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The youngest executive of Manhattan's Martillo Family Camorra. Firo is surprisingly friendly, humorous and compassionate, but he is a skilled fighter and can be ruthless or even vengeful towards those who bring harm to the people he cares about. He became a complete immortal in November 1930 after accidentally drinking Szilard Quates' immortality elixir. In the same year he falls in love with Ennis, and despite the fact that she initially is unsure of her affection due to her underdeveloped emotional capabilities, they live together as roommates and gradually she reciprocates Firo's feelings.

Firo has unconscious issues with femininity due to a childhood incident wherein he was mistaken for a girl and nearly molested. His complex about being called girly suggests a lingering stigma, but this does not mean he dislikes women—on the contrary, he treats them with chivalry, never harming them and considering any man who does so as scum. He is also shy when it comes to romance and refrains from taking the initiative even when his feelings are obvious, as shown with his relationship with Ennis. All of these traits can be traced back to the incident from his childhood, making it one of the most formative events of his life.

Firo marries Ennis in the early 1980s and the two honeymoon aboard the yacht ship Entrance in 2002.