The Bloody Baron is one of the ghosts of Hogwarts. He is the Slytherin House Ghost. According to the Slytherin welcome message, he is quite friendly but towards the rest of the school he has a creepy reputation.
The Bloody Baron in the books
The Bloody Baron appears several times in the books.
Nearly Headless Nick tells the Gryffindor students that the Baron has been insufferable about the consistent Slytherin victory in the House Championship over the past 6 years. The Baron is seated next to Draco Malfoy at the feast.
The Hogwarts ghosts held a council meeting to decide whether Peeves would be allowed to attend the start-of-term feast. The Bloody Baron put his foot down at the suggestion, and refused to allow the poltergeist to go.