Boos are mischievous ghosts hidden on every floor of the Last Resort. There's one Boo per floor, and they only appear after you've completed the main objective for that floor. To find a Boo, you'll need to use your Dark-Light Device in suspicious areas. When you hear a distinct rattling sound and your Poltergust vibrates, you're close! Once revealed, stun the Boo and vacuum it up. Catching all Boos unlocks a special reward.
