Nergal Jr.

Nergal Jr. is the son of Nergal and Aunt Sis, debuting in the episode "Son of Nergal". He is technically Billy's cousin. Junior's true form is a horrific monstrosity that is never seen, but he possesses the ability of shape-shifting into the form of any human he has encountered, usually killing/incapacitating them before he takes their form. His first victim was a nameless boy who he killed and replaced in Son of Nergal and has permanently used the appearance of as his default form ever since.