Asenath Waite is the daughter of the Innsmouth mystic Ephraim Waite. She studied occultism at Miskatonic and married the pliant poet Edward Derby. Using mind-transference sorcery, she repeatedly displaces Derby's consciousness, progressively destroying him.
The deeper horror is that "Asenath's" body is itself already occupied by the transferred mind of old Ephraim; the real Asenath was dispossessed long before. This layered truth is revealed in the story's chilling climax.