Born between an Italian father and English-American mother, Firo's early childhood was mostly spent in Manhattan's Italian slums.
Orphaned at the age of 9, Firo was found by the Martillo Family in the streets one day and stayed with them ever since. Being one of the series' best at knife combats, Firo became an executive staff of the Family at the age of 18. He's also indirectly responsible for the birth of a brand new generation of Immortals in 1930.
Firo is claimed to be the official protagonist of Baccano! by its author despite not being featured at all in some of the books.