The youngest Advenna Avis immortal. His guardian, Fermet, tortured him to test the limits of immortality. Czesław manages to devour Fermet, ending his suffering. Unfortunately, he learns from Fermet's memories that he was simply being tortured for Fermet's own self-satisfaction. As a result, Czesław becomes paranoid and trusts no one, making it his mission to devour other immortals before they can devour him.

In the 1930s it appears he cares little for the lives of others, even requesting that a large group of people be killed so he could find an immortal among them. However, after meeting Isaac and Miria and being forgiven by Maiza, he stops hunting other immortals and moves in with Ennis and Firo.