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!").