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- “Seriously, though, who would actually believe that mansions get given away in contests?! Talk about stupid! What do they feed you Mario brothers anyway... gullible soup?”
- —King Boo, Luigi's Mansion
King Boo is a Big Boo wearing a crown. He is one of Bowser's Boo generals that has appeared in many Mario spin-off titles.
History
Super Mario series
Super Mario Sunshine
King Boo makes his first appearance as the leader of the Boos on Isle Delfino and appears as a boss in Sirena Beach, the sixth world of Super Mario Sunshine. He is apparently made out of goop, as can be seen during his defeat, meaning that Bowser Jr. might have drawn him. This game also notes that he dislikes spicy food, which serves as the key to defeat him.
He is named King Boo by the American localization team. However, the original Japanese version assigns a unique name to this character (?????, meaning Boss Boo), signifying that this King Boo is a separate character from the King Boo that appears in Luigi's Mansion and its sequel.
Super Mario 64 DS
King Boo appears in the Super Mario 64 remake, Super Mario 64 DS. As the boss who locked Luigi away, he captures a part of Princess Peach's Castle. However, in the English version of the game, King Boo is incorrectly called Big Boo, even though the Big Boo from the original game also appears in the game.
Regardless, King Boo aids Bowser in his conquest of Peach's Castle by capturing Luigi and imprisoning him in Big Boo's Haunt. Specifically, Luigi is imprisoned behind a painting of himself inside the haunted mansion. Mario heads to the Big Boo Battle level to save Luigi. Upon entering the level, he is greeted by King Boo, and from there he must listen for King Boo's cackle and enter the door where it is loudest. Eventually Mario reaches the end and fights King Boo to free Luigi. To defeat King Boo, Mario has to follow King Boo's movements in the arena's mirror and stomp him when he comes close enough. Ultimately, Mario saves his brother from King Boo and together (with Yoshi and Wario once he is saved) they defeat Bowser and save Princess Peach.
New Super Luigi U
King Boo makes a small cameo in New Super Luigi U in the Secret Exit of Vanishing Ghost House, where his art from Luigi's Mansion: Dark Moon can be seen in the form of a sign hanging from the side of the building[1].
Mario Kart series
Mario Kart: Double Dash!!
King Boo is an unlockable character in the racing game Mario Kart: Double Dash!!. This is his first appearance in a spin-off, and also his first playable appearance. His appearance in this game results in his frequent appearances in the Mario Kart series, as he appears in Mario Kart DS as a boss and in Mario Kart Wii as a playable character again. Despite being a ghost, King Boo is classified as a heavyweight driver due to his large size. His partner is Petey Piranha, and both have the ability to use any other character's special item (if they are going to get a special item, it is randomly chosen, based on their race position).
King Boo's personal kart is the Boo Pipes, which is a gray version of Petey's Piranha Pipes kart, but with an image of a Boo instead of a Piranha Plant where the item-using partner stands. However, the more important difference between the two is that, while the Piranha Pipes has high speed and low acceleration, the Boo Pipes is vice versa, with low speed and high acceleration, which is quite odd for a heavy kart. In fact, the only other heavy kart like this is the Offroader from Mario Kart Wii.
Oddly, when partnered with Petey Piranha, King Boo appears smaller than Petey. This is most noticeable before the race starts if he is not the driver. Yet, if partnered with another character, he appears much larger than the other character. He also appears to slightly shrink when he becomes the driver. On the second "Thanks for Playing!" screen (after beating the Special Cup with a Gold Trophy while all other cups already have a Gold Trophy), King Boo appears in the background, chasing Wario and Waluigi.
