventually falls asleep, allowing the protagonist to push the box so that they can select the FIGHT option. Sans dodges the first attack, but the second attack (unprovoked by the protagonist) hits. He appears to bleed from his wound, though it's speculated that the "blood" is truly ketchup, since Sans is known to drink entire ketchup bottles. Sans reminds the protagonist that he warned them before getting up, and weakly walking off-screen. Sans's death grants the player enough EXP to reach LV 20.

In Battle

 * See Sans/In Battle

Frisk
Sans is the first monster the protagonist meets outside The Ruins. When he first meets the protagonist in Snowdin, Sans is friendly to them, regardless of the play style they are following. He plays jokes on them, using a "whoopee cushion in hand" trick when they first meet. He is able to tell whether the protagonist has experienced the events of the game before, and his attitude toward the protagonist changes drastically depending on how merciful, or otherwise, they are during the current playthrough. Sans only battles the protagonist at the end of a Genocide Route, where he believes that the existence of the world's timelines are in danger.

If the player dines with Sans in the MTT Resort, then Sans tells the protagonist that had he not promised to protect them, they would be "dead where they stand", implying that he would've killed them himself.

Papyrus
Sans cares for and admires his brother. He reads Papyrus bedtime stories, and even gifted him action figures. Nonetheless, he likes to prank and joke with his brother- which is typical of him. Sans is worried about other people taking advantage of Papyrus' innocence; he assumes that Papyrus talking about a flower talking to him is actually someone using an echo flower. If the player kills Papyrus, then Sans disappears until the final corridor, where (on a Neutral Run) he harshly judges the player for this. On various neutral endings, Sans is also afraid to tell Papyrus about the people you have killed, so he lies about them going on a vacation.

Toriel
Toriel and Sans are close friends, sharing a love of bad jokes. He never sees her in person until at the end of a True Pacifist run. While not being one to keep promises, in all runs except the Genocide Run Sans complies with her request to not harm any human that passed through the door exiting the ruins. Toriel keeps record of Sans' knock-knock jokes in her diary.

If the protagonist reaches him at the end of the Genocide Run, he indirectly apologizes to Toriel, as he is forced to break his promise in order to stop the player.

Flowey
Sans doesn't seem to know much about Flowey himself beyond that he's the talking flower that appears before Papyrus; " talking flower " and " echo flower " are in different colors when Sans mentions it and then claims somebody's using an echo flower for a prank.

On the other hand, Flowey states in one of the Neutral Route Endings that he tried to experiment with Sans using his SAVE ability. However, Sans "caused more than [Flowey's] fair share of resets" as if Sans was just about the only thing capable of stopping him, much the same as it is for the player on the Genocide Route. Flowey also calls Sans a "Smiley Trashbag."

