Sans

The brother of Papyrus, Sans first appears in Snowdin Forest, outside his hometown of Snowdin. The Snowdin shopkeeper says that he and his brother 'just showed up one day'. He works as a sentry at several posts in the Underground.

S POILER W ARNING : Plot details follow.

Personality
Sans is shown to be very laid-back, sleeping on the job as often as he takes breaks. He enjoys making poor skeleton puns, to the annoyance of his sibling. He is kind and reassuring, 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 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.

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.

Appearance
Sans is a short and stout big-boned skeleton, wearing a hoodie or simple jacket, unzipped at all times, with a white turtleneck underneath, black shorts and slippers.

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 iris appears in his left eye during 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). However, it appears he does not actually retain knowledge when the player loads a SAVE. 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 a "Gaster Blaster," a device which resembles a goat skull.

Pacifist Route
Frisk/the player first meets Sans soon after exiting the Ruins into Snowdin Forest, where Sans first appears to be a silhouette which stalks the player, and to the surprise of the player, is a friendly and welcoming character. He then hides the player from his brother, Papyrus, and explains that despite being a sentry, he has no interest in capturing humans. However, he says that Papyrus does, though assures that he is not dangerous. The puzzles he sets, such as the "Monster Kidz Word Search", are very plain and easily avoided, showing his true 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 or hangs out with Papyrus. During this, the player can access several items around the house that demonstrate Sans' comical nature, including a sock he refuses to put away, and his "humerus" 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 they should pay for the food, presenting an unreasonably high price for it. Regardless of if the player says yes or no, he laughs and asks it to be put on his tab.

Sans is encountered again in the Waterfall, letting the player use an odd looking telescope.

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', with a water sausage instead of an actual sausage.

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 expresses his support for the protagonist. He mentions that he has been friends with a woman for a long time, meeting her first while practicing 'knock, knock' jokes at the gate of the Ruins. She enjoyed his bad jokes and they became friends, although never meeting each other until in the Pacifist Route's ending. Sans then told the player that the woman made a request: to watch over and protect any human who came through the door. Sans says he made a promise to do so, hinting that he would have killed the player himself if he didn't make that promise. He changes the subject to how he must have done a good job protecting the player (from his perspective, the protagonist hasn't died a single time), telling them that someone really cares about them before leaving.

At the end, when passing through the last corridor, the player meets Sans, where he says he will judge the player for every thing they have 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). He cuts his speech short to reveal himself and talk normally, telling the player that they never gained any LOVE. He goes on about how he appreciates the player for having kindness in their heart, telling the player that the monsters' fate is in their hands: let Asgore take the player's soul for their freedom, or take Asgore's soul for the player's escape. He tells the player that he believes their determination will help them do the right thing. He then leaves.

The truth
Loading the game and returning to the last corridor will result in Sans telling the player that he suspect they have some sort of time travel ability. He whispers a secret codeword to the player and asks them to 'come back' and tell him the codeword. Loading the game and returning two more times, Sans gives the player the key to his room in Snowdin, telling them that it's time they learned the truth.

In Snowdin, Sans' bedroom contains the Silver Key, which unlocks Sans' Workshop behind the house. The room has a different decor to the rest of the house and it contains blueprints, photographs, a badge and a broken machine.

Pacifist ending
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 " (possibly because he is aware of his inability to manipulate the time loops and SAVEs), "why even try ", and "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 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 motorbike. Papyrus seems angry when Sans overtakes him.

Neutral Route
If the player kills Papyrus, Sans won't appear until the Final Corridor. If the player spares Papyrus, Sans' appearances in the neutral route are entirely the same as those in the pacifist route except in the Last Corridor.

If the player has killed at least one monster, Sans will deliver his speech about the player's judgment in full. 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.
 * If the player spared Papyrus but killed other monsters, Sans tells the player that it will be left up to the player to decide what will happen. He then leaves without saying anything more.
 * If the player killed Papyrus, Sans goes on to tell the protagonist that he suspects they have some sort of special power, and he asks if they think it should be their responsibility to do the right thing. If the player says "Yes," he then asks why they killed his brother. If the player says "No," he says that he won't judge the player for their viewpoint, but in the same breath, he calls them a "dirty brother killer." Either way, he will leave after reminding the player of Papyrus's death at their hands.

