Grim

 The Grim Reaper or just Grim is the main protagonist of the franchise. He is an undead skeleton personification of Death, who speaks with a Jamaican accent. After having lost to Billy and Mandy after Mandy cheated in a limbo contest over the soul of Mr. Snuggles (and also the soul of Billy himself), Grim is now forced to be their best friend forever. If Grim does not live up to his commitment, he will be stripped of his powers and be forced to spend eternity in Underworld Jail.