Billy is the main protagonist. He is an extremely happy-go-lucky child with an IQ of -5. He has an extremely large, pink nose, wears a blue and white striped shirt, and covers his ginger-red hair with a red hat. His main affiliations are Grim and Mandy. Mandy is Billy's best friend, though she treats him more like a servant than a friend. It has been hinted several times that Billy may have underlying feelings for Mandy that even he (in his stupidity) may not fully realize. Billy is definitely kinder to Grim than Mandy is, and while he may go against the Reaper's advice and use him as a plaything, he appears to genuinely like him. Of the two children, Billy is the one who most often tells Grim that he's his "best friend".
Due to Billy's foolish desires, which he orders Grim to bring about, he is often the cause of Grim and Mandy's mishaps. Although generally good-natured, he has occasional fits of stubbornness, violence, and rage, which shows insanity that even Mandy seems to fear. It is suggested that while Billy is the most innocent character of the three, he ends up causing the most problems due to his irrational and often impulsive choice of actions. Billy is terrified of clowns to the point of having recurring nightmares about them, although eventually, due to an encounter with his inner frat boy, his fear is transformed into a psychotic loathing and hate for them. He is also scared of the mailman and spiders, which is quite unfortunate because he is Jeff's father.
Billy's family is Harold (his father), Gladys (his mother), Nergal (his uncle) Aunt Sis (his aunt), Nergal Jr. (his cousin), Jeff the Spider (his son), DeUglio (his brother) and his grandma, who lives in the Netherworld (for reasons unknown). He is also very good friends with Irwin and, occasionally, with Pud'n.