King Boo (Super Mario Sunshine)

King Boo is the leader of the Boos on Isle Delfino and appears as a boss in Sirena Beach, the sixth world of Super Mario Sunshine. He was apparently made from Goop, as can be seen during his defeat. His game of origin notes that he dislikes spicy food, which serves as the key to defeating him.

He was named King Boo by the American localization team. However, the original Japanese version assigned a unique name to this character (ボステレサ, meaning Boss Boo). Players have no control over the camera during the King Boo boss fight.

Super Mario Sunshine
King Boo primarily appears as a boss in episode five, King Boo Down Below, of Sirena Beach in Super Mario Sunshine.

While Mario is searching Sirena Beach for Shine Sprites, King Boo takes up residence inside the basement of Hotel Delfino's casino. Down there he produces enough noise to alert the Hotel Manager, who proceeds to inform Mario of the ghost's presence. The player is able to access the casino's basement by having Mario Ground Pound on the one tile on the giant roulette wheel that glows purple. The inner ring of the wheel then descends and the battle against King Boo starts.

King Boo's primary strategy in this boss battle is bringing out a giant slot machine with images of pineapples, coins, Stus, and question marks. Mario needs to spray the boss with water, who will then activate the slot machine with his tongue. If the player waits too long to do this, King Boo will start to send out smaller Boos to attack Mario. Once the slot machine is activated, there are five possible outcomes that determine how the battle will proceed. After each round, King Boo will wait for a short time and then bring out the roulette wheel again until defeated. To get a chance to damage King Boo, the player needs to get three pineapples on the slot machine. To harm him, the player needs to have Mario pick up a pepper to set the ghost's tongue on fire, and then hit him again with any other available fruit. If he is hit without having his tongue disabled beforehand, King Boo will not take damage from the attack. After three direct hits with any fruit other than peppers, King Boo will be defeated and release a Shine Sprite.
 * Three pineapples make various fruits appear that can be picked up and thrown.
 * Three coins make 10 coins appear which allows the player to restore Mario's health.
 * Three question marks summon a horde of randomly chosen enemies. Mario needs to defeat them or evade them until they disappear to proceed.
 * Three Stus make six Strollin' Stus or Smolderin' Stus appear that chase Mario.
 * Any other combination causes a bunch of bubbles to appear. They leave behind Water Bottles after being sprayed with water. If they are not sprayed, they will hurt Mario if he touches them.

Trivia

 * This is the only main boss of Super Mario Sunshine which is 100% absent from any future game, Petey Piranha and Gooper Blooper have appeared in various Mario Spinoffs, Wiggler's voice Clips can be heard in Petey Piranha's battle in Mario & Luigi: Paper Jam and the names Eely-Mouth and Phantamanta can be seen in Mario Golf: Advance Tour. He was most likely absent after Super Mario Sunshine because another character with the same name replaces him.