spelled as "Sibyll". However, in the American edition of Half-Blood Prince, it is re-spelled as "Sybill", matching the UK edition. The name "Sybill" is a reference to Greek and Roman prophet priestesses, while Cassandra refers to the Trojan seer Cassandra, whose doom was that her prophecies would never be believed. Professor Trelawney is portrayed by Emma Thompson in the Prisoner of Azkaban, Order of the Phoenix, and Deathly Hallows Part 2. Hogwarts ghosts Hogwarts houses have at least twenty ghosts, but when people in the novels speak of the ghosts at Hogwarts they usually refer to one of the four resident ghosts of the Hogwarts houses: Sir Nicholas de Mimsy-Porpington (or, as the students refer to him, Nearly Headless Nick), the almost beheaded ghost of Gryffindor house; the Bloody Baron, who is the ghost of Slytherin house; the jovial Fat Friar, who is the ghost of Hufflepuff house; and the Grey Lady, who in life was Helena Ravenclaw, the ghost of Ravenclaw house. These ghosts seem to act as something like advisers and aides to the students; Nick frequently helps Harry during moments of uncertainty or crisis. Despite animosity between the houses, their respective ghosts get along well with each other. Nearly Headless Nick is respectful of the Bloody Baron and claims that he cannot imagine starting a fight with him, while the Fat Friar pleads on behalf of Peeves the Poltergeist to allow him to come to the welcome feasts despite his past wrongdoings. During The Deathly Hallows, Nick is protective of Helena and only reluctantly tells Harry how to find her. The Bloody Baron 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". In Harry Potter and the Deathly Hallows, Helena Ravenclaw (the Grey Lady) explains to 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