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 if you looked away they will often come to scare you. From Super Mario 64, they would also become invisible.