Billy Rocks is an assumed name; he chose it at random when he decided to remain on the earth and keep an eye on Goodnight… at least, that’s what he tells his superiors when questioned. Honestly, he just enjoys spending time with the young demon. Besides, he has his underlings to do the dirty work for him.
Billy’s true name is Azrael, and he is an angel of death. He enjoys the irony of being friends with the demon who wears the same moniker.