Poe uses in The Fall of The House of Usher, what can be called a First Person Point of View. The narrator is one of Roderick's old friends who comes back to his house to attend to him in his days of melancholia and his sister Madeline's fatal illness. So, in a way he is one of the three major characters in the story, but there is a curious distance that is maintained between him and the brother-sister which makes him resemble a Third Person perspective in a way. The method of narration is panoramic and despite the strong posychological interest of the narrative perspective, the mental worlds of the Ushers remain a structure closed down and thus inaccessible to him.
The narrator in the story is one who questions. He is a rationalist as opposed to the irrational bents in the Ushers. He doubts it all, especially the supernatural possibilities. But at another level, the role of the narrator is to realize his own otherness through a horrific identification with the paranoid Ushers. Above all, I think, the narrator's role in the story is to make the story of the Ushers survive. He is the only surviver. He has to live on for the strange tale to be told.