A Boo is a fictional
ghost that appears in the
Yoshi series of
video games. Boos first appeared in
Super Mario Bros. 3.
 Since then, they have become a common enemy in the Mario games. They sometimes appear as playable characters in the Mario
spin-off games. In some games, such as
Paper Mario, they help Mario during his adventure. They are named after the sound that a person might make when they want to scare someone ("boo!").
Boos most often attack the player by sneaking up behind the player while the player's back is turned to them.
 When the player looks at them, they will hide their faces. They cannot be defeated without certain items or abilities.