Who is Stefan Stern from The Case of Comrade Tulayev and what is their importance?

Stefan Stern is a revolutionary. He is a big man with green, slanted eyes, and his forehead is covered with messy, rust-brown hair. He meets Annie in Barcelona, and they live together in a pink house in Gracia and write papers, articles, and theses and meet regularly with comrades to discuss the progress of the Revolution in Spain. Stefan is around thirty-five years old at the time he is living in Spain. He is kidnapped and questioned and dies under shady circumstances on the boat where he is held captive.