So it seems, to a casual viewer, that Grim is somehow tied to Billy and Mandy and can’t just leave. What’s the deal?
The story:
Grim came to get Billy’s hamster and Mandy challenged him to a contest. If he won, he got Mr.Snuggles. If they did, they kept the hamster and he had to be their friend forever. Mandy tricked him and he lost at the Limbo. Now he is bound to them. The end.