If Poe separates the mind, body, and spirit, which character represents each in "Fall of the House of Usher?"

The narrator is certainly the representation of the mind.  He approaches the house with a scientific air, and reports in detail on all that he sees.  He catalogs all the nervous fits of Roderick Usher and the physical symptoms that accompany these fits.  He refers to his visit as a "mystery" that he is trying to solve.  He also makes certain to reassure his audience of his calmness in comparison to Roderick's nervousness.

Roderick represents the spirit.  He is described physically as being pale, with weblike hair.  It is as if he is a ghost.  He is also all senses, twitchy because of his sensitivity to light and the sounds he is convinced he is continually hearing.  He exudes a supernatural air constantly, which slowly works upon the narrator.  It is as if the spirit Roderick is invading the mind of the narrator.

This leaves Madeline as the body.  She is not presented as being a sentient being - she barely appears in the story, and she never speaks.  She exists as a body in the house, and Poe makes her the victim of some unknown physical disease.