Hitman is a Thief type Dark Hero introduced in Boss Monster: The Next Level. He is one of four Dark Heroes.
Due to the nature of this card, a player can discard a Thief room from their hand to give this hero a health boost, thus more likely ending in an opponent getting a wound. Because of the special effect of this card, it earns a Star, and is thus completely optional to play with. It also cannot equip Item cards.
Hitman seems to be a reference to the video game of the same name, which features a hitman that is shaved bald, that also wears all black and has a proficiency in weapons.