Grim is over 137,000 years old. He's clever and distinguished, and his previous job was to collect and deliver spirits to the underworld. By losing a limbo contest to Billy and Mandy, Grim is unfortunately fated to be their "best friend" forever.
Mandy is Billy's closest friend. Almost always angry, she hardly ever smiles (and denies it when she does), and usually gets what she wants by being scary. Grim sometimes refers to her as "horrible demon child" and even Mandy's own parents are scared of her!
Billy is a happy-go-lucky, hyperactive idiot. He's far too stupid to be scared when faced with monsters, and his curiosity leads him into all kinds of trouble. Also, strangely, Billy is terrified of clowns and has recurring nightmares about them.