Mario Kart DS
In Mario Kart DS, King Boo, along with six other bosses from Super Mario 64 DS, appears as one of the bosses of the Mission Mode. King Boo is the boss of the fourth level, with the playable character being Peach. Peach attempts to collect fifty coins on the Lava Arena within the time limit, but after a few seconds King Boo appears, surrounds her, and then attacks her, stealing ten of her coins and making all the other, uncollected coins disappear, before teleporting to another area of the arena. To take them back, Peach has to drive and bump into King Boo, making him transparent. Then, she can drive through him again, retrieving her coins and making the other coins reappear. However, King Boo soon reappears a few moments later to get the coins again. After collecting all fifty coins, Peach finally defeats King Boo.
Mario Kart Arcade GP 2
King Boo makes a cameo appearance in Mario Kart Arcade GP 2 on the Yoshi Park 2 course in the Horror House. Here, a large picture of him appears near the end of the course where it inhales many ghosts and the racers. As the player drives into his mouth, they are warped back to the beginning of the course.
Mario Kart Wii
King Boo returns in Mario Kart Wii as a large-sized driver, and is unlocked by completing the Star Cup in 50cc. He features a good handling bonus and a small off-road bonus, which comes in handy for vehicles like the Offroader, Wario Bike, and Phantom. He is used as the Staff Ghost on the SNES Ghost Valley 2 course. In the credits, he appears driving the Jetsetter on the SNES Ghost Valley 2 course.
King Boo is one of very few characters to have his standing animation on the Character Select screen altered from Mario Kart: Double Dash!! In this case, his tongue is not sticking out of his mouth the whole time like before; now, before selecting him, his tongue is still in his mouth. Other characters whose sprites were altered include Birdo, whose bow was made larger, Wario, who now has short sleeves, Waluigi, who, when selected, does something somewhat different from what he did before, and Donkey Kong, who does not have his mouth as wide open as before while looking in another direction.
Mario Baseball series
Mario Superstar Baseball
King Boo is a playable character in Mario Superstar Baseball and a member of Wario's baseball team, the Wario Greats. He does not need to be unlocked. He is a power hitter, using his white, crowned, Boo-like baseball bat to hit long outfield drives and even homeruns. King Boo has chemistry with Boo and Petey Piranha, and he has bad chemistry with Luigi. However, like Boo, his hits are usually very curved, and this causes them to be fouls most of the time.
Mario Super Sluggers
King Boo reappears in Mario Super Sluggers and is one of the few characters that were on Wario's team in the last game that are still on it. He is once again a power hitter, and he also excels in pitching, with both stats being his highest. In Challenge Mode, he appears immediately after the player defeats Bowser Jr. at the Wario City stadium, stating that he turned invisible and was watching the whole game. However, he is uninterested in joining the player's team unless the team has some Boos with them. This is accomplished by finding both Boo and the Mini Boos item. In this game, King Boo has chemistry with Petey Piranha, Boo, King K. Rool, and Wiggler, but he has bad chemistry with Mario, Luigi, Baby Mario, and Baby Luigi.
His baseball bat does not have a physical crown on it anymore. In this game, it is represented as a crown picture above the "Boo mouth" picture. As well, in this game only the colours of the jewels on his crown are reversed.
Super Princess Peach
King Boo aids Bowser in Super Princess Peach. In this game, Bowser's forces capture both Mario and Luigi with some Toads. Although King Boo does not directly watch over Mario or Luigi, he is given a captured Toad to guard, as Mario is Bowser's prisoner while Luigi is Kamek's prisoner.
After adventuring through Vibe Island, Peach discovers where King Boo is and engages him in combat. During the battle, King Boo fires blue homing fireballs at Peach. Peach can damage King Boo with direct light, which is made by having his own flames light the candles spread throughout the room. After a few attacks, King Boo uses his Gloom vibe, which causes him to cry, which extinguishes the candle flames. After enough attacks, King Boo is eventually defeated. With King Boo eliminated, Peach rescues the Toad and continues against Bowser's forces. Ultimately, Peach defeats both Kamek and Bowser and rescues Mario and Luigi.
