Helena Ravenclaw, known after her death as the Grey Lady, was the daughter of Hogwarts founder Rowena Ravenclaw. In life, she stole her mother's diadem in a bid to surpass her, and after her tragic death, she returned to Hogwarts Castle as the official ghost of Ravenclaw House. For centuries, she kept the location of the lost diadem a secret, but she was twice persuaded to reveal its location: first to a young Tom Riddle, who defiled it by turning it into a Horcrux, and years later to Harry Potter, to whom she provided the crucial information needed to find and destroy it during the Battle of Hogwarts.
Born to the celebrated witch Rowena Ravenclaw, Helena grew up in her mother's shadow, envious of her wisdom and the respect she commanded. Desperate to become more clever and important, Helena betrayed her mother by stealing her enchanted diadem, which was reputed to enhance the wisdom of its wearer. She fled with the artifact to a remote forest in Albania. Ashamed of the theft, Rowena Ravenclaw told no one that the diadem was missing and continued to pretend she possessed it. However, when Rowena fell fatally ill, her final wish was to see her daughter one last time. She sent a man who had long loved Helena to find her: the Bloody Baron. The Baron tracked Helena to her hiding place in Albania and pleaded with her to return to her mother's deathbed. When Helena refused, the Baron, who had a violent temper, became enraged by her rejection and stabbed her. Overcome with remorse for his horrific act, he then used the same weapon to take his own life. Their spirits returned to Hogwarts Castle as ghosts. Helena became the Ravenclaw House ghost, taking the name the Grey Lady, while the Bloody Baron became the Slytherin ghost, forever carrying the chains he wore in penance for his crime. For centuries, Helena refused to speak of the diadem's location, having been shamed by another student, Tom Riddle. The charming and handsome Riddle flattered her, and she confessed the story of the diadem and its hiding place in a hollow tree in Albania. Riddle later retrieved the diadem, murdered a peasant to splinter his soul, and transformed the historical artifact into one of his Horcruxes. He later returned to Hogwarts and hid the Horcrux in the Room of Requirement. During the Second Wizarding War, Harry Potter sought the diadem as part of his mission to destroy all of Lord Voldemort's Horcruxes. After learning of her connection to the object from the ghost of Nearly Headless Nick, Harry found the Grey Lady in a corridor. Initially haughty and reluctant, she was eventually persuaded by Harry's compassion and his genuine desire to destroy the evil that had defiled her mother's legacy. She revealed that she had been the one to tell Tom Riddle about the diadem and that he had hidden it within the castle itself, specifically in the place “where everything is hidden”: the Room of Requirement. This information allowed Harry to find the diadem, which was later destroyed by Fiendfyre.
As a ghost, Helena Ravenclaw is described as a tall and beautiful young woman with long hair, a stately carriage, and a haughty expression. Her form is pearly-white and slightly transparent, and she glides silently through the castle. In life, Helena was ambitious and envious, driven by a desire to eclipse her mother's fame. As a ghost, she is melancholic, quiet, and aloof, rarely speaking to students and preferring solitude. She carries the shame of her betrayal and her trust in Tom Riddle as a heavy burden. However, she proved to be capable of trust and a desire for redemption when she confided in Harry Potter, recognizing his sincere intent to vanquish Voldemort. Her voice is described as a whisper that is barely audible.
Little is known of her magical abilities when she was alive, though her ability to steal a powerful magical object from a formidable witch like Rowena Ravenclaw and flee to another country suggests she was resourceful. As a ghost, she possesses the standard non-corporeal abilities, such as floating through the air and passing through solid objects like walls and doors.