Who is the narrator of the story and why is he visiting the House of Usher.

The narrator is one of Roderick Usher's friends. He is visiting Usher because he received an invitation (letter) from Usher. Usher said that he was suffering from a "nervous agitation" and wished to visit with him..... thinking the visit would improve his condition.

Its proprietor, Roderick Usher, had been one of my boon companions in boyhood; but many years had elapsed since our last meeting. A letter, however, had lately reached me in a distant part of the country --a letter from him --which, in its wildly importunate nature, had admitted of no other than a personal reply. The MS. gave evidence of nervous agitation. The writer spoke of acute bodily illness --of a mental disorder which oppressed him --and of an earnest desire to see me, as his best, and indeed his only personal friend, with a view of attempting, by the cheerfulness of my society, some alleviation of his malady.