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

Killing only Papyrus will result in Sans informing the player that Toriel's attempt to welcome humans to the Underground was stopped by Undyne, who knew of Papyrus' death. In closing he tells the player that they are not welcome to the Underground.

If the player only killed bosses, 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 final corridor, Sans warns the player that their path will lead them to 'a bad time' and makes a speech, implying that he knows about the player's SAVE ability.

After the speech, a battle starts. Sans attacks the player several times while contemplating why he has not done much, whether realizing everything will eventually be reset or admitting he is just too lazy to care, only to finally jump in to this battle after foreseeing what happens next after his defeat. After this speech and the next turn, he offers to spare them, telling them that it will make his job a lot easier.

If the player spares him, he tells them that he won't let the player's work go to waste. He then kills them with an unavoidable attack while winking at the player with his left eye, and asks them in the game over screen not to return if they really are a friend.

If the player continues to fight Sans, he continues to attacks them, while resetting over and over again. He completes his attacks by slamming the player's soul against the walls of their box, but the player survives this. He then uses his 'special attack', which does literally nothing, though 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 reminds the player that he warned him before casually getting up, weakly walking offscreen before dying. Sans' death grants the player enough EXP to reach level 20.

Battle
Sans is widely considered the most powerful monster in the game among players, and he is 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 faster and trickier attacks, randomly switching between Blue and Red Mode. He also uses beam projectiles that emerge from skeletal faces. In the game files, this beam move is called "gasterblaster".

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. Similarly, while he genuinely does have only 1 HP and 1 DEF, he will dodge the player's every attack without fail until the very end.

If you accept his offer of Mercy... prepare to get dunked on. You must kill Sans to continue past him in the Genocide route.

Flavor Text
The easiest enemy. Can only deal 1 damage.

(After he dodges a few attacks) Can't keep dodging forever. Keep attacking.

The Player
Sans is the first monster the protagonist meets outside the Ruins. When he first meets the player 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 player has experienced the events of the game before, and his attitude toward the player changes drastically depending on how merciful they are during the current playthrough. Sans only battles the player at the end of a Genocide Run, 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 player that had he not promised to protect them, they would be dead, with the implication that he would have killed them himself.

Papyrus
Sans cares a lot for Papyrus, he reads Papyrus bedtime stories, and even gifted him action figures, dressed as 'Santa'. 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.

Toriel
Toriel and Sans are close friends, sharing a love of bad jokes. He never sees her in person, until the ending where Toriel survives. Sans adhered to her request to not harm any human that passed through the door exiting the ruins, since he had 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 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- but during Flowey's own resets and travels through the Underground it seems Sans was just about the only thing capable of stopping him, much the same as it is for the player on a Genocide run.

Trivia

 * 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, his dialogue is silently given instead of being accompanied by his voice beeps.
 * Almost all of Sans' dialogue is in lower case, except for when he is speaking in 8bitoperator instead of Comic Sans.
 * Sans can be found in the Hotland selling hot dogs. If the player buys a hot dog when they have no room, Sans will place the hot dog on their head. The hot dogs on your head can be stacked to thirty.
 * Sans, although looking quite laid back, easy going and useless at first - he seems to be one of the most learned and strong characters in game, being the only one other than Flowey to know about the ability of SAVE.
 * Many things hint that Sans might formerly be from a scientific background; his workshop, his speech in Genocide boss fight, the quantum physics book, time machine, and Papyrus mentioning his love for 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' 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 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 player if they're following him.
 * There is a cell conversation with Papyrus (at the ice and X/O puzzle that requires the player to activate a switch to cross a gap) that canonically confirms Sans' 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 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 demo if the player does a perfect pacifist playthrough, where he talks to the player at the end, along with Papyrus.
 * Flowey calls him Smiley Trashbag, and admits that Sans has caused him to reset numerous times.
 * In the game data, the sprites for his weapon are labeled "gasterblaster."
 * 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 an Earthbound mod called "The Halloween Hack". This track was later used again in Andrew Hussie's webcomic "Homestuck", and finally used in Undertale.
 * There is a somber piece of music in the game's files labelled "mus_sansdate," implying that a date with Sans may have at one point been implemented in the game. The piece contains what sounds like waves, a storm, or a crackling fire.