Who is Carlo Vivaldi from History: A Novel and what is their importance?

We first meet Davide Segre when he appears at the shelter where Ida and Useppe are living after their building is destroyed. He presents himself as a Catholic Italian, Carlo Vivaldi, but he is actually an escaped Anarchist Jew.

Davide does not believe in violence. Eventually, however, he joins Nino in partisan fighting after learning his entire family has been killed at Auschwitz. In direct violation of his own values, Davide murders a young German, something that will haunt him for the rest of his life.

After returning to Rome, Davide is a shell of his old self. He becomes increasingly addicted to hard drugs and even severs his relationship with little Useppe, who wants to help him but does not understand his condition. Davide finally dies from an overdose, his suffering and early death making him another victim of a hideous war.