The narrator of the story is definitely a man named Montresor who is the main character and the viewpoint character. Since the story was published with the byline of Edgar Allan Poe, it would seem--to me at least, if not to everybody--that the story is represented as a translation of an old manuscript that somehow came into Poe's possession. Ostensibly, Montresor wrote out this detailed confession with the intention of giving it, or mailing it, to someone whom he addresses in the story as "You, who so well know the nature of my soul..." The original manuscript may have been written in Italian or French, perhaps even in Latin. It may or may not have ever been received by the man or woman to whom it was directed. It is quite possible that Montresor, an very old man, might have written out the entire confession one night while intoxicated and put it away without mailing it or having it delivered. No one will ever know the identity of "You, who so well know the nature of my soul." My guess is that is was a woman. The manuscript might have been found among Montresor's papers after his death. Or it might have been found among the papers of the recipient, if it ever reached that person. In publishing it in English in Godey's Ladies' Book, Poe was implicitly conveying the idea that he somehow got hold of this old manuscript, translated it, and was now making it available to the public.
Poe liked to experiment with different ways of telling a story. In his "Ms. Found in a Bottle," he uses the fiction that he, Edgar Allan Poe, is publishing the manuscript of a journal found in a bottle thrown overboard while a ship was sinking (see reference link below). This is similar to the fictitious acquisition of Montresor's manuscript in "The Cask of Amontillado."
Montresor is undoubtedly dead by the time Poe publishes his confession, and Fortunato has been dead for over fifty years. The perfect crime has been committed because Montresor is out of reach. The concluding words of the story are:
Against the new masonry I reerected the old rampart of bones. For the half of a century no mortal has disturbed them.
In pace requiescat!
The words "In pace requiescat!" are not meant to be ironic or sardonic. Montresor is undoubtedly sincere in wishing Fortunato to rest in peace. Montresor explained his ideas of the perfect revenge in the opening of the story. The perfect revenge would have to provide perfect satisfaction of the hatred that prompted thoughts of revenge in the first place. Once Montresor has finished walling Fortunato up in the niche, he has cleansed himself of his long-pent-up feelings of hatred. He has achieved what is so often these days called "closure." So he no longer harbors any ill feelings for the dead man but feels well disposed towards the skeleton hanging in rusty chains. The words "In pace requiescat!" make a perfect ending for this story of revenge, because they prove that Montresor has accomplished his diabolical plot to his complete satisfaction.
Poe is the actual writer of the story, but his fictitious character Montresor is presented as the narrator, the protagonist, and the viewpoint character.