In addition to Montresor and Fortunato, the man who keenly desires to exact revenge and the man on whom this revenge will be exacted, respectively, there is potentially another character in this story as well: the person to whom Montresor is telling the story. Montresor seems to be telling the story of his revenge on Fortunato to someone about fifty years after the events actually took place. In the second to last sentence of the story, he says, "For the half of a century no mortal has disturbed [Fortunato's bones]." Therefore, it seems as though Montresor is now an old man reflecting back on this episode from fifty years earlier. Further, in the second sentence of the story, he refers to his auditor as, "You, who so well know the nature of my soul [...]." Thus, there is another person in the story as well: this person to whom Montresor is telling his story. Though some might classify this unknown person as the audience, an argument could be made that they are another character.
It is possible that, after all these years, Montresor is getting ready to die and that a priest has come to his death-bed to give him his last rites. At this time, he might take the opportunity to confess any sins for which he has not atoned. After all, he claims that this person knows his soul -- which a priest, theoretically, would -- and, in the final line of the poem, Montresor says, "In pace requiescat!" (seemingly about Fortunato) which is exactly what a priest would say to a parishioner as he passes: Rest in peace. Therefore, it seems plausible that Montresor is telling his story to a clergyman; however, we don't know this for sure. Either way, there is a third character here: whoever is listening to Montresor tell this story.