Sans

Sans is the brother of Papyrus and a major character in Undertale. He first appears in the forest after the protagonist exits the Ruins. He serves as a supporting character in the Neutral and True Pacifist routes and as the final boss and heroic antagonist in a Genocide Route.

Appearance
Sans appears as a short skeleton with a dimpled smile, large eye sockets, and perfectly aligned teeth. He wears an unzipped blue hoodie, a white t-shirt/sweater, black shorts with white stripes, and a pair of sneakers/slippers. He has white pupils that disappear when he is serious or angered. When Sans uses telekinesis, his left eye flashes light blue and yellow, the colors for patience and justice, and his right pupil disappears.

Personality
Sans is laid back, often sleeping on the job and taking breaks. His laziness is a combination of fatalism and apathy. He wonders if his indolence stems from his knowledge that any progress he makes will be erased when the timeline resets. Conversely, he suggests that this knowledge could be "a poor excuse for being lazy."

Though Sans is usually agreeable, he becomes eerily serious at particular moments. He is also observant; Sans reads the protagonist's expressions and can often tell when they have already done certain tasks. He enjoys making skeleton-related puns and the fact that he performs at MTT Resort hints that he may be a talented comedian. He enjoys science fiction and loves to drink ketchup. He also hates making promises.

Sans may have a scientific background. Evidence includes the quantum physics book, workshop, his relationship to Alphys, affinity for science, and his timeline research.

Abilities
Sans can quickly travel or teleport using what he calls "shortcuts."

In combat, he uses "Gaster Blasters," devices that resemble a skull, to shoot large beams. Sans excels in combat and fights with unique abilities that include: KR, applying directional forces directly upon the protagonist's SOUL, and attacking the protagonist in their own selection menus.

Because of his "research," he has knowledge of timelines and the SAVE function. However, he does not retain his memories when the protagonist loads a SAVE, and he cannot use the SAVE function. His awareness of time travel makes him scrupulous, and he recognizes when the protagonist is behaving unusually. Sans reads the protagonist's expression and determines if they have experienced events before and are repeating them. He also knows how much EXP the protagonist has and comments on it when judging them.

Sans also plays the trombone, though he is only seen playing it once. Papyrus finds his "incidental music" annoying, implying that he has played many times before.

Like other characters, Sans is aware of when the game's code suffers bugs and calls the protagonist a "dirty hacker" if they achieve an impossible ending of a Neutral Route.

Neutral Route
Sans introduces himself to the protagonist in Snowdin Forest after he initially appears as a silhouette. He explains that he has no interest in capturing humans, but that his brother, Papyrus, does. Then, Sans hides the protagonist behind a lamp whose shape is identical to the protagonist's. Papyrus rushes onscreen, and the brothers discuss Sans's laziness and the importance of puzzle upkeep. Sans appears multiple times throughout Snowdin Forest, observing and commentating on the puzzles but not engaging in any japery, except for the word search.

Before Papyrus's battle, Sans is absent in Snowdin Town. He appears briefly to play trombone during the date/hangout with Papyrus but does not appear again until the Last Corridor if the protagonist kills Papyrus.

After entering Waterfall, Sans is found manning a sentry station and asks the protagonist if they want to take a break with him. If the protagonist agrees, he takes them to Grillby's via a "shortcut." The patrons warmly greet Sans, and he proceeds to treat the protagonist to a burger or fries. After some dialogue, a spotlight falls on him and the protagonist and time appears to stop as Sans tells the protagonist that a flower has been talking to Papyrus. Sans believes that someone is tricking Papyrus with an Echo Flower when the flower in question is Flowey. Sans can be seen again in Waterfall and pranks the protagonist with a red-eye telescope. Later, during Shyren's battle, "a hooded figure" selling toilet paper concert tickets is described in the flavor text, most likely being Sans.

The protagonist passes him sleeping at a sentry post in Hotland when fleeing from Undyne. Undyne notices him sleeping and berates him mid-chase.

Later, Sans sells Hot Dogs in Hotland for 30G. If the protagonist attempts to buy a hot dog when their inventory is full, he stacks a maximum of 30 hot dogs on their head for free.

