King Boom Boo is a very large Boo from Sonic Adventure 2. He is a giant ghost, and the leader of the Boos that reside in all spooky levels in the game as well as the pyramid based levels. He resides in the Death Chamber level yet his presence is seen at the giant pillars of the Pumpkin Hill level and Aquatic Mine level, and like other ghosts in the Sonic universe he is repelled by light.
After Knuckles obtains all the keys in the Death Chamber, he heads into a room and finds himself face to face with King Boom Boo.
In the boss battle, Knuckles runs around a circular arena while followed by King Boom Boo who throws blue fireballs at him (and may also attempt to bite him at close range). Occasionally the ghost stops to spew a huge blue wave of flames, and during that time Knuckles must stun King Boom Boo by activating an hourglass which opens panels in the ceiling, making light fill the room. After that you must chase his shadow across the room and dig it up, then run after him and punch him. He can be damaged up to three times in one hourglass flip. If you go too fast when running from him, he will come the other way round. King Boom Boo has not appeared since.
King Boom Boo appears when the Boos kidnap Sonic and the others. He pretends to be Sonic and fools Amy into placing an hourglass on the moon side on the space on the wall. One of his Boos then posesses Amy and he is held back when Chris turns the hourglass to the sun side. Sonic beats him, but he tunnels after them. He is then sealed away by the hourglass once again.