King Boo

King Boo is said to be the ruler of all evil Boos and weilds awesome power when other Boos are nearby. King Boo first appeared in Luigi's Mansion where he tricked Luigi into thinking he won a mansion. When Mario and a bunch of Toads arrived at the mansion before Luigi, King Boo attacked them and sealed Mario in a portrait. It was up to Luigi to save Mario and capture all the ghosts in the mansion. After he caught all the Boos, Luigi confronted King Boo. King Boo sucked Luigi and himself into a portrait of Bowser and the battle began. King Boo attacked Luigi in a the reanimated corpse of Bowser (or at least, a fairly accurate representation of the Koopa King). Luigi could damage the monstrosity by blowing it's head off. When that happened King Boo would come out and Luigi got the chance to weaken him. When King Boo was finally sucked up, the Bowser corpse fell lifelessly to the ground.

King Boo somehow came back in Super Mario Sunshine where Mario fought him in the casino. He attacked using a roulette which flung either enemies, coins or fruit. He could also spit Boos. If fruit came from the roulette, Mario would have to toss a chili pepper at King Boo's tongue. His tongue would then catch fire. While King Boo is trying to put his tongue out Mario could smash him in the head with another fruit.

King Boo appeared again in Super Mario 64 DS where he replaced Big Boo. Mario had to rescue Luigi from King Boo. He made another appearance in Super Princess Peach where he could be hurt using light.

King Boo made an appearance as a secret character in Mario Kart: Double Dash!! and, despite being a ghost, was a heavyweight. He also appeared as a character in Mario Superstar Baseball.