He is a very sad skeleton.

He used to be very hostile and aggressive until Frisk fell down to the underground. They had a curse that everytime they died and came back, a flower grew on them. Sans had killed them a lot but after they still showed mercy he felt bad and they eventually became lovers. But, other monsters killed them so many times that... They died permanently. Sans begged and pleaded for them to come back but they never did. He was crushed after the death of the human.