Sans

NOTE: THIS PAGE CONTAINS SPOILERS, IF YOU ARE AVOIDING SPOILERS, DO NOT READ THIS.
Sans is the brother of Papyrus. Sans' name was revealed in the demo if the player does a perfect pacifist playthrough, where he talks to the player at the end.

Sans and Papyrus first appear in the outskirts of Snowdin, their home town. The Snowdin shopkeeper says that they 'just showed up one day'.

Personality
Sans is shown to be very laid-back, having little interest in fighting. He enjoys making poor skeleton puns, to the annoyance of his sibling. He is kind and re-assuring, but becomes eerily serious at abrupt moments (particularly when angry), demonstrating that his casual nature is perhaps a guise.

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 Frisk/the player (in genocide route). He tends to say he hates to make promises, whether it is a result of a past incident, or his own laziness, is a mystery.

Appearance
Sans is short and stout skeleton, wearing a hoodie (some believe he might be wearing a simple jacket), unzipped at all times, with a white turtleneck underneath, black trousers and either slippers or tennis shoes.

Sans is seen with a grin at all times, and suggests that it might not be possible for him to make many facial expressions or open his mouth like his brother. He has white eyes, which tend to disappear when he is serious. A bright, flashing pupil appears in his right eye during the the battle against him, as well.

Abilities
He has obtained the knowledge of multiple timelines and the SAVE Function, presumably through his scientific research, as can be found in Sans' Room (potentially with the aid of W. D. Gaster). 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 a device which looks like some bird's skull.

Pacifist Route
(WARNING: SPOILERS AHEAD)

Frisk first meets Sans soon after exiting the Ruins into Snowdin Forest, where Sans approaches the player quite friendly. He hides the player from his brother, Papyrus, and explains that despite being a sentry, he has no interest in capturing humans, but says Papyrus does. He assures the player that Papyrus is not dangerous. The puzzles he sets are very plain and easily avoided, showing his lack of interest in catching humans

After random encounters with him before reaching Snowdin, his house (which he shares with Papyrus) can be accessed if the player goes on a date with Papyrus. During the date, the player can accessible several items around the house that demonstrate Sans' comical nature, including a sock he refuses to put away, and his humorous entrance with a trombone. When Undyne asks Papyrus how they can afford to have such a big house in Snowdin, Papyrus says that Sans pays for it, when questioned further about it, he says it's a mystery.

Just after entering the waterfalls, Sans offers player to hang out with them- if the player agrees, Sans takes them to Grillby's- saying he knows a shortcut and reaches it instantly, demonstrating his capabilities of teleportation. Sans is greeted warmly, showing he's quite popular with people. Sans tells the player that he is concerned about his brother who is being 'pranked' by a flower telling him motivational things (it is strongly hinted to be Flowey, as Flowey later says that Papyrus is in his fan club). He passes it off as an echo flower. Before leaving, he pranks the player saying that he should pay for the food, saying an unreasonably high price for the food, if the player says no, he laughs and asks it to be put on his tab.

The player can see him sleeping at one of his sentry posts in Hotland when fleeing from Undyne, when Undyne notices him sleeping, she becomes quite angry, causing her to slow down a little. Sans is encountered later on selling 'hot dogs' - which have a water sausage in place of an actual sausage - buying three lets him joke about the player liking 'hot animals'.

The player can then meet him outside MTT Hotel, where Sans offers yet another option to hang out, if agreed, Sans takes the player (through another 'shortcut') to MTT Restaurant. There Sans mentions that he has been friends with Toriel for a long time, meeting her first while practicing 'knock, knock' jokes at the gate of the Ruins. Toriel enjoys his bad jokes and they become friends (although they don't see each other until the Pacifist Route's ending), and due to her request to take care of any human that passes through the door, Sans hints that he hasn't killed the player yet because of her request, and then leaves.

At the end, when passing through the last corridor, the player meets Sans, where Sans says he will now judge the player for every thing he has done, showing how Sans has been observing player since the start. He mentions the true meaning of EXP (execution points) and LOVE (level of violence). Sans appreciates the player for having kindness in his heart, and tells the player that the monsters' fate is in his hands, trusts him and leaves.

He can be seen coming during the cutscene while preventing the battle with Asgore, saying "hey guys, what's up?" Toriel recognizes the voice, and they meet face-to-face. Toriel recognizes Papyrus as his brother, saying that Sans has told her a lot about him.

During the final battle against Asriel, Sans in his lost soul form, says things like "just give up, I did", "why even try", "we'll never see 'em again". During this, if the player does a bad joke with Papyrus' soul, he hates it, but Sans' soul likes it. After Asriel is defeated, the barrier is destroyed, and the cast returns to the surface, Papyrus leaves to make a 'good first impression', Sans says he will leave to keep Papyrus away from trouble, but takes a different path, away from the rest of the group.

