By ghost standards, Boos have a somewhat cutesy look that we've come to expect from Nintendo. They basically resemble a rounded marshmallow in appearance, with two short stubby arms that seem to have no purpose other than to cover their eyes. These ghosts usually wear a frown, and cruise around with their tongue stuck out of their large mouth. Also they have large (and sometimes spiky) teeth sticking out of their upper jaw, probably as a last-ditch effort to make them look threatening. Starting with Super Mario 64, Boos emit a high-pitched giggle when Mario's around.
You'll find Boos sometimes hanging out in haunted house levels in the Super Mario games, such as the Ghost Houses in Super Mario World, where you'll find large swarms of them floating around. Another notable location is Big Boo's Haunt in Super Mario 64, which has a surprisingly creepy atmosphere and humorous enemies such as haunted pianos and books. Here you'll even get a chance to do battle with Big Boo himself, who is literally a big Boo (he easily dwarfs Mario).
|Big Boo pursues Mario.|
Despite being malicious ghosts, Boos are actually pretty shy. Anytime Mario looks a Boo directly in the face, it will cover its eyes and go invisible. However, as soon as Mario turns his back on it, the Boo will reappear and begin to pursue him. Naturally a good strategy when riding floating platforms is to always have Mario face as many Boos as possible.
Boos definitely pose a challenge in Mario games, due to their tendency to hide when you try to attack them head-on. Also they almost always will gang up on Mario by attacking from different angles, making it difficult and sometimes impossible to stare them all down at once. Still, they are a refreshing enemy to face every once in a while, when you need a break from beating down countless Goombas and Koopas. Sure they aren't a scary enemy, but they have that Nintendo charm that we all know and love.