Boos

"Ghosts...don't...die! Heh heh heh! Can you get out of here...ALIVE?"

- The Big Boo, one of the Boos.

The Boos are nasty ghost-like creatures that are seen a lot in the Super Mario Bros. series.

Their main method of attack is to attack Mario only while his back is turned. If Mario looks at them they freeze and turn invisible and do not move at all until he turns his back to them again.

The most powerful and most clever Boo mong them is their vile leader, none other then King Boo. The second most powerful of Boos are the Big Boos.

Trivia

 * Boo's Mario Strikers Charged Football outfits resembles the Spooky Speedster's outfit.
 * The manual for Mario Kart DS refers to the item Boo as "Boo Hoo".
 * Creator Shigeru Miyamoto got the idea for Boo from the wife of his co-worker and close friend Takashi Tezuka, as she is shy but one day got very angry at him for coming home late from work.
 * Boo's old laugh is just a sped up version of Bowser's laugh with echoes added.
 * Strangely, in Mario Golf: Toadstool Tour, Boo hits right-handed, but in the Mario Baseball series', it hits left-handed.
 * In the Animal Crossing games, a character named Wisp appears, who bears a resemblance to Boo. The main difference is the facial appearance, and headgear.
 * In Super Mario Sunshine, the Boos are the only Boos that can be defeated by jumping on them outside of the Mario RPG titles.
 * Mumbies, enemies from the Kirby series, attack in an identical way to Boos.