William "Billy" is the secondary protagonist also the deuteragonist of The Grim Adventures of Billy and Mandy. Billy lives with his overprotective parents, Harold, and Gladys, in Endsville. He does a lot of stupid things. He has a spider-like son named Jeff.
He deeply cares about his friends and will do just about anything for them. He at sometimes can be an antagonist when he gets very angry and delusional. He has a fear of spiders, (especially towards Jeff his spider son), clowns (he overcame it in season 3, though in the "big Boogey Adventure" special shows that he still dreads them) and mailmen.
He is enthusiastic and very curiously and dimwitted like his father, Harold.