Ghosts...don't...die! Heh heh heh! Can you get out of here...ALIVE?
~ The Boos in Super Mario 64.
The Boos are an antagonistic group of nasty ghost-like creatures that are seen a lot in the Super Mario Bros. series and they are also the main antagonists of the Luigi's Mansion 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 among them is their vile ruler, King Boo. The second most powerful of Boos are the Big Boos.
Big Frame Boo
Big & Little Boo
Boo Buddy Snake
Boo Buddy Swarm
Boo Man Bluff
Calm Fishing Boo
Circling Boo Buddies
Dark Dark Boo
Disappearing Boo Buddy
Mad Big Boo
Boo's Mario Strikers Charged Football outfits resemble 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 coworker and close friend Takashi Tezuka, as she is shy but one day got very angry at him for coming home late from work.
In both Super Mario 64 and Luigi's Mansion, Boo's old laugh is just a sped up version of Bowser's laugh from Super Mario 64 with echoes added. It was also reused in Mario Tennis 64 as well as the Nintendo DS remake of Super Mario 64 and in the first seven Mario Party games.
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.
According to Professor E. Gadd in Luigi's Mansion, Boos grow in power when they are together.
Boo appears in Super Smash Bros. Ultimate as a Primary Spirit. Its battle is against Kirby who can temporarily go invisible. Boo can them be enhanced to King Boo, who slows the falls of jumping fighters.