Grim

Grim is the main protagonist of the Cartoon Network TV show, The Grim Adventures of Billy and Mandy. Originally the Grim-Reaper for Earth, Grim came upon Billy and Mandy in his capacity as the Grim Reaper when Billy's hamster died and it's spirit needed to be reaped. Billy tried pleading with Grim to no avail at which point Mandy decided to offer him a bet for the fate of the hamster's soul, with Grim casually remarking that if they actually beat him he'd be their best friend forever.

Grim was beaten by the crafty Mandy and determined Billy and in addition to having to allow the hamster to live, Billy and Mandy's interpretation of the term "best friend" meant Grim had to stay by their sides and do whatever they asked of him. The series revolves around the children's prolific abuse of Grim's vast powers and supernatural connection.