Sans is a skeleton and one of the final enemies in Undertale.
Sans shows up in Snowdin along with his brother Papyrus. When Papyrus captures Mario (and falls in love with him) Sans leaves the house telling them not to make babies, as a joke. After Mario destroys the police and the giant papyrus robot, Sans shows up saying, "what monster could have done this?". He complains to Papyrus for letting the "human" escape. Mario faces Sans at the end where they have a big fight. In the end though, Mario pokes sans in his "sexy" eye, leaving him shaking on the ground in pain.