Boos (japanese: テレサ Teresa) is a fictional ghost that appears in the Mario and 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. Boo are voiced by Sanae Susaki.