Boo

Boos, as seen to the right, appear in Super Smash Flash 2's Adventure Mode. It is not confirmed where they will be appearing, but it obviously appears to be a level made for Mario.

Boos first appeared in Super Mario Bros. 3, where if you look at them they would stay completely still and cover their faces. But if you looked away, they would come to scare you. In Super Mario 64, they could also become invisible.