King Boo

“Seriously, though, who would actually believe that mansions get given away in contests?! Talk about stupid! What do they feed you Mario brothers anyway... gullible soup?” —King Boo, Luigi's Mansion

King Boo is the leader of the boos and he is the main antagonist of the Luigi Mansion's series and is the arch enemy of Luigi.He is an ally with Bowser who has helped him in his various evil schemes.

Luigi's Mansion
Before the events of Luigi's Mansion Professor E.Gadd  used his ghost portrificationizer and E.Gadd trapped Boolossus into a portrait then he hung it inside his gallery which is inside the lab. King Boo is angered so he devises a plan in order to free Boolossus. King Boo's forces attacked the laboratory. Then the boos reserved the settings of the Portrificationizer and freed Boolossus.King Boo ordered that all the portait ghosts that E.Gadd had captured be released as well. After all the portrait ghosts are released King Boo decides to make a huge mansion and places it next to the Professor's lab. Afterwards the recently freed portrait ghosts decided to inhabit the mansion. With a huge mansion and an army of ghosts he decided to get revenge on Mario and Luigi by capturing them. King Boo tries to trick Luigi Hoping to capture him King Boo sends him a letter. When the letter gets to Luigi it says that he won a mansion in a contest and provides the location of the prize.Luigi asks his brother Mario to investigate the mansion.and says that he will arrive shorty after.Mario and Toad travel to the mansion but Mario is easily overwhelmed by King Boo's forces.King Boo imprisons Mario in a portrait similar to how Boolossous was imprisoned before.King Boo then places Mario in the Altar which is the basement.