Ghosts

Bloody Baron - Slythern Ghost. The only person that can controll Peeves.

Fat Frair - The Hufflepuff Ghost

Grey Lady - Ravenclaw Ghost

Moaning Myrtle - Girl who haunts a bathroom. Killed by a Basilik 50 years ago.

Peeves the Pollygist - He has a wicked mind. He floats around the school causing trouble. He only listens to the Bloody Baron and no one else except in the 5th book he listens to the twins when they say to make Umbridge's life Hell.  

Professor Binns - He is the History of Magic Teacher. He is also the only Ghost teacher. He died one night in his sleep and woke up the next day as a ghost and continued teaching. Hermione asked him about the Chamber of Secrets and that is the only time the class was hanging on his every word.

Sir Nicholas De Mimsey - Ghost of Gryffindor. Also Called Nearly Headless Nick. His head is just barely hanging on.