Sans is the brother of Papyrus and a major character in Undertale. He first appears in Snowdin 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 the Genocide Route.

Sans appears as a relatively short and paunchy skeleton, with mitten-like hands. Sans is always seen with a wide, toothy-grin on his face. Sans wears a blue hoodie, a white t-shirt underneath, black shorts with white stripes, and a pair of slippers. He has white dots in black eye sockets 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) while his right pupil vanishes.