King Boo is an enemy character in "Luigi's Mansion" and other Mario games. He is often the boss in a stage, since he is larger and more powerful than the other Boos, common ghostly enemies of Mario and Luigi.
In "Luigi's Mansion," King Boo is the final boss and commands the other Boos Luigi has been fighting throughout the game to rescue Mario. He also appears as a boss in "Super Mario Sunshine," where he can be defeated by tossing chili peppers into his mouth, and he's a boss in "Super Princess Peach." In the "Mario Kart" and "Mario Baseball" series of games, he is often a playable character.
King Boo dislikes Mario and Luigi and sometimes works with another Mario villain, Bowser, to defeat the Mario brothers. He usually wears a crown and often has powers based on the number of other Boos in his vicinity.