I'm sure the person's name is Luchesi. Somehow it got spelled Luchresi in a few editions of the story, but most critics refer to him as Luchesi. This character never appears but is referred to by Montresor several times throughout the story. He is a wine connoisseur. When Montresor first encounters Fortunato in the street he tells him he needs someone to advise him whether the cask of wine he just bought is genuine Amontillado and says:
"Since you are engaged, I am on my way to Luchesi. If any one has a critical turn, it is he. He will tell me"--
This bit of dialogue has a dual purpose. Montresor is fishing: he wants to find out if Fortunato is expected anywhere that evening. If so, he would probably decide not to commit the murder on that occasion. He would like to leave a cold trail. He would prefer to have at least that night and the following day elapse before anyone missed his intended victim. He doesn't get the information he wants immediately. But when he repeats that he will consult Luchesi, Fortunato says:
"I have no engagement; come."
I have suggested elsewhere that all three men are "gentlemen-brokers" who survive in the declining city of Venice through buying and selling expensive things such as paintings, statues, gemmary (i.e. jewelry), and probably gourmet wines. The "pipe" of Amontillado Montresor claims to have bought would contain 126 gallons of wine. Fortunato is greatly interested because he would like to buy some himself--but he has to be sure it is genuine Amontillado. He knows that Luchesi would also be keen on buying some if he heard about it, and then he would be competing with Fortunato in bargaining with the seller. Montresor's apparently naive, innocent intention of consulting Luchesi motivates Fortunato to go to Montresor's palazzo that every night. Both Fortunato and Luchesi could probably afford to buy the entire shipment of Amontillado, whereas Montresor is a poor man and could probably only afford another two or three casks if he were assured by an expert that it was genuine.
Although Fortunato repeatedly disparages Luchesi's connoisseurship, he is deliberately lying in the hope of dissuading Montresor from consulting the other man. Fortunato is not eager to taste the nonexistent Amontillado because he is such a great wine-lover, or because he wishes to accommodate a friend, or because he wants to show of his expertise: he is hoping to make a lot of money by buying many casks of gourmet wine and reselling it in quart bottles at a big profit. The good thing about wine as merchandise is that it improves with age if it is kept in wooden casks, so Fortunato (or Luchesi) could take their own sweet time about disposing of the Amontillado (if it really existed).
Finally, it is important to note that Fortunato would not have to go to Montresor's palazzo to taste the wine. Once he knows it is available for sale, he would have no trouble finding the ship that brought it into Venice. But if he makes some excuse (such as his cold) for not going that night, Montresor will go straight to Luchesi--and that is what Fortunato wants to forestall.