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

Davide Segre appears first in the story when he comes to the shelter where Ida and Useppe are living after their building is destroyed. He presents himself as a Catholic Italian, Carlo Vivaldi, but is, in actuality, an escaped Anarchist Jew. While doing some propaganda work, Davide is caught by the Germans and arrives at Pietralata under the assumed name of Carlo Vivaldi.