King Boom Boo

King Boom Boo is a boss character in Sonic Adventure 2. He is a giant ghost and the leader of the Boos that reside in every haunted level in the game. He is encountered in Death Chamber, yet his presence can be seen at the giant pillars in Pumpkin Hill and in Aquatic Mine, and like other ghosts in the Sonic universe, he is repelled by light.

Sonic Adventure 2
Knuckles encounters King Boom Boo whilst searching for the keys to a door which Sonic, Tails, and Amy are trying to get through. During the boss battle, Knuckles is chased around a circular arena by King Boom Boo, who throws blue fireballs at him and attempts to bite him at close range. Occasionally, the ghost pauses and spews a large blue wave of flames. During this attack, Knuckles must activate an hourglass which opens panels in the ceiling, flooding the room with light and causing King Boom Boo to shrink. His shadow sinks into the floor or the wall, and Knuckles has to dig it up and bring the ghost to the surface. At this point, the player must run after King Boom Boo and punch him before the hourglass runs out, clearing the light from the arena and causing him to grow back to his normal size. He can be damaged up to three times in one hourglass flip. If the player runs too fast, he moves in the opposite direction.

Sonic Rivals 2
King Boom Boo appears as a collectible card in Sonic Rivals 2. It can be unlocked by winning the Tails Cup in Single Player mode. The card unlocks Tails' Captain suit.

Sonic X
In Sonic X, King Boom Boo appears in an episode before the Sonic Adventure 2 adaptation. He haunts an ancient castle along with the smaller Boom Boos, but is permanently trapped there by Sonic and his friends.

In the anime, King Boom Boo is shown to be capable of coherent speech (whereas he could only babble, grunt, and roar "BRAAAA!" in Sonic Adventure 2). He can manipulate the sound of his voice; for example, he imitated Sonic in order to trick Amy into setting the hourglass moon side up.

Sonic X (comic series)
In Issue 14 of the Sonic X comic series, King Boom Boo makes an appearance. After the incident at the castle in the Sonic X anime, the hourglass stone was stolen from the castle and taken to one of Linsey Thorndyke's summer homes. This, along with the disturbance from his rest by the endless activity from a recently-built tennis court nearby, is a great annoyance to King Boom Boo. His loyal horde of ghosts consequently possess everyone on the island, including Cheese, Linsey and Vanilla. King Boom Boo claims that once his ghosts possess enough living people, he'll be able to enter the world of the living for good. Cream and Chris manage to reason with King Boom Boo that his time on Earth is over. The ghost king concedes to retract his presence and his ghosts release their hosts, but extracts the promise of the stone hourglass being returned to its proper place by dawn. Linsey plans to convert the tennis court into a garden, to further ensure King Boom Boo's continued endless, dreamless sleep.

Trivia

 * King Boom Boo was suggested as a new character for issues of the Archie Comics following #134, but the plot line in which he was intended to feature appears to have been cut.
 * King Boom Boo is the only boss in Sonic Adventure 2 to have green text appear on-screen before facing him.
 * Although it's never shown, it's arguable that Knuckles is afraid of King Boom Boo, as when he notices him, he says "What the... a ghost?!", but he only says it with a little shock and disbelief. However, in an unused line, Knuckles says "What?! A ghost!" in a more frightened tone, and in the Japanese version, Knuckles says "O-Obake!", meaning "G-Ghost!", and he is clearly scared.
 * King Boom Boo resembles the Boos from Nintendo's Mario series. His name even contains "Boo". Enemies known as Boom Boos later appeared in Super Mario Galaxy.
 * When fighting him, the player can see a picture of a Nintendo Boo and a regular Boom Boo on the wall.
 * King Boom Boo actually came before Nintendo's King Boo. King Boo's first appearance was in Luigi's Mansion, which was released the same year as, but several months after, Sonic Adventure 2.
 * King Boom Boo and the other ghosts in Death Chamber look similar to Hyudoro in Sandopolis Zone in Sonic & Knuckles. Also, both of them inhabit pyramids.
 * King Boom Boo appears in winning fan art in Sonic and the Black Knight.
 * There is a glitch in the Playstion network port, when the windows on the roof of Boom Boo's arena are opened if the player pauses the game. The sky will change from blue to black. If the player restarts the game from the pause menu, the sky will stay black for the rest of the fight.