Rowena is the narrator's second wife, married after the death of the never-to-be-forgotten Ligeia. As such, she becomes the unwitting (and unwilling) fleshly vehicle for the return of Ligeia from beyond the confines of death. Other than that, within the terms of the story, she isn't anybody at all. Poe was not so sufficiently master of the short story that he knew how to give a secondary character independent life, and Rowena Tremaine is purely a vehicle for the return of the true subject of the tale he tells.