Billy is Grim and Mandy's best friend; He is an 8-year-old happy-go-lucky child. He has an extremely low IQ of -5, having been outperformed by a shovel and two candy bracelets on an IQ test. He has an extremely large, pink nose, wears a blue and white striped shirt, and covers his orange hair with a red cap. 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 far kinder to Grim than Mandy is, and while he almost always goes against the Reaper's advice and uses him as a plaything, he appears to genuinely like him and 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 and Jeff is a huge spider. His greatest fears were combined by Horror's Hand into a monstrous clown-spider-mailman hybrid in the Big Boogie Adventure movie.
In Underfist, the origin of his fear of spiders is shown through in a flashback. A yellow marshmellow bunny named Bun-Bun used to sneak into Billy's room when he was a baby and tormented him with real spiders. This is explained by Bun-Bun during the flashback so as to torment Jeff about his problems with his dad.
Billy's family is Harold (his father), Gladys (his mother), Nergal (his uncle) Aunt Sis (his aunt), Nergal Jr. (his cousin), and his grandmother, who lives in the Netherworld (for reasons unknown). He is also very good friends with Irwin and, occasionally, with Pud'n, however his best friends are Mandy and Grim.