Further on, Sans waits outside of MTT Resort and asks the protagonist if they want to grab a bite to eat. He escorts the protagonist inside via "shortcut" and talks about a woman he has befriended that shares his affinity for puns. He tells the protagonist that this woman made him promise to protect any human that left the Ruins and that, if she had not said anything, the protagonist would be "dead where [they] stand." He dismisses this as a joke immediately and says that he has done a great job protecting the protagonist. If the protagonist has not died before, Sans takes credit for their success. If the protagonist has died, Sans questions the validity of his statement. Before leaving, he tells the protagonist to take care of themself because someone really cares about them.

Judgment
The protagonist encounters Sans for the last time in the Last Corridor. He explains that EXP and LOVE are acronyms - "EXecution Points" and "Level Of ViolencE" and judges the protagonist for their current EXP.

If the protagonist gained no EXP, Sans skips his judgment, instead praising the protagonist for embracing compassion and actively choosing to be merciful to the monsters throughout their journey. He reinforces the idea that the protagonist's actions shall decide the fate of the world in one of two ways: the protagonist lets Asgore take their SOUL and allow the monsters to break the Barrier or they take Asgore's SOUL to escape the Barrier themselves. Sans concludes that the protagonist now has enough determination and believes that they can do the right thing before leaving. If the protagonist has heard his speech before, Sans notices their bored expression and offers a secret codeword that he uses to prove that they are a time traveler. After the protagonist loads their SAVE a few times, Sans's suspicions are affirmed and he gives them a key to his room. Inside Sans's room is a key to his workshop.

If the protagonist gained EXP, Sans allows them a moment to think about their actions. He then emphasizes the importance of integrity and, if Papyrus is alive, concludes that whatever happens next is up to the protagonist. If the protagonist killed Papyrus, Sans expresses his suspicion that they have some special power. He then asks if the protagonist thinks it is their responsibility to do the right thing. Regardless of their answer, Sans reminds the protagonist of his fallen brother and leaves.

If the protagonist returns to the Last Corridor after loading a SAVE, Sans judges the protagonist on their earned LV. Subsequent attempts to talk to Sans after judgment prompts him to tell the protagonist to "consider our session over."

Endings
After the protagonist defeats Photoshop Flowey, Sans calls and informs them of the events that transpired after their departure. The content of this phone call varies depending on the protagonist's actions; a list of the endings that Sans describes is on the Neutral Route page.

True Pacifist Route
Sans is absent from the Last Corridor in a True Pacifist Route. There is also no SAVE point upon entering the corridor. He appears during the pre-Asriel cutscene and meets Toriel face-to-face for the first time. Flowey binds Sans and the protagonist's other friends. However, the protagonist's friends protect them from Flowey's attacks before Flowey absorbs their SOULs.

During the battle against Asriel, Sans appears as one of the Lost Souls.

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

When Papyrus runs off to introduce himself to the humans, Sans says that someone must keep him away from trouble and runs off in the opposite direction, presumably to use a "shortcut."

In the credits, Sans rides a tricycle on a highway, while his brother races alongside him in a car. Papyrus seems irritated when his brother overtakes him.

Genocide Route
Sans stalks and introduces himself to the protagonist as usual but requests that they continue pretending to be a human. He does not comment on the protagonist turning around before he asks them to. Sans goes along with Papyrus's japery and does not appear as an NPC as he does on other routes. After the protagonist crosses the bridge before Snowdin Town, Sans warns them not to fight his brother or else they will "have a bad time." He then disappears and is absent until the Last Corridor.

If the protagonist aborts a Genocide Route by sparing Papyrus, Sans reappears at the Waterfall sentry station and tells the protagonist he respects them for doing that. He continues to appear as if on a Neutral Route but does not take the protagonist to Grillby's since there is no one left in Snowdin. Alternatively, Sans follows the protagonist if they kill Papyrus.

The protagonist encounters Sans in the Last Corridor for his battle.

After defeating Sans, the protagonist's EXP is set to 99999, granting them LV 20. Sans notices if the protagonist has killed him before and is re-fighting him.

Sans does not count as a kill in the Stats menu, and the game does not record whether or not he is killed.

The Protagonist
Sans's attitude towards the protagonist depends on their actions, though he usually enjoys pranking them and occasionally hanging out with them unless they kill Papyrus. Near the end of a Neutral Route, he judges them on their EXP. At the end of the True Pacifist Route, Sans considers them a good friend. During the Genocide Route, he does not bother to encounter the protagonist until the end where he has no choice but to fight them, expressing subtle fury toward them during the period together.