Mario Party series
Mario Party 8
King Boo makes his first appearance in a Mario Party game in Mario Party 8. He gives out Stars on his own course King Boo's Haunted Hideaway. He hides in one out of three altars and gives the player a Star for ten coins if they can find him where he is hiding. If the player is able to buy the Star, he will "summon the bizarre" and disappear, causing the player to jump into a Warp Pipe that returns them to the starting point. This "makeover mojo" causes the whole layout of the course to change. However, if they do not have enough coins, King Boo will cause a pitfall to appear under the player, causing them to return to the starting point and allowing the next player that reaches him to buy the Star. Additionally, if the player fails to steal anything from the board event Pickpocket Pinball, the hosting mouse sounds like King Boo. The player can purchase King Boo sounds from the Fun Bazaar, along with a figurine titled "King Boo's Revenge". When the player presses 
on the figurine during the Figurine Parade, King Boo floats up and gives off a brilliant glow, as if he is a light bulb.
Mario Party 9
King Boo appears as the stage boss of the board Boo's Horror Castle in Mario Party 9. He is fought in the minigame, King Boo's Puzzle Attack, where the competitors line up power-ups in lines of three or more to deal damage to him. Once his health reaches halfway, he throws Boos onto the field which can stun the player's cursors, making them unable to move until they shake the Boo off. What is unique about his boss battle is that if a player gets the Final Attack, they get ten points instead of the usual three, likely because the players can easily accumulate points up to fifty. In the final fight against Bowser, King Boo can also appear to hinder the players' progress. If Bowser throws the die with all the stage bosses' faces on it and King Boo's face appears on top, King Boo will materialize above the stage. Then, he will summon five Boos that move in a counterclockwise direction while moving outwards. When the Boos reach the boundaries of the stage, King Boo summons another five Boos, except this time they move clockwise. After that, King Boo disappears from the battle unless his face appears on the top of the die again. King Boo also has a constellation that can be returned to the sky; it is called the Ghost Major. It is located in the farthest left portion of the sky, near Big Bob-omb's constellation and right next to Boo's constellation. Like all constellations, it can be bought for fifty Minigame Points.
Mario & Sonic series
Mario & Sonic at the Olympic Winter Games
In the Wii version of Mario & Sonic at the Olympic Winter Games, King Boo appears as the first and sixth (in Individual Festival mode only) rival. King Boo races the player in the Alpine Skiing Downhill event in both Individual and Group Festival modes but he only races the player in Giant Slalom in the Individual Festival mode. King Boo has no skis so he goes down the hill smoothly. If the player loses to him, the guide (either Cream or Toad) says that it is unfair because King Boo has no skis.
In the Adventure Tours mode in the DS version, King Boo is the mid-boss of Polastraits. When the challenger approaches him, King Boo tells Mario/Sonic that he must race someone afraid of him. He selects Luigi and they must race in Giant Slalom. In this version, King Boo is transparent and the player can ski through him. He also appears slightly larger than in his other appearances.
Mario & Sonic at the London 2012 Olympic Games
King Boo appears in the next installment of the Mario & Sonic series, Mario and Sonic at the London 2012 Olympic Games. In the Wii version he is one of the characters that can be found in the London Party mode, and when he is encountered, he will challenge the player to the Dream Sprint event. Since he has no feet, though, it is questionable how he gets the ball to roll. If the player reaches the goal line before him, they will get a sticker as an award.
In the 3DS version, King Boo teams up with a Boo to face Sonic and Tails in the Badminton Doubles event. After winning, the player finally completes Sonic's second episode.
Mario & Sonic at the Sochi 2014 Olympic Winter Games
King Boo appears in Mario & Sonic at the Sochi 2014 Olympic Winter Games as a rival in the Legends Showdown mode. He is found in Area 4, and is raced against in Short Track Speed Skating 1000m.
General information
Physical appearance
He resembles a white Boo with a golden, five-pointed crown, though he is noticeably larger. As well, the crown has a blue jewel on the front and four red jewels around the sides in every game except Mario Super Sluggers. He first appeared in this form in Mario Kart: Double Dash!!, and it remains his current appearance to this day.