There are several different directions you can go in your interpretation of Madeline Usher. One theory is that she doesn’t fully exist from the start, but is some sort of supernatural shade, a spiritual doppelganger half of Roderick. (Doppelganger means ghostly double.) This is why the narrator rarely sees her and why she doesn’t acknowledge or interact with him during those times. It’s why she can come back from the dead – because she wasn’t fully human in the first place.
If you like the psychological approach we discuss in Roderick’s “Character Analysis,” then Madeline and Roderick are two halves of the same person. Naturally, a person cannot live divided into two pieces, much as the House of Usher cannot stand with that crack running down the middle.
Another approach, this one blending psychology with the supernatural, argues that Madeline, at least the Madeline who returns from the dead, is the physical manifestation of Roderick’s worst fears. In fact, when Roderick is foreshadowing his death, he says "…the period will sooner or later arrive when I must abandon life and reason together, in some struggle with the grim phantasm, FEAR." Does FEAR = Madeline? In “What’s Up With the Epigraph” we discuss the way that Roderick’s artistic creations either predict or create the tale’s spooky outcomes. Notice that Madeline doesn’t appear at the door until Roderick claims that she is standing there – some good evidence for this last interpretation.