Sans initially considers the protagonist to be an "anomaly"; his intentions of befriending them were in hopes that they would stop resetting the timeline. However, he also says that the protagonist did not die after their initial encounter because of his promise to Toriel. This may imply he would have killed the protagonist or that he protected them from life-threatening harm after their first encounter.

Papyrus
Sans cares for and admires his brother. He reads him bedtime stories and gives him presents under the identity of Santa. Nonetheless, Sans likes to mess with Papyrus, sending him bad puns 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.

Toriel
Toriel and Sans share a love of bad jokes and are friends even though they never see each other face-to-face until the epilogue, the Family Ending, or the Exiled Queen Ending. Before the protagonist's arrival, Sans and Toriel shared jokes through the door to the Ruins.

In the Exiled Queen Ending, Sans becomes Toriel's roommate if Papyrus has been killed. In this route, Sans also states that he "can't bear to tell her what [the protagonist] did." If Papyrus is alive, the brothers visit Toriel often, play board games together, and sometimes convince Toriel to leave the Ruins.

While not being one to make promises, Sans complies with Toriel's request to watch over and protect any human that exits the Ruins. At the end of a Genocide Route, he indirectly apologizes to her, as he is forced to break his promise to stop the protagonist.

Alphys
Though there is evidence that Sans and Alphys have a relationship, Sans only alludes to this in the epilogue.

It is also noteworthy that if Papyrus and Undyne are called just before entering the Laboratory, Sans appears in the call and state that there may be dogs inside the lab. This hints that he knows about the true lab since Endogeny, an Amalgamate composed of several dogs, can be found there.

W. D. Gaster
A few things hint at a connection between Sans and W. D. Gaster. Evidence includes the blueprints and veiled machine found in his workshop, the sprites used for his skull-like laser guns (labeled gasterblaster in the game files), and his unexplained scientific research on the "anomaly" and timelines. Sans and Papyrus are named after a font, which W.D Gaster is assumed to also be, so it is believed they are related.

Trivia

 * Sans's name alludes to the typefaces in which he speaks in: Comic Sans and Sans Serif. This is a reference to Helvetica, a webcomic about a skeleton named after a font.
 * In Japanese phonetics, Sans's name is homophonous with the name of the Sanzu River, a boundary between death and the afterlife in Buddhist lore which, like Sans, judges all who cross it for their deeds. The Sanzu River is also a recurring locale in the Touhou series, from which Toby drew inspiration for Undertale ' s bullet hell battles.
 * Sans is a Mii costume for the Mii Gunner in Super Smash Bros. Ultimate. The Gaster Blaster is used as the Mii Gunner's arm cannon. The Sans Mii Costume and MEGALOVANIA remix can be bought for $0.75.
 * Sans is the first Mii Costume in Super Smash Bros. Ultimate to include a song with it.
 * If the player attempts to name the fallen human "Sans," the game responds with "nope." and prevents the name from being used.
 * He makes an appearance alongside Papyrus at the end of the Undertale Demo if the protagonist gained no EXP.
 * Sans appears to be left-handed. Evidence includes:
 * Greeting the protagonist with a handshake using his left hand.
 * Holding and drinking a ketchup bottle with his left hand while dining at Grillby's.
 * Steering a tricycle using his left hand in the True Pacifist Credits.
 * Manipulating gravity with his left hand in his battle.
 * When the "fun" value is between 45 and 50, Sans calls the protagonist in Snowdin Forest to ask if their fridge is running.
 * There is a glitch in which Sans walks up off the screen at Grillby's when he takes the protagonist there. His dialogue still appears normally.
 * There are two unused overworld sprites of Sans in the game files, one showing him with his pupils (spr_sans_dangerous_0) and one showing him holding an ice cream.
 * Sans speaks in all lower case, not even capitalizing names or sentence starters, with a few exceptions. This may be a representation of how lazy he is.
 * In a deleted tweet, Toby Fox said that Papyrus would use the term "aniki," a Japanese honorable term used for older brothers or superiors. There are a multitude of interpretations for this as it doesn't give a clear answer. However, in the official Japanese translation released in 2017, Papyrus refers to Sans as "niichan", a somewhat immature way of addressing one's older brother.
 * Sans's line " on days like these, kids like you... Should be burning in hell." could possibly be a reference to a line said by Captain Strong in EarthBound, which goes "At times like this, kids like you should be playing Nintendo games.".