Trivia

 * Sans is shown wearing sneakers as shown in his Steam Trading Card art . However, in the Mercy T-shirt, he is shown to be wearing slippers.
 * His name is based off the font he speaks in, Comic Sans, just like his brother, Papyrus.
 * Not only does he speak in comic Sans font, but Sans himself is a comic.
 * At other times, Sans will drop his font quirk and instead speak in 8bitoperator (a Sans-serif font) with extra spacing between letters, and it is properly capitalized instead of being entirely in lower case. When he is talking this way, specifically at his most serious, his dialogue is silently given instead of being accompanied by his voice beeps, and cannot be skipped by using "x".
 * In the Genocide route, when Sans falls asleep, the font used for his "zzz"s is the Aster font, which could be a connection to W.D Gaster.
 * Sans can be found in the Hotland selling Hot Dogs and Hot Cats. If the protagonist buys a hot dog when they have no room in the inventory, Sans will place the hot dog on their head, which can stack up to twenty nine.
 * Many things hint that Sans might formerly be from a scientific background; his workshop, his speech in the Genocide Route boss fight, the quantum physics book, time machine, and Papyrus mentioning his love for "real" science fiction. When talking to Alphys at the end of a Pacifist Route, there's an implication that the two know each other.
 * If Sans's room is accessed, his bed can be seen in too poor a state to sleep (with his sheets rolled up in a pile and the mattress lying uncovered), which may explain the fact that he is sleepy in some parts of the game. He also has a treadmill, which may be the reason due to his fast speed and being able to dodge the protagonist's attacks.
 * Sans is one of three characters who will try to convince the protagonist to stop playing the game; Flowey is the second to make an attempt, and the other is Chara.
 * Although Sans makes and likes bad jokes, he is a performer in the MTT Resort, hinting he might be skilled at comedy.
 * Sans has the power to teleport, as portrayed in game.
 * Near Snowdin, on the path to the mysterious door, Sans seems to teleport from one side of the screen to the other, and jokingly asks the protagonist if they're following him.
 * In Genocide route, he sometimes teleports visibly, such as after warning the protagonist on a potential bad time while being in Snowdin forest.
 * There is a cell conversation with Papyrus (at the ice and X/O puzzle that requires the protagonist to activate a switch to cross a gap) that canonically confirms Sans's teleportation ability. Papyrus notes that he, himself, never solves the puzzle, but just jumps the gap. He then mentions that Sans never solves the puzzle either, but instead seems to just... be on the other side. Papyrus explains this strange behavior as, "I THINK HE HAS A SHORTCUT OR SOMETHING? "
 * Sans also has the ability to teleport other people, as demonstrated by the "shortcuts" he takes the protagonist through, and by Papyrus's comment when entering Sans's room: "IS SANS PRANKING YOU ACROSS TIME AND SPACE? I HATE IT WHEN HE DOES THAT! "
 * Sans loves to drink ketchup, as seen in his interaction with Frisk in Grillby's, where he does not eat his food, but drinks the entire bottle of ketchup if Frisk refuses it.
 * In the Snowdin Forest, examining the sentry post nearest to the Ruins exit reveals that there are a number of condiment bottles kept in the post.
 * This might be why in the boss fight with Sans on a genocide run that he is the only one seen actually bleeding in the game, it's believed to be due to the fact he only drinks ketchup and ended up 'bleeding' out the ketchup from his wounds.
 * He makes a brief appearance in the Undertale Demo if the protagonist does a perfect pacifist playthrough, where he talks to the protagonist at the end, along with Papyrus.
 * In the game data, the sprites for his weapon are labeled "gasterblaster." This is most likely a reference to Gaster.
 * If you type "Sans" for the character's name, the response becomes "nope." and prevents you from using the name.
 * The theme that plays when you fight him on the Genocide Route is "MEGALOVANIA", the trademark song of Undertale's creator Toby "Radiation" Fox. He originally made the song for a Halloween themed EarthBound ROM Hack. This track was later used again in Andrew Hussie's webcomic Homestuck, and finally used in Undertale.
 * The piece of music that plays when dining with Sans in the MTT Resort is named "It's Raining Somewhere Else", and is also labelled "mus_Sansdate" in the game files, implying that a date with Sans may have at one point been implemented in the game (or that the dinner was the date). The piece contains what sounds like waves, a storm, and a crackling fire.
 * Sans knows if you cheated your way though the game, as found in the 'Dirty Hacker' ending. Given that this also counts as an error handling message, he is aware of when the game's code suffers bugs as well.
 * If one interacts with Sans's crossword puzzle in the genocide route, Papyrus will comment on how he can't work under these conditions, after the music has changed back to normal. Sans will then say "if you like crosswords you can't be that bad? " (The music will change back if one encounters "But nobody came.")
 * You are able to kill Sans multiple times via reloading prior saves, which also grants you some extra dialogue.
 * After killing Sans once:
 * "... that expression on your face... ...well, i won't grace it with a description. "
 * After killing Sans two or more times:
 * "... that expression that you're wearing... ... you're really kind of a freak, huh? "
 * Likewise, Sans also counts the number of times he manages to defeat you, getting progressively more cocky the more you fall to him in battle. He stops counting after the 11th time. Afterwards, he simply says "let's just get to the point. " before the battle.
 * Interestingly, the kill count does not increase after killing Sans, leading some players to speculate that he did not die at all.
 * This is confusing, as the player's LOVE reaches 20 after defeating Sans, contradicting the speculation. Furthermore, the sound effect of a dying monster can be heard after he dragged himself off-screen, suggesting that Sans did indeed die.
 * When encountering Sans after a reset, the player will immediately turn around and shake his hand after he says "Human.". Sans will quickly notice this, which he labels as weird. When Papyrus meets the player later on, he feels that the human is familiar and wonders to Sans about it.
 * There is a random chance of getting a call from Sans in which he asks if your refrigerator is running after the second encounter with Sans and Papyrus. This only happens when Flag[5] is in range 40...50.
 * Sans may be left-handed.
 * In Snowdin Forest, Sans greets you with a hand shake using his left hand.
 * At Grillby's, Sans holds and drinks a ketchup bottle with his left hand if you refuse to use some of it.
 * During the Sans boss fight in the Genocide route, he relies on his left hand when using magic.
 * According to one of Toby Fox's older tweets, in the Japanese version of Undertale, Papyrus would use "aniki" when referring/talking to Sans. Aniki is a honorable term exclusively used for older brothers, meaning that canonically Sans is the older brother.