Boo

Boo, known in Japan as Teresa, is a ghost enemy hailing from the  franchise.

Character description
Boos are common ghost enemies in the Mario franchise that first appeared in Super Mario Bros. 3. Normally found in Ghost Houses and other haunted places, they are a type of white, spherical ghost with menacing and normally open mouths. Known for their large tongues and small "arms" of some sort, they float in the air and pursue Mario or Luigi when not being looked directly at. However, the most distinctive characteristic of the Boos is their shy personality, where they cover their faces when someone looks them directly at the face, uncovering it once the subject turns back around. They also cannot usually be defeated through normal means such as jumping on them and often can only be defeated when hit with a Super Star power-up or when light shines on them.

In A Super Mario World
A Boo appears in the McLeodGaming cartoon A Super Mario World, where it briefly appears in the first episode as one of the monsters that Mario fights with the Master Sword. It becomes visible in the air and attempts to attack Mario before he slices it, killing it quickly.

As a background character
A Boo appears in Super Smash Flash 2 as a background character on the Waiting Room's Halloween theme, one of multiple holiday-inspired versions of the stage. It can be seen floating above the left couch in the background.

As an enemy
Boos were initially confirmed to appear as enemies in the level of SSF2's Adventure mode. Any further information is unknown and whether it will appear is no longer confirmed.