It has been suggested that Montresor is confessing to a priest. It has also been suggested that he is writing a letter to a friend. There are other possible answers to the many questions about the identity of "you, who so well know the nature of my soul."
One of them is that "you" means you, the reader. This puts you close to the narrator. It also makes you an accomplice, because you know where the body is buried. You are to imagine that you have known Montresor all your life and that he is confiding in you because you are the only person he can trust with his guilty secret. By the time you finish reading this tale you will indeed know the nature of his soul very well.
Another possibility is that Montresor is addressing a totally imaginary confidante, or perhaps a private diary.