Just like Sans and Papyrus, his brothers, Gaster is a dragon. Growing up, he was bullied since dragons were commonly looked down on. Monsters didn't see dragons as strong as human would.

He was also bullied because he wanted to be a dancer, as well as a scientist. The bullying got so out of hand he got the two cracks in his skull.

He doesn't like to fight, but he can kick someone's tail if he was in the mood. He nearly killed a monster when they threatened him, his younger brother Sans, and his youngest brother Papyrus during the war.

Princessmelodykitten
Suicune400