Fermet was one of the original immortals from 1711. He was guardian to the orphaned Czeslaw Meyer. Sometime after they became immortal, Fermet began to "experiment" on Czeslaw by torturing him, claiming that this was all in the name of determining the limits of immortality; Czes reports that Fermet's memories show that he was actually motivated by pure sadism. This went on for an unknown number of years before Czes managed to slip his right hand out of his bindings and devour Fermet.

Spoiler

In the novels, Fermet is revealed to be the main antagonist of the series. Despite having apparently been eaten by Czes and by another alchemist who was later devoured by Szilard, Fermet remains alive in the 21st century. Fermet is also responsible for murdering Monica, Huey's wife. His full name is revealed to be Lebreau Fermet Viralesque, and he somehow overcomes the no aliases rule by going by parts of his name.

(Source: Baccano! Wikia)