king boo
As the name suggests, he is the king of all Boos. He debuted in Luigi's Mansion, where he served as the main villain and final boss. Since then, he has appeared in numerous Mario spinoff games (such as Mario Kart) and has reprised his role as the villain in Luigi's Mansion: Dark Moon and Luigi's Mansion 3. He is often shown as Luigi's main nemesis, much like Bowser vs. Mario.