Mini Boo

Mini Boos are smaller variants of Boos. Though they have very similar appearances, with the exception of size, Mini Boos are often not used as actual enemies, and when they are, they do not act as Boos. As an item, they may be used to assist the character who summons them.

Mario Power Tennis
In Mario Power Tennis, Boo’s power shot involves him summoning many Boos into a large tennis racket, hitting the ball with it, then allowing the Boos to cloak the ball in hopes of earning a point. As is the case again, opposing players are still able to return the ball if they know where to be.

Super Mario Galaxy
In Super Mario Galaxy, small, transparent Boos can be spotted on the finish line planet of Ghostly Galaxy during the mission A Very Spooky Sprint. They merely come from the planet's surface a short distance before disappearing.

Mario Super Sluggers
In Mario Super Sluggers, Mini Boos serve as an item, making the ball invisible to the player in the outfield. They are found in Wario City and are required for King Boo to join the player’s team in Challenge Mode.