King Boo (Luigi's Mansion)

"Hey, remember when you trapped me in a painting for all eternity? Good times. But I got out! And now I'm painting the town red!"

- King Boo to Luigi This version of King Boo is the the main antagonist of the Luigi's Mansion sub-series. He retains his position as the absolute ruler of the Boos, but this version works independent from Bowser and is notably much more evil than his main series counterpart.

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.

Personality
This version of King Boo is notably darker than the main series version. King Boo initially was much more fair in his rule, going out of his way to save a high-ranking Boo and generally acting fond of his own troops. He despised the Mario Brothers, and so plotted to get rid of them by capturing Mario and then tricking Luigi, so he could store them both into paintings. He also hates Professor Gadd for similar reasons, his Poltergust the only thing he fears.

In the sequel, all King Boo's altruistic traits have been rendered null. King Boo, after his first defeat, started to loathe Luigi even more than Gadd and Mario, to the point of conducting an elaborate revenge scheme involving shattering the Dark Moon and causing the benevolent ghosts to fly out of control. King Boo is consumed by revenge, and notably cares little for his troops, experimenting on the ghosts he enslaved and broke into submission and treating his Boos as common casualties. It reaches the point where King Boo willingly opens a portal to the ghost's homeworld with the intent to destroy the universe as an act of spite. King Boo is noticably less comedic and more openly cruel to Luigi, as well as sadistic, clearly relishing the terrified Luigi's pain with his mind games.