There are two main characters in Poe's "The Cask of Amontillado," Montresor, the narrator and agent in this tale of revenge, and Fortunato, his victim.
Montresor is an aristocrat who has fallen on difficult times. Although he has a palazzo and moves in the upper class of the city in which the story takes place, there are several indications that his family is no longer at the top rung of society. Fortunato, however, is a wealthy aristocrat who now considers himself better than Montresor, most likely because the Montresor family has moved down the social ladder.
At some point in the recent past, Fortunato has insulted Montresor so seriously that Montresor has decided to murder Fortunato in a horrific manner, and the action of the story consists of Montresor luring Fortunato to his fate in the catacombs underneath Montresor's palazzo.
We learn a great deal about Montresor from his own lips because he is narrating the action, and from what we hear, most readers would concluded that Montresor is an exceedingly proud man who has become unbalanced by his desire for revenge for the insult from Fortunato. Fortunato, on the other hand, is depicted as vain, pompous, and so self-absorbed that he never realizes his danger. Because the story takes place during Carnival, Fortunato is costumed as a jester or fool, and that is entirely appropriate for his nature.