King Boo is a large, purple eyed Boo, who is the leader of the Boos and the main antagonist of all 3 Luigi's Mansion titles. (excluding the Arcade.)
King Boo exhibits powers such as teleportation, controlling lightning, spirit balls, shape-shifting, and materialization of objects along with ghost-like abilities such as phasing through physical objects and invisibility.
Prior to the events of Luigi's Mansion, Professor E. Gadd captured Boolossus. Using a device called a Ghost Portrificationizer, E. Gadd trapped Boolossus in a portrait and hung it in his Gallery inside his lab in Boo Woods. This angered King Boo, so he devised a plan to rescue Boolossus.
King Boo and his forces attacked the laboratory. The Boos reversed the settings of the Portificationizer and released Boolossus. King Boo then ordered that all the ghosts that E. Gadd had captured should be released as well. After releasing all the portrait ghosts, King Boo created a huge mansion and placed it next to the professor's lab. King Boo, his Boo minions, and the recently freed portrait ghosts decided to inhabit the mansion. With a huge mansion and an army of ghosts, King Boo decided to exact revenge upon Mario and Luigi for all the Boos they attacked during their adventures by capturing them.
Hoping to capture Luigi, King Boo sends him a letter. The message states that Luigi has 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 is arriving shortly thereafter. Mario travels to the mansion, but right after he enters, he is instantly overwhelmed by King Boo's forces and captured. King Boo then imprisons him in a portrait, similar to how Boolossus was imprisoned. Mario is placed in the Secret Altar in the basement of the mansion. With Mario captured, King Boo is ready to capture Luigi.
While waiting for Luigi, King Boo and his Boo minions wait behind a secret door in the floor of the Storage Room. When Luigi enters the room and reveals the secret door, the Boos spring out from the floor. However, the Boos (including King Boo) stop their surprise attack when they notice the Poltergust 3000 on Luigi's back. Luigi had been given the Poltergust 3000 by E. Gadd so he could recapture the Portrait Ghosts and save his brother. The Poltergust 3000 is one of the few things King Boo fears, and he and the other Boos scatter and hide in the mansion. King Boo travels to the Secret Altar in the mansion's basement and hides there with the Mario painting.
Later on, after Luigi successfully captures all 50 Boos, Luigi makes his way to the secret altar and King Boo begins speaking to Luigi. King Boo makes fun of Luigi, mocking how gullible and stupid Luigi is for believing Mansions get given away in contests. King Boo then forcefully pulls Luigi into a painting of Bowser, bringing Luigi into an illusionary world. (Paranormal Dimension?) He then utilizes a Bowser mech suit to attack Luigi. Luigi uses the spiked balls King Boo throws as weapons and shoots them at the Bowser mech's head, causing the head to pop off. Luigi then sucks up King Boo with the Poltergust 3000, and saves Mario.
Luigi's Mansion: Dark Moon
Luigi's Mansion 3
Trivia / Fun Facts
• It is unconfirmed whether or not the King Boo seen in the Luigi's Mansion titles is the same King Boo seen in the spinoff and mainline Super Mario titles, although they are believed to be separate due to their different colors and physical attributes. (e.g. the Luigi's Mansion King Boo has larger more puffed out "eyebrows".) This is further backed up by the dialogue in the Luigi's Mansion sequel games, where King Boo explains he's been in a portrait since the previous game, leaving no room for separate activity in-between. It is also possible that King Boo changes his own look to match the crown he is wearing, since he has shape-shifting abilities. He's been shown to alter his own appearance before, such as in Super Princess Peach where he gives himself blue eyes.
• When King Boo says, "I will not give up my favorite decoration. I like Mario just where he is.", he references a quote from Star Wars VI: Return of the Jedi, when Jabba the Hutt says, "I will not give up my favorite decoration. I like Captain Solo where he is."
• It is heavily, though falsely rumored that the jewels on his crown change his appearance, though this is proven false, as when Luigi sucks King Boo up, he loses his jewel though retains the same design. It is also proven false due to him having a different jeweled crown in Luigi's Mansion 3DS remake, but retaining his purple eye color and look. It's also heavily indicated otherwise by the character Boolossus, as Boolossus has unique eye colors and shades, similar to King Boo, but has no crown.