Leon emerges in the Sicily arc as a member of the Mancini family, locked in a bitter generational feud with the rival Barbazza clan over control of vital fishing territories. His fierce protectiveness centers on his younger sister, Monica Mancini, driving his initial vehement opposition to her burgeoning romance with Jack Barbazza, a direct consequence of the families' entrenched hostilities.

His background cements his loyalty to the Mancini cause, aligning him firmly with his father, Don Mancini, in their efforts to dominate local fishing operations. This allegiance propels him into violent clashes with the Barbazza family, escalating to an attempted assassination of Jack Barbazza.

Throughout the Sicily storyline, Leon undergoes a significant transformation. Witnessing Jack's authentic devotion to Monica and recognizing the destructive futility of the feud, his stance shifts from outright antagonism towards cautious cooperation. He ultimately brokers a fragile truce between the warring families, enabling Jack and Monica to be together without further bloodshed. This resolution necessitates the difficult concession of relinquishing contested territorial claims.

Leon's role culminates in the families' hard-won reconciliation, marking a profound departure from his earlier hostility and demonstrating a new priority placed on familial peace over the weight of historic enmity.