In the credits, Sans is seeing riding a motorbike. Papyrus seems angry when Sans overtakes him.

Neutral Route
The neutral route is entirely the same, except since Frisk has the potential to have killed monsters or Boss Monsters, his judgement in the hall before Asgore will vary in degree of condemnation. Additionally, Sans never appears in the confrontation with Asgore, and only shows up after the hallway scene post credits, where the player receives a phone call from him, telling them about who is taking Asgore's place after his death. His description of the Underground's new leadership is dependent on who the player has killed, spared or befriended.

In case Frisk kills every boss, and has killed at least one monster, Sans, in his call mentions how everyone capable of being a leader died overnight and about the bad condition of monsters due to this. He says how he is not the King because he likes to take it easy, then says that the circumstances they are in is because he took it easy. He tells Frisk to "go to hell" and hangs up.

Killing only Papyrus will result in Sans telling the player that they are not welcome to the Underground. He also tells them that Toriel was mistaken about the player.

If Frisk killed bosses only, Sans is oddly in a lighter mood despite his brother's death. He tells the player that a white dog has taken the throne, and that everything is peaceful.

Genocide Route
Sans meets the player as usual in Snowdin's forest, but is surprised by the player refusing to play along with him or Papyrus. After the player crosses the bridge, Sans warns them not to fight his brother.

In the hall before Asgore, Sans warns the player that their path will lead them to 'a bad time'. They enter battle. Sans attacks the player several times before offering to spare them; sparing him will lead him to kill the player with an unavoidable attack, then he will ask the player in the game over screen to not return if they sincerely wish to stop their run.

If the player continues to fight Sans, he attacks them some more, bringing their HP to 1 before using his 'special attack', which is doing nothing during his attack turn and letting it continue indefinitely, in the hope that the player either quits out of boredom or stays trapped. Sans eventually falls asleep, allowing the player to use their soul to push their box into the ATTACK option. Sans dodges the first attack but the second mortally wounds him. He appears to bleed from his wound and warns the player a final time before casually getting up, weakly walking offscreen before dying. Sans' death grants the player enough EXP to reach level 20.

Battle
Sans is a very powerful monster, and the final boss of the Genocide Route, but his CHECK claims that he is "the easiest enemy". Much like his brother, Sans uses bones in his attacks and uses Blue Mode. Sans, however, uses more faster and tricky attacks, randomly switching between Blue and Red Mode. He also uses beam projectiles that emerge from skeletal faces.

While his attacks only deal 1 damage at a time, they ignore invincibility, attacking 40 times per second, allowing him to deal a punishing amount of damage.

Papyrus
Sans is supportive of his brother's endeavors, even calling him cool. Sans seems concerned about a flower talking to Papyrus, and he assumes that it is someone using an echo flower to talk to him. In a genocide run, he warns the player not to kill Papyrus.

The player
When he meets them in Snowdin, Sans is friendly to the player, no matter what gameplay they are following. He plays jokes on them, using a whoopie cushion when they first meet. He is able to guess from the player's behaviour that they may be able to SAVE and that they have experienced the events of the game before. He only battles the player at the end of genocide run, where he believes that the existence of the world's timelines are in danger.

Toriel
Toriel and Sans are close friends, sharing the love of bad jokes. Although he never sees her in person until the ending (if Toriel survives), he is concerned for her when she is upset, and adheres to her request to not harm any human (although he originally planned to kill them instantly). Toriel keeps record of Sans' knock-knock jokes in her diary.

Flowey
According to Flowey, Sans "caused more than [Flowey's] fair share of resets". Sans does not seem to know anything about Flowey, considering Flowey used the reset, he does gets curious about SAVE due to Flowey's resets.

Trivia

 * His name is based off the font he speaks in, Comic Sans, just like his brother, Papyrus.
 * Sans can be found in the Hotland selling hot dogs. If three or more are purchased, he declares that "You like hot animals don't you? It's okay, if it weren't for people like you I would be out of job."
 * If the player buys a hot dog when they have no room, Sans will place the hot dog on their head.
 * Sans, although looking quite laid back, easy going and useless at first- seems to be one of the most learned and strong character in game, being the only one other than Flowey to know about the ability of SAVE.
 * Many things clue that Sans might formerly be from a scientific background, namely his workshop, his speech in Genocide boss fight, the quantum physics book, and Papyrus mentioning his love for science fiction.
 * If Sans' 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 player's attacks.
 * Although Sans makes and likes bad jokes, he is a performer in MTT hotel, hinting he might be skilled at comedy.
 * Sans may have the power to teleport, as portrayed in game.
 * One example of him teleporting is seen in a genocide run, where after warning the player not to fight Papyrus, Sans appears to cause the screen to black out before he disappears.