Boo

The Boo is an item who has appeared in every Mario Kart game until Mario Kart DS.

Super Mario Kart
Boos made their first appearance in the Mario Kart series, in Super Mario Kart. Though not as obstacles, they only appear in the three Ghost Valley courses. In each race, they just laugh when the player drives by. They also appear as items, under the name, "Ghosts". When the player uses him, they will become transparent temporarily, while the "Ghost" steals the opposing player's item, if available. This item is only obtainable in "2 Player" mode.

Mario Kart 64
Boos are used as an item, in Mario Kart 64. When activated, the Boo selects a racer with the best item in reach, and steals the item and gives it to the player that released him. At the same time, the player obtains total invisibility and invincibility for a short period of time. While invisible, the player can pass the items, the course obstacles and the other racers more easily by simply going through them. If no other players have any items, the Boo will return empty handed. Boos also appear in Banshee Boardwalk.

Mario Kart: Super Circuit
Boos appear as items again in Mario Kart: Super Circuit. When activated, they can also slow down the leading racer. Starting in this game, if a CPU racer uses a Boo, he can also steal items from the player and make the player lose Coins.

Boos also appear in two new courses, Boo Lake and Broken Pier. At Boo Lake, Boos hold candles which guide the player through the track. On Broken Pier, they attack the players and take their Coins.

Mario Kart DS
The Boo most recently appears in Mario Kart DS. The only differences are, they do not slow down the leading racer, and they no longer steal the best item in reach. Instead, they just steal the nearest racer's item. Also, using the Boo won't affect off-road racing, so various shortcuts can be made. Some pictures of Boos from the game, Luigi's Mansion, appear in the Luigi's Mansion race course. Banshee Boardwalk reappears as a race, along with the Boos. They do not do anything that affects the racer; they just laugh when a racer drives by.