One good quote that sums up Montresor's character comes in the opening paragraph. Montresor explains, briefly, that Fortunato has injured or insulted him, possibly many times, and that he is planning revenge.
At length I would be avenged; this was a point definitely, settled -- but the very definitiveness with which it was resolved precluded the idea of risk. I must not only punish but punish with impunity. A wrong is unredressed when retribution overtakes its redresser.
(Poe, "The Cask of Amontillado," eNotes eText)
This quote shows Montresor's intelligence and cunning nature; he is willing to let his revenge take a long time as long as it comes and he is not suspected. If his revenge was public -- say, a duel -- he could easily be tried for a crime. By causing Fortunato to vanish completely, with no one else suspecting his involvement, Montresor has his revenge and remains free and clear. From this quote, Montresor's character can be seen as plotting, meticulous, and cautious; he is not subject to reckless, emotional action, but instead sits back, plans, and waits for the opportune moment.