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 created a spooky mansion and lured Mario and Toad into the mansion. King Boo sealed Mario in a portrait. He then tricked Luigi into thinking he had won the mansion. It was up to Luigi to save Mario and capture all the ghosts in the mansion. At the final level of the game, Luigi confronted King Boo, who was wearing a Bowser disguise.
Whether or not King Boo was inhabiting Bowser's reanimated corpse or was in a artificial suit is a very debateable topic. Madame Clairvoya, a fortune teller in the game, confirms that Bowser was indeed "soundly defeated" before the events of Luigi's Mansion and asks how he could have been revived. This is further confirmed when we see both Bowser and King Boo's spirits trapped in a painting. However, there are arguements that it is indeed a disguise, but all information about this come from outside-game sources, such as the Player's Guide. It can be inferred that King Boo either possessed Bowser's corpse or a highly detailed disguise.
Luigi could damage King Boo in his Bowser costume by blowing its head off. Once that happened, King Boo would emerge and Luigi had the chance to weaken him. When King Boo was finally sucked up, the Bowser disguise fell to the ground.
On a note similar to the subject: Bowser was revived before the events of Super Mario Sunshine, possibly by either Bowser Jr. or King Boo.
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 tried 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. In both games he was partner and friend to Petey Piranha, although the reason for this is unconfirmed. They were probably chosen to be friends because they were both misfit characters and were both bosses in Super Mario Sunshine.
