After losing a bet to Billy and Mandy, the Grim Reaper is fated to be their "best friend" forever. Treated like a slave, Grim is forced to do everything from creating portholes to other dimensions to household chores.
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.
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!
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.