the Grey (or gray) Lady

Ghost of Ravenclaw House, the daughter of founder, Rowena Ravenclaw, Helena Ravenclaw.

harry: who's the Grey (or gray) Lady?!
quirrell: the house ghost for ravenclaw, like nearly-headless nick is gryffindor's, the bloody baron is slytherin's, or the fat friar is hufflepuff's, harry my man.