The Bloody Baron is the Slytherin House ghost. He is the only person besides Dumbledore and Fred and George Weasley who can exert any control over the poltergeist Peeves; Peeves is terrified of him for some unknown reason, referring to him as "Your Bloodiness" and "Mr. Baron."
The Baron's nickname comes from the fact that he is covered with blood, which appears silvery on his ghostly form. When Nearly Headless Nick is asked in the first book why the Baron is so bloody, Nick delicately comments that "[he has] never asked". However, this is explained in Harry Potter and the Deathly Hallows, when Helena Ravenclaw tells Harry that the Baron had been in love with her when the two were alive, and when she ran off with her mother's diadem, Rowena Ravenclaw sent the Baron after her, knowing he would not stop until Helena was found. When Helena refused to return with him, however, the Baron killed her in a fit of rage, and then, in remorse, killed himself with the same weapon. He has thus haunted Hogwarts ever since, wearing his ghostly chains as a form of penitence.
Terence Bayler portrayed the Baron in the first film. In contrast to his book counterpart, the Baron is quite mirthful in the film, playfully swiping through the Sorting feast with his sword, much to the amusement of those within his house.