Who is Abraham and Mandelbaum from The Complete Maus and what is their importance?

Mandelbaum is a friend of Vladek and Abraham is Mandelbaum's nephew. Abraham and Mandelbaum, like Vladek and Anja, manage to avoid the various deportations and survive in hiding in the Sosnowiec area even after the Nazis close out the various local ghettos. They then, along with Vladek, meet with some Polish smugglers who offer to transport them all to Hungary for a huge sum. Mandelbaum and Vladek are cautious and decline the offer but Abraham is intent upon taking his chances at escape.

Abraham and Mandelbaum evolve a plan, speaking in Yiddish to supposedly avoid being understood by the smugglers. Abraham will write a letter upon achieving freedom, and this will signal to Mandelbaum that the smugglers have kept their bargain. Abraham thus goes off with the smugglers. Within a few weeks an authentic-looking letter arrives, with Abraham's signature, stating that he is doing well and has arrived at his anticipated location. Mandelbaum, Vladek, Anja, and others then accompany the smugglers. The smugglers double-cross them, however, and hand them over to the Nazis who transport them to Auschwitz.

Mandelbaum and Vladek meet Abraham again in Auschwitz. He explains that the smugglers understood Yiddish after all, and after betraying him to the Nazis held a gun to his head and dictated the letter which he wrote and sent. In Auschwitz Mandelbaum does not fare well—his pants are enormously oversized and his shoes are completely inadequate. He walks around trying to hold on his clothes in search of shoes as his spoon and bowl are stolen. Vladek manages to acquire a belt and new shoes for Mandelbaum