Sans is the King of the Underground. He is said to be very lazy and he used to be good friends with Toriel Dreemurr, the Captain of the Royal Guard, until Toriel thought he tried to hurt a human whom he was only slowly setting down to save them from a fall. He needs seven human souls to break the barrier and free the monsters from Underground, same as his role's origin in Asgore.
With everyone urging him to break the barrier, he keeps to himself in his home and throne room and is seldom seen outside of them. One of places he is most comfortable with is his throne, decorated with a pillow.