Boo

A Boo is a type of Ghost that appears in A Mario Game. They first appeared in Super Mario Bros 3. Then they appeared in Super Mario World, Super Mario World 2:Yoshi's Island, Super Mario RPG, Super Mario 64, Paper Mario, Luigi's Mansion, Super Mario Sunshine, and secondary games such as Super Mario Kart, Mario Kart 64, Mario Golf, Mario Tennis, and The Mario Party Series. Even though in most games Boos are considered to be Mario's enemies in Paper Mario the Boos are all on Mario's side. In fact their princess Lady Boo actually joined Mario in his battle against Bowser in that game. In Luigi's Mansion King Boo was the main villain in that game who captured Mario and Luigi had to defeat him to rescue him.