Sans is a short and big-boned animated skeleton, with a dimpled smile on his face. He wears an unzipped blue hoodie, a white t-shirt or sweater, black shorts with white stripes and a pair of sneakers or slippers. He has white pupils, which tend to disappear when he is serious. When using magic, his left eye flashes cyan and yellow, and his right pupil disappears. spoiler
Near the end of the game the player encounters Sans in a long hallway on the way to Asgore's throne room. Sans will judge the player, commenting on how many monsters the player has killed.
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If the player has killed no monsters Sans will commend them for getting this far without hurting anyone and the game will be permanently locked into the Pacifist ending.
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If the player has killed a small amount of monsters Sans will speculate on if these deaths were intentional, or the player being frightened into self-defense.
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If the player has killed a large amount of monsters Sans will condemn them for their cruelty. He is especially harsh if you kill his brother Papyrus.
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If the player has killed every monster and boss in the game then Sans will not judge the player at all. Instead he will attack the player in a gruesomely difficult boss battle. If the player dies and returns he will repeatedly taunt and insult the player before attacking again. Sparing him doesn't work until the half-way point of the battle. But if you choose to spare him he will insta-kill you and tell you to get dunked on.
Sans' dialogue in game is unique. Similar to how his bother Papyrus's text is written exclusively in papyrus font, Sans' dialogue is written exclusively in comic sans font. He likes to drink ketchup out of the bottle and knows how to play the trombone. Sans is widely considered to be Undertale's most famous and recognizable character and is scheduled to appear in Deltarune Chapter 3 or 4.