Who is Berel Jastrow from War and Remembrance and what is their importance?

Berel Jastrow is Aaron's cousin. Berel is not as fortunate as Aaron early on in the war and winds up at Auschwitz well before Aaron is interred. Berel, however, is strong and is not willing to give in to the German torment and torture without a fight. He manages to land on a working team headed by a man he knows named Sammy Mutterperl.