King Boo

King Boo is a character in the Mario franchise and a heavyweight driver in the Mario Kart series.

Koopa Kart: Double Trouble!!
King Boo is a new racer in Koopa Kart: Double Trouble!!, and he is a default driver in this game. His partner is Petey Piranha, and their special item is the Boo.

Koopa Kart DS
King Boo's next appearance is in Koopa Kart DS, where he can be unlocked by winning every 50cc Nitro Grand Prix cup, the same method for unlocking Dry Bones in Mario Kart DS.

Koopa Kart Wii
In Koopa Kart Wii, King Boo is the hardest character to unlock, as he is unlocked by earning at least a 1 Star Rank for all Mirror Grand Prix Cups, the same method for unlocking Rosalina in Mario Kart Wii.

Koopa Kart 7
King Boo returns in Koopa Kart 7, where he can be unlocked by playing Time Trials on 24 different courses.

Koopa Kart 8
In Koopa Kart 8, King Boo can be unlocked by unlocking an Expert Staff Ghost in Time Trials.