Sans appears to be wearing a jacket with shorts and slippers.
He helps hide Frisk from Papyrus near the beginning of the game. His name is a pun on the typing font Comic Sans, in which all his dialogue is. He is the final boss of the Genocide Route. He also explains that he exchanged jokes with Toriel at one point, until she disappeared.
Sans appears consistantly throughout the game, first appearing in Snowdin. His dialogue and actions stay the same, although he does appear in the date with Papyrus, where he plays the trombone when the Annoying Dog runs away with Papyrus' present bone. He also appears later in Hotland, when the player is running from Undyne. He is sleeping on the job, and is unable to help Frisk's escape. He also appears in Hotland selling hotdogs and hotcats.
He also appears with everyone else near the end of the run, where he shows up in New Home. Flowey also captures his soul, and must be saved along with everyone else.
During the Genocide Runs, Sans is a lot more cold and blunt about how he feels, telling you to keep pretending to be a human. After killing Papyrus, Sans disappears for most of the game. He reappears in the Judgement Hall, where he fights the player. Despite him being called "the easiest enemy", most people, including Jack, call him the hardest boss in the game.