How would one describe Montresor's character in "The Cask of Amontillado"?

The easiest way to describe Montresor would be as vengeful, single-minded, and unforgiving. We know from the very beginning of the story that he is intent on exacting revenge for the “injuries” and “insults” of Fortunato, and that he is patient, willing to wait “at length” for an opportunity to arise to allow him to achieve his end. He does not once waver in his plan to murder Fortunato, hesitating only once as he is bricking up the recess in which Fortunato is chained; and even then he hesitates not out of indecision, but out of fear that Fortunato has escaped his bindings. And consider the Montresor family’s motto: Nemo me impune lacessit—“no one attacks me with impunity.” Here we have confirmation that the man is vengeful, and learn that this vengeance is rooted in his family’s values and history. The man is proud—he comes, after all, from “a great and numerous family,” with a reputation that he is burdened with upholding.

Given Montresor’s unerring performance in tricking Fortunato into his cellar and his chosen method of murdering his “friend,” we can confidently say that Montresor is a creative man, and clever. His tactics involve a good amount of knowledge about his acquaintances—he knows that Fortunato, a “quack” in many ways, is a genuine connoisseur of wine, and that he believes Luchresi to be “an ignoramus” who “cannot tell Amontillado from Sherry.” Montresor plays off of Fortunato’s own biases and pride to lure him deeper and deeper into his trap, until there is no escape. The fact that Fortunato is led very deep into Montresor’s cellar without having any doubts or suspicions about the events at hand is testament to Montresor’s acting abilities—all this time, harboring the most sinister of intentions, he manages to disarm Fortunato and maintain a friendly demeanor. Also, we know that Montresor is never punished for his crime, which indicates that he feels little to no guilt, and that he has a very strong command of his emotions, such that he can keep his secret and raise no suspicions for decades after the crime is committed.

Montresor is a scheming and intelligent man, patient and careful. He is true to himself and loyal to his family’s legacy, unafraid—indeed, determined—to do anything necessary to free his name from what he perceives as unjust insult. He is strong-willed and innovative, utilizing the resources available to him to maximum effect. And, of course, he is a murderous villain.