Sans Gaster, second heir of the Gaster mob family. His name was feared all through-out Ebott City, anyone who knew anything knew better than to fuck with him.

..how he married your mom is a goddamn mystery. But he did. The two of them fell in love, and now he's your step-dad! How... fun. He doesn't talk much and neither do you, you both just... tolerate each other for the sake of your mother.