Lady Madeline Usher is portrayed as an almost ghostly presence in the home. When the narrator first sees her, she seems to appear and disappear like a phantom:
While he spoke, the lady Madeline (for so was she called) passed slowly through a remote portion of the apartment, and, without having noticed my presence, disappeared.
She takes no notice of the narrator and never attempts to engage with him. The narrator is left feeling both astonished and filled with dread but can't explain his feelings adequately.
Lady Madeline is ill, and the disease has never been properly diagnosed. She almost seems to be disappearing, her body "wasting away" and somewhat "cataleptical." By the time the narrator arrives, she has become fairly confined to her bed. Her brother does not expect his sister to be seen again, as she has given in to the "power" of the disease that is destroying her.
Lady Madeline dies while the narrator visits Roderick, and the two men carry the body to the family tomb. Just before sealing the lid of the coffin, the narrator catches his first glimpse of Lady Madeline's face. He is taken aback by how similar she and Roderick look and learns that the two are actually twins.
In the end, Lady Madeline appears in a supernatural way. She is either a ghost, stuck in some place between life and death, or has possibly clawed her way out of her coffin, which would take incredible energy even for a healthy person. She appears at the door of Roderick's chamber, wearing blood-covered robes; her frame is still "emaciated." She trembles as she flings herself on top of her brother, and the two seemingly pass from this world forever.