For the Super Mario Bros. enemy, click here.
Boos all have white, spherical bodies with a pair of small arms that have clawed fingers. They also have manic-looking eyes with yellow irises on red sclera with dark rings around them. Their mouths are filled with black, dagger-like fangs.
Boos are mischievous and sneaky. In some cases, they can be plainly seen and will advance on a victim menacingly. Other times, they will lurk in the shadows or inside objects, bursting out to frighten a victim when they approach. As shown in the Sonic X anime series, Boos are also capable of possessing people and will manipulate their victims into attacking their friends.