King Boo (Luigi's Mansion)

King Boo is the king of all Boos as his name suggests. He is a disturbing individual who has a particular contempt for scaring. He is also the arch-nemesis of Luigi, and as such serves as the main antagonist for the Luigi's Mansion sub-series.

Luigi's Mansion
He made his first major appearance in the game Luigi's Mansion. He is angry with the Mario Brothers for causing them so much trouble in the past, so he designs a plot to get rid of them both. He sends Luigi a note telling him he has won a mansion... in a contest that he didn't enter. Luigi calls Mario and tells him the good news, asking him to go there and meet him so they can celebrate. However, when Luigi arrives, Mario is nowhere to be seen, and the mansion is infested with ghosts.

Later in the game, we find King Boo staring at a painting with Mario trapped inside. Also, we find out that he was the one who created the mansion and released all the ghosts within it. At the end of the game, Luigi fights what appears to be Bowser, revived from his previous (unseen) defeat by King Boo, but is really King Boo in a Bowser costume. Luigi defeats King Boo and sucks him into the Poltergust, and later turns him into a portrait with the Portrificationizer.

Dark Moon
King Boo reappears in the sequel to Luigi's Mansion on the 3DS and seeks revenge against Luigi for his imprisonment. At same point in this canon, E. Gadd lost the painting at a yard sale. King Boo initiates the events of the game by destroying the Dark Moon, a purple crescent-shaped artifact that keeps the ghosts of Evershade Valley in a docile state. Shattering the Dark Moon into pieces, King Boo incites the ghosts to turn on E. Gadd which leads to his summoning of Luigi for help.

Eventually, King Boo reveals himself to Luigi after intercepting E. Gadd's teleportation and entraps Luigi in an alternate dimension. There, King Boo demands the Dark Moon pieces from Luigi so that he may conquer the Mushroom Kingdom with a spectral army in exchange for releasing Mario, once again trapped in a painting. Luigi naturally refuses and the battle ensues, with Luigi triumphant. However, after catching King Boo in the Poltergust 5000, no specific mention of King Boo's fate is ever brought up.