King Boo is one of three main antagonists and the leader of one of the three armies. King Boo has appeared in Luigi's Mansion, Super Mario Sunshine, and Luigi's Mansion Dark Moon. As his name suggests, he is the King of all Boos.
He appears as the main antagonist and final boss in the game Luigi's Mansion along with Bowser In Super Mario Sunshine, King Boo has a minor role as a mini-boss and can be defeated using the chili peppers. In Dark Moon, he once again appears as the main antagonist and the final boss.
Luigi's Mansion and Boss Battle.
In the game: Luigi's Mansion, King Boo is the final boss and the main antagonist of the game. He is first seen when Luigi accidentally released all the Boos in the storage room. He is once again seen when Luigi sees them in the Secret Altar through the Bottom of the Well. After Luigi defeats the Artist portrait ghost and captured 40 Boos, He confronted King Boo. King Boo then battles Luigi with a ghostly version of Bowser (Who died in Paper Mario but adds up to a theory). Luigi had to use the Bombs thrown by Ghost Bowser and launch them at Bowser's head. King Boo will exit his head and Luigi can suck him up. King Boo has 500 HP. However Bowser will shoot frozen shards at Luigi making it difficult. The process must be repeated several times to win. Afterwards, King Boo is sucked up but escapes to seek revenge on the Mario bros later....
Super Mario Sunshine
King Boo appears as a mini-boss in the mission: "King Boo Down Below". He has a different appearance and has drool on his tongue. He can be defeated by using the Chili Peppers he spits out. When he eats them and his mouth is set on fire. This process must be repeated thrice and becomes increasingly difficult after every time. After defeating King Boo, Mario is awarded with a Shine Sprite. King Boo was the cause of why many Boos invaded the Hotel. He seems to be weaker in this game.
Luigi's Mansion: Dark Moon
King Boo once again returns as the main antagonist of the game. He is responsible for the violent behavior of the ghosts in Evershade Valley because he destroyed the Dark Moon. Luigi had to recollect the pieces and then fight King Boo once more. King Boo was fought in the final arena known as "King Boo's Nightmare". After defeating King Boo, Evershade Valley returned to it's original peaceful
Boolossus is King Boo's strongest underling. He serves him with brutal force and scares anyone who denies the brutal might of King Boo.
Mario & Luigi
Unknown to many people, King Boo has hated the Mario Bros. way before the game of Luigi's Mansion. How? In Super Mario World, Mario and Luigi have killed many Boos (despite being ghosts) in the ghost houses. King Boo since then vowed to destroy the brothers....
Once Again, unknown to many players King Boo may have a relation with Bowser that has developed into a theory.
Ghosts & Boos
King Boo has developed friendly relations with the ghosts. He now has an entire army of ghouls
, boos, and undead to destroy the Mario bros....
This video explains a theory behind the connection between King Boo and Bowser.
It is summed up as this: King Boo is the dead version of Bowser, due to the various people similarities, such as: Power and Monarchhood, and the fact that they almost never appeared in the same games together. (or atleast appeared onscreen at the exact same time that is.)