Sans works for his brother all the time. He is hated by everyone in Snowdin because he is the brother of Papyrus, though he tends to spend his time there anyway, at Grillby's. He has had to watch Papyrus kill innocent monsters nearly every day. At first, he hated the attitude everyone had towards him, but after a while, he stopped caring. He was not previously as mean-spirited as he is now, but since Papyrus became a member of the Royal Guard, Papyrus has been abusing Sans's power and using it mostly for his own cruel, selfish desires.
Sans hates humanity because they sent the monsters to the Underground, and because of this, he just wants the whole of humanity to perish. While he respects his brother as his master, he secretly wishes they could at least be more brotherly.
AppearanceUnderfell Sans looks very similar to his Undertale counterpart. He is a skeleton with sharp teeth, with one being gold. His left eye is seen to be glowing red almost constantly, and he has bags under his eye sockets. He wears a black winter jacket with a fur hood and a gray cotton hoodie, and a red sweater with black pants that have yellow stripes going down. He wears black shoes with red socks or even red slippers. Sans wears a black jacket, with the "fluff" at the collar being notably more ruffled than the original Sans'. His shirt is a red color in contrast to the normal white shirt. His shorts have a yellow strip instead of a white one and he wears black/red/white shoes or red slippers, as opposed to his counterpart's light blue slippers. His body structure appears the same as his counterpart (besides his teeth).
He is aggressive with others and quite hateful when seen in Snowdin. This is shown especially when he is around the player. He often teases the player which usually results in Flowey getting very annoyed. The only time he is ever really seen to be hurt is when his brother takes credit for something he has done as he doesn't try very often and feels sad that he can't be acknowledged for what he has done. He is portrayed as sweaty, possibly due to emotional stress or strain, though it might also be a result of his frequent magic usage. He is mentally unstable due to the abuse his brother gives him and does frequently have nightmares, but he doesn't feel like he can tell anyone because he'll get labeled as weak. He is typically ruthless and brutal. His knowledge of the timelines and resets is shared among all AU's (excluding some interpretations of Underswap ) but he hides this information very well. He is not as "evil" as most of the other characters are portrayed, but still believes in the "Kill or Be Killed" motto. Mustard is his favorite food, rather than ketchup. The monsters in Snowdin hate him because he is Papyrus' brother and so they want him dead. He is portrayed to not care but deep down he's always terrified as he knows how easily most could kill him.
Powers and Abilities
Sans is a little more powerful than his Undertale counterpart. He can manipulate sharp bones and telekinetically move a living soul at ease. He can also manipulate Gaster Blasters, and is sometimes shown to be able to manipulate gravity and can recall each timeline that Frisk resets and saves.
On the true Pacifist route, Sans keeps an eye on Frisk. He hates how Frisk keeps being nice and waits in Hall Judgment. This fight is similar to the fight against Undertale's boss, it is only more brutal, tougher and more deadly. Underfell Sans gets tired much faster in his fight as a result of using more powerful attacks. Once Frisk manages to 'reform' Sans, he apparently refuses to fight any more.
They share a very tense relationship. Papyrus is Sans's little brother, but Sans has to deal with Papyrus as a boss, which he hates. Very rarely do they speak to each other as equals and Papyrus always speaks before him and often is attributed the merit of the sans what has been done among them some of the puzzles that are seen in Snowdin. Sans wants his brother to at least take care of him, even though some speculate Sans battered Papyrus as a child.
True Pacifist Timeline
Sans wishes Frisk would die but later becomes a good friend of them. After the meeting in the Hall Judgment, Frisk achieves the 'reform' Sans. Sans then says he will not harm Frisk any longer.
Sans first assumes that Frisk is weak and will eventually be killed. After losing his brother, Sans becomes broken. He doesn't feel hatred since, in this world, monsters follow the motto "kill or be killed." Sans then goes to the Judgement Hall and waits for Frisk. Instead of fighting or threatening Frisk, he instead thanks Frisk for believing in the law. Sans is reluctant to fight Frisk because of how powerful Frisk becomes. Sans tries to calm Frisk but to no avail. When it is clear that Frisk will not back down, Sans begins to fight with his full strength, but in the end, Sans dies after 24 rounds. During the fight, Sans loses his fear of Frisk and comes to terms with his imminent death.
Sans strongly dislikes Flowey because of his frailty. sans knows of Flowey's existence and wishes to kill him.
While most monsters hate Sans and Papyrus, Grillby is friendly towards Sans, possibly because of his drinking.
Sans is a good friend of Toriel. She in begged Sans to at least let her kill Frisk if anyone so that she can give them a peaceful death. Sans was reluctant and felt pity for her, so he decided to end Frisk himself.
Undyne is Sans's real boss. She hired Sans as a sentry in the forest because Papyrus asked her to.
Sans is shown to have met Alphys before, possibly from when Gaster was the royal scientist. He doesn't like her very much and wwant's to keep his distance from her.
- In the game Underfell, Sans takes on the name of Mega Sans during combat and the game.