Grim

The Grim Reaper or just Grim for short, is a character from Grim & Evil. In Car Wash, he is played by MightyMario, only appearing in a Halloween arc and a couple posts afterward. 
Grim appears in Cayde's apartment for her mouse, Pericles, who was sick. Frightened, Cayde and Zerris decide to play a game of billiards for Pericles. Zerris ends up winning and saves the poor mouse, while also becoming their "best friend". He later appears at a Halloween party at the Car Wash. It is assumed that Grim returned to the underworld or to Endsville, where he resides in the Cartoon Network show he originates from.