Fortunato has added what Montresor terms "insults" to these purported injuries, causing Montresor to now seek revenge. But, because he does not support his exaggerated claims against Fortunato with real proof, Montresor is an unreliable narrator. For instance, there is no enumeration of "the thousand injuries" supposedly committed against Montresor by Fortunato. Also, Montresor may consider Fortunato boorish simply because of an ethnic bias against Italians. In one instance of this bias, Montresor claims, "few Italians have the true virtuoso spirit." Asserting that Italians feign enthusiasm in the finer things only for the sake of deceiving "British and Austrian millionaires," Montresor, nevertheless, acknowledges that although Fortunato is a "quack" like his countrymen, he has been "sincere" and knowledgeable of the Italian vintages. So, capitalizing on Fortunato's pride in his real ability as a connoisseur, Montresor lures the intoxicated man into the family catacombs on the pretext of tasting the Amontillado. 

Also in the exposition of the story, Montresor depicts Fortunato as somewhat arrogant since he wishes to be recognized by Montresor as superior to Luchesi in his abilities. For when Montresor suggests that Fortunato is too busy to come to his vaults, or when Montresor feigns concern for Fortunato's health, Fortunato insists that he is quite capable of accompanying Montresor. He tells Montresor that "the cold is merely nothing," and he asserts that he is the better judge of the Amontillado: "Luchesi . . . cannot distinguish sherry from Amontillado."

Montresor describes Fortunato as someone who has injured and insulted him one too many times, and so now Montresor has sworn revenge.  Montresor also says that Fortunato has only one flaw, and though he never names it explicitly, it seems likely that this flaw is pride because it is Fortunato's pride that Montresor exploits in order to lure Fortunato to his vaults so that he can kill him.  However, he also claims that Fortunato, other than this one weak point, is actually a man who many respect and even fear.  He is a true connoisseur of wine, and he takes great pride in his expertise (which Montresor admits is real), though he has no real skill in painting or gemmary.  When Montresor finds Fortunato on the night the story takes place, Fortunato is dressed as a jester and has been celebrating Carnival by drinking heavily. 

Montresor's attitude towards Fortunato throughout "The Cask of Amontillado" can best be described as obsequious. This word is defined in various ways. Dictionary.com defines it as:

characterized by or showing servile complaisance or deference...fawning

"Fawning" is defined as:

displaying exaggerated flattery or affection

Montresor certainly displays exaggerated affection. Although he hates Fortunato and intends to murder him in a horrible way, he calls him "My friend" and refers to him as "my good friend" and "my poor friend" throughout the tale. Here are some examples of obsequious speech and behavior intended to reassure his intended victim of his good will:

I said to him—“My dear Fortunato, you are luckily met. How remarkably well you are looking to-day."

“My friend, no; I will not impose upon your good nature."

I took from their sconces two flambeaux, and giving one to Fortunato, bowed him through several suites of rooms to the archway that led into the vaults.

“Come,” I said, with decision, “we will go back; your health is precious. You are rich, respected, admired, beloved; you are happy, as once I was. You are a man to be missed. For me it is no matter. We will go back; you will be ill, and I cannot be responsible. Besides, there is Luchesi—”

Montresor's behavior is that of an inferior towards a superior. This suggests an answer to the questions: Why did Montresor put up with a "thousand injuries"? Why didn't he just stay away from him? Montresor is dependent upon Fortunato for money. The third paragraph suggests that these two men may invest or trade in luxury items.

He had a weak point—this Fortunato—although in other regards he was a man to be respected and even feared. He prided himself on his connoisseur-ship in wine. Few Italians have the true virtuoso spirit. For the most part their enthusiasm is adopted to suit the time and opportunity, to practise imposture upon the British and Austrian millionaires. In painting and gemmary, Fortunato, like his countrymen, was a quack, but in the matter of old wines he was sincere. In this respect I did not differ from him materially;—I was skilful in the Italian vintages myself, and bought largely whenever I could.

Perhaps Montresor put up with Fortunato's injuries because he needed him for business purposes. Meanwhile, Montresor's hatred keeps building, but he continues to act obsequiously while intending to take his revenge when the time is ripe.

It must be understood that neither by word nor deed had I given Fortunato cause to doubt my good will. I continued, as was my wont to smile in his face, and he did not perceive that my smile now was at the thought of his immolation.

It is interesting to note that even when Montresor has his "friend" chained to the granite wall, he does not gloat or vent his hate and anger. He maintains the same obsequious manner he has been exhibiting all along. Fortunato will understand that Montresor has been deceiving him all these years.

“Pass your hand,” I said, “over the wall; you cannot help feeling the nitre. Indeed, it is very damp. Once more let me implore you to return. No? Then I must positively leave you. But I must first render you all the little attentions in my power.”

According to Montresor, Fortunato is someone that deserves to die.  Not really a lot of specific details are mentioned in the story, and it is a bit difficult to trust Montresor.  I mean he lures a guy into his wine cellar to bury him alive and feels no remorse about it.  That's a bit twisted.  

There are some character traits that the reader can infer about Fortunato though.  First, the guy has an addiction to wine.  He's already quite drunk by the time Montresor approaches him, and Fortunato is hungry for more wine (especially if its a really good one as promised).  Second, Fortunato is a trusting individual.  If he wasn't he wouldn't have gone down into the cellar.  Third, Fortunato is likely insensitive to the feelings of others.  

THE thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult, I vowed revenge.

I still think Montresor is crazy, but it is clear that Fortunato is a bit of an insensitive jerk.  A thousand injuries he has done to Montresor.  But then he finally insults Montresor, and that is the last straw.  I'm telling you right now, I wouldn't be super nice to someone that has wronged me a thousand times.  I would imagine that Montresor is the same way, but Fortunato is too insensitive to notice he is causing others pain, he's too insensitive to notice that Montresor is mad enough to kill him, and he's too insensitive to even pay a compliment to his host.  He's just greedy for more wine. 

Montresor wants revenge on Fortunato, so we must understand that we are getting a biassed view of Fortunato. Montresor says he has put up with a "thousand injuries" from Fortunato, though he doesn't specify what they are. He says too that Fortunato has "insulted" him, though he doesn't say how. 

Fortunato, Montresor continues, is a man to be respected and feared but has a "weak point," which is his pride in his knowledge of wines. Fortunato obviously thinks he knows far more about wines than his rival, a man named Luchresi, and Montresor plays heavily on Fortunato's jealousy of that rival to get him to enter the dank catacombs. 

Through Montresor's eyes, we see Fortunato as a figure respected by the rest of the world, but also as a drunken fool, easily baited and flattered, who Montresor plays expertly and gets the best of, luring him to his death.