Billy is the titular main character of the franchise. He is Mandy and Grim's best friend. He is a dim-witted, happy-go-lucky boy living in Endsville. He has an extremely low IQ of -5, having been outperformed by even inanimate objects. Good-natured and incredibly dopey, Billy's curiosity tends to get him into trouble he can't get out of by himself. Billy enjoys the company of Mandy and Grim and would do just about anything for them.