Boo

Boo is a type of ghost in the Mario games. They first appeared in Super Mario Bros. 3, but in Super Mario World, there were Ghost Houses, which contained many Boos. These were easy areas, so they were useful to go through after a hard level to save.

The Boo's Mansion in Paper Mario was a bigger version of the Ghost Houses, which had many puzzles involving Boos in it. After solving these puzzles, Bow, the Princess of the Boos, joins Mario's team.

In Luigi's Mansion, there are 50 Boos which Luigi must suck up in his Poltergust 3000. The final Boo is King Boo, in a Bowser suit.

In Paper Mario: The Thousand Year Door, the Boos live in a tower called Creepy Steeple. When Mario Spin Attacks them, they merge into the Atomic Boo.

Boo is also playable in many Mario sports games and in Mario Party 5 & 6.