Sans

Sans is a major character in Undertale Logic and a minor character in Friday Night Funkin' Logic. He is Papyrus's brother and famous for being one of, if not the hardest boss in video game history.

Appearance
Sans appears as a short, paunchy skeleton with a wide toothy grin, glove-like hands and large eye sockets. He wears an unzipped blue hoodie, a white shirt, black shorts with white lining, and a pair of pink 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 and flippant, 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." On the other hand, when Papyrus becomes the leader in the Papyrus ending, Sans is implied to be working harder despite not denying his laziness upon being called out on it. This is because while Sans is apathetic towards life, he is never apathetic towards the people he cares about.

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 various strange jokes that tend to involve pranks and things that just mess with people's heads, such as "selling fried snow" which on its own makes no sense, but then raising the price to ludicrous degrees until saying he does not want to sell it because it has "too much sentimental value." His pranks involve juvenile things such as eye paint on a telescope, an unscrewed ketchup bottle, and whoopee cushions. He occasionally makes some low-brow puns and the fact that he performs at the MTT Resort hints that he may be a talented comedian. He enjoys science fiction and loves to drink ketchup. He also hates making promises. This is possibly because he seems unable to stick to his own moral compass as he congratulates the Player on keeping this idea in their heart, but he does everything in his power to kill him by any means necessary during the Genocide Route or during BOYFRIEND vs.SANS?! Friday Night Funkin' Logic, even faking mercy on them. This is supported by the fact Sans is clearly not afraid of killing when he mentions to the player that if "the old lady (Toriel)" had not made him promise to protect them, "they'd be dead where they stand." He does show incredible restraint however, as he never tries to fight the protagonist even if they kill the people closest to him, only outwardly shaming them with disturbing implications about their character and direct judgement on their actions, coldly asking them "then why'd you kill my brother?" or calling them a dirty brother killer. His pledge to this promise even goes so deep that if Papyrus is killed but not Toriel, he does not mention the protagonist's slaughter to Toriel in order to not emotionally damage her.

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

Sans offhandedly mentions to the protagonist that he understands that they would "want to go back" and suggests that they already have everything they could need. He seems to be longing and searching for a life that he cannot go back to as he "gave up trying to go back a long time ago" and as a Lost Soul tells the protagonist that "you'll never see 'em again." He is implied to have given up on trying to go back as he had projected acceptance in staying in this new life to the protagonist, although what Sans is trying to go back to is quite vague.

In Undertale Logic
Sans introduces himself to Player 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. He then takes Player across the bridge to Papyrus, assuring him it'll be funny.

In Friday Night Funkin' Logic
Sans appears in BOYFRIEND GETS A FAN CLUB?! Friday Night Funkin' Logic as one of the people Player gives a signature to. He asks Player if he will play the Dusttale Mod, but Player says no.

Sans also appears in BOYFRIEND vs.SANS?! Friday Night Funkin' Logic as a supporting character. He accompanies Player and Girlfriend, who offer to help him kill Chara, as they have been on a Genocide Route and have killed every other monster in the Underground

Relationships

 * Player - Sans' attitude towards Player depends on his actions, though he usually enjoys pranking them and occasionally hanging out with them unless they kill Papyrus. Near the end of a Neutral or Genocide Route, he judges him based on the amount of EXP he has.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, he protected them from life-threatening harm after their first encounter, or he was simply joking. 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. If the protagonist pursues pacifism, Sans begins to lose his previous apathy and comes to genuinely support the protagonist in their mission, as indicated in his MTT Resort date and Last Corridor judgement.
 * Papyrus - Sans cares for and admires his brother. He keeps up with Papyrus's temper tantrums and he knows how to deal with Papyrus's childish outbursts. Sans also 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 the Papyrus Ending of the Neutral Route, 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. Near the end of the Genocide Route, he indirectly apologizes to her, as he is forced to break his promise to stop the protagonist.  Sans and Toriel are only seen together on screen in the True Pacifist Ending. If the protagonist decides to go back through the Underground before going to the surface, Sans sends them texts for Toriel since her thumbs are too big for the buttons on the phone. It is implied that they soon fight for control of the phone when they begin to send embarrassing texts of the other person.
 * 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 is called just before entering the Lab, Sans appears in the call and suggests that there may be dogs inside the lab. This hints that he may know about the True Lab since Endogeny, an Amalgamate composed of several dogs, can be found there. According to Alphys in an aborted Genocide Route, she is forced to rule the Underground and Sans helps her lead; she calls Sans "a good guy".
 * 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 also has an unused pose for his battle graphics, similar to his shrugging pose, but with clenched hands.
 * 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 does not give a clear answer. However, in the official Japanese translation released in 2017, Papyrus refers to Sans as "Nii-chan", 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."
 * In addition, the first part of this quote is a direct reference to a quote of Asgore's on the Neutral Route.
 * 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.
 * In the 5th Anniversary Alarm Clock text, Undyne was talking about a hockey game everyone had played against her. Once Sans scored a goal, he went over to Alphys and did a "finger-pointy arm-crossy dance," implied to be the Fortnite emote Dance Moves, more commonly known as the Default Dance.


 * Sans also has an unused pose for his battle graphics, similar to his shrugging pose, but with clenched hands.
 * 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 does not give a clear answer. However, in the official Japanese translation released in 2017, Papyrus refers to Sans as "Nii-chan", 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."
 * In addition, the first part of this quote is a direct reference to a quote of Asgore's on the Neutral Route.
 * In the 5th Anniversary Alarm Clock text, Undyne was talking about a hockey game everyone had played against her. Once Sans scored a goal, he went over to Alphys and did a "finger-pointy arm-crossy dance," implied to be the Fortnite emote Dance Moves, more commonly known as the Default Dance.