A prankster is someone who plays pranks — a person who sets up tricks to make other people look silly. The word can sound almost fond, like a class clown who loves a joke, or it can sound less kind, like someone who never knows when to stop. News stories and online videos often call a repeat joker a prankster. It is informal to neutral, a little cheerful and story-like in tone, but still in use. You would not put it in a very serious report unless you wanted that flavour.