Both Catalans, Mercedes and Ferdinand Mondego are cousins. Ferdinand is desirous of Mercedes and jealous of Edmund Dantes.
Mercedes and Ferdinand are descended from Spanish and Moorish sailors who settled in the promontory on the Mediterranean Sea, but they did not mix with the French residents of Marseilles. In Chapter III, Ferdinand sits watching Mercedes with vexation and discomfiture. Finally, he speaks to her:
"Tell me for the hundredth time that you refuse my love, which had your mother's sanction. Make me understand once for all that you are trifling with my happiness, that my life or death are nothing to you. Ah, to have dreamed for ten years of being your husband, Mercedes, and to lose that hope, which was the only stay of my existence!"
Mercedes tries to reason with Ferdinand, explaining that a woman cannot be a good wife to anyone if she loves someone else. She urges her cousin to accept the love of her friendship as this relationship is the only thing she can promise him. As a result, Ferdinand feels hatred toward Edmund Dantes, and for this reason, he enters the conspiracy against Edmund which leads to his imprisonment.
Years later, Ferdinand convinces the grieving Mercedes to marry him, and they have a son, Albert. Ferdinand has become an officer and then a count, the Count de Morcerf. He had the letter delivered that implicated Edmund as a Bonapartist, a damning letter which sent Dantes to the Chateau d'If. Many years later, Dantes, as the Count of Monte Cristo, wreaks his revenge upon Ferdinand, exposing his treachery while he was a officer as he betrayed the Pasha of Janina, causing his death. Ferdinand also sold the pasha's wife and daughter into slavery. Disgraced in court and before his contemporaries, Ferdinand commits suicide.