One of the youngest members of the Martillo family and is a skilled fighter. He was orphaned at the age of ten and picked up by the head of the Martillo family after Firo attempts to rob him. In the Martillo family, he is tasked with monitoring casinos to prevent cheating, and later manages those casinos after becoming an executive member of the family.
When Maiza leaves the Martillo family in the 1970s, Firo is given the position as the family’s bookkeeper until Maiza returns.
In 1930, he mistakes the immortality elixir for alcohol and takes it. He later distributes the elixir at a party, becoming immortal himself. That year, he falls in love with Ennis and devours Szilard Quates, saving her and the new immortals. Despite the fact that she initially does not reciprocate his feelings, due to her underdeveloped emotional capabilities, they live together and eventually marry.