Madeline only appears three times in "The Fall of the House of Usher." How do her appearances, explicit and implicit, develop the plot and symbolism of the narrative?

We really have to question if Madeline actually exists. Madeline can be seen as kind of like a doppelganger of Roderick. You know, kind of like Norman Bates and his mother in the movie Psycho. Perhaps Madeline's return from the dead is only a metaphor of Roderick's fears. In any case, Madeline serves as a plot device to give a manifestation to Roderick's fears and anxieties.