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

King Boo is 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.

He is voiced by Kazumi Totaka who also voices Professor E. Gadd and Yoshi.

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.

Theories
Many fans speculated due to Bowser Robot that King Boo used in his first battle against Luigi, as comparing to the other mechs in Mario universe, this one looks quite realistic. As Mario universe has no set timeline, fans say that it is possible for Bowser to get killed and King Boo inhabiting his corpse. As the last game before King's debut was "Paper Mario" set in Japan as "Mario Story", people say that one of the ghost in the first mansion is reading "Mario Story", which can be historical book. Also, fortune-teller in the mansion exclamed that King Boo could possibly revive Bowser, which means that King Koopa was already dead. There are several visions of the situation.
 * King Boo revives Bowser and takes his place as Koopa King, so in future games Bowser is gain supernatural abilities to stay alive even when burned to the bones like in "New Super Mario Bros." with his Dry Bowser form. Then, King Boo which we see is actually just a Big Boo with a fancy crown, imitating King Boo. This can be canon due to the blue supernatural fire of Bowser, his Dry Bowser form (which also explains that regular Boos inhabit Dry Bones and are confused, when player beats them only for Boos to return into skeletons), and the gold portrait of King Boo, showing him and Bowser having a confrontation.
 * King Boo revives Bowser and they make an alliance against Mario Bros, so King Boo is able to revive Bowser anytime when he gets killed and takes his body until Bowser recovers. This version means that King Boo looks like a regular Big Boo because of the lose of his enchansed power-crown, which gives him incrediable battle powers and creepy look. This can be canon due to the pose of Bowser in the portrait, like having sleep before King Boo inhabits him, and also gold portrait of King Boo, showing his confrontation with Bowser.
 * King Boo and Bowser are one and the same person. After his death, Bowser turns into a ghost, namely Boo, but still has some of his magic powers, which allowed him to bow Boos to his will. Then he recreates his body (or creates a very similiar mech) to inhabit it. However, he wasn't able to fully recover, so he traps Mario, though fails in capturing Luigi. This theory provides a thought of Boo-Regent like the one in the first variant. This can be canon due to Bowser having good relationship with Boo and also quote of King Boo of Mario "giving him much troubles in the past". Also, this is can be consider canon, as Bowser still has powers to be revived, as it is done not only by himself, but by Bowser Jr, Koopalings and Kamek on several occassions.