TYPE: Undead, Ghost.
TURF: Castles, Forests.
AFFILIATIONS: Koopa Troop.
SKILLS AND ATTACKS: Can create illusory copies of itself.
WEAKNESSES: Just as weak as Shy Guys.
Boo Guys are undead Shy Guys. It's currently unknown whether they are the spirits of dead Shy Guys or an entity of unknown origin.
They were first employed by Kamek during his attack on Yoshi's Island usually appearing in the castles operating machinery or underground wandering out of pipes. Later, Bowser employed them to help capture the Star Children.
After their last defeat on Yoshi's Island, they moved to Toadwood Forest where they attacked any adventurers that entered the forest.
They don't seem to support the Shroob invaders although they didn't attack them either. A stronger version of appears within Shroob Castle.