A Boo, a haunting enemy in the Mario series, makes its appearance in Mario Kart series as an item, first in Super Mario Kart, and then in Mario Kart 64, Mario Kart: Super Circuit, Mario Kart DS, and Mario Kart 8 Deluxe. The Item is also in the arcade games Mario Kart Arcade GP and Mario Kart Arcade GP 2.
Boos are also harmless (but still scary) characters that fly around in courses such as Ghost Valley 1, Ghost Valley 2, Ghost Valley 3, and Banshee Boardwalk. However, they are dangerous on the track, Broken Pier in Mario Kart: Super Circuit, because running into one will cause one to grab onto you and make you lose speed and Coins.
The Boo item temporarily left the Mario Kart series for 12 years, and then came back to Mario Kart 8 Deluxe.
A Boo, probably the sneakiest Item in Mario Kart, temporarily turns the racer who is using it completely invisible to all other opponents. The player themselves sees their Kart turn into a ghost, similar to the ones in Time Trials. While invisible, the racer can't be hit by any Items whatsoever, and they are also not hindered by driving off-road. However, they still slow down when they crash into walls (Sorry guys, no passing through walls. That'd be, like, cheating.) and you can drive off the track. But still, it is extremely convenient for cutting corners and avoiding incoming Items, making a Boo a great recovery and defensive Item.
In addition to invisibility, a Boo will go and fly off to another racer who has an Item. They can steal it, and then fly back to it and bring it to you (yeah, stealing is perfectly legal in Mario Kart)! It could be anything from a Star (great Item) or a Green Shell (an OK Item)-it just depends on whatever the other opponent has. In all games except for Mario Kart 8 Deluxe, it will steal the strongest item in play, and in Super Mario Kart, the steal will work even if the opponent's Item Roulette has not yet stopped. If no one has anything, well.. too bad. No Item for you! But don't worry, Boo will still come back to say goodbye. It just won't deposit anything in your Item inventory.
In Mario Kart 8 Deluxe, Boo will steal from the main item slot of a random racer who is ahead of the user, and place it in the user's reserve slot. Fire Flowers, Boomerang Flowers, Golden Dash Mushrooms and Triple items will be fully restored as if the player had just got them from the Item Box. It will not steal Piranha Plants or Bullet Bills that are currently in use, nor will it steal Crazy 8s. It also will not steal other Boos, as it is impossible to have more than one in play at a time. If there is nothing to steal, he will give the user a free Mushroom. The invisibility no longer prevents slowdown from driving off-road.
Finally, exclusive to Mario Kart: Super Circuit, a Boo will do all the things mentioned above, but it also is a nice attack Item as it will fly over to the first place racer and latch on to them, slowing them down and stealing your Coins (similar to the Shy Guys on Sunset Wilds in the same game). Too bad for the first place driver, being haunted and all.
- In the Super Mario Kart instruction booklet, Boo was called a Ghost.
- Players can actually race as a Boo, specifically the regal King Boo, in Mario Kart: Double Dash!!, Mario Kart Wii, and Mario Kart 8 Deluxe.
- Nintendo removed the Boo from Mario Kart Wii and so forth. It had a long absence of 3 games, but it has returned in Mario Kart 8 Deluxe and has the same purpose stealing items and bringing them back to the user while making them invisible but this time, Boos are much more common (and WAY more annoying) with sometimes up to six appearings in one race. It's also notable that King Boo has been a returning racer.
- In the Mario Kart DS kiosk demo, the item texture used the Mario Kart 64 Boo sprite.