Who is Yulek Schwartz from The Cage and what is their importance?

Yulek Schwartz is a seventeen-year-old who was sent by the Skif to offer help to Riva. He was responsible for his younger sister, Faygele, who was fourteen, and felt that responsibility as heavily as Riva. Yulek was eventually deported and, fearing Faygele's fate without him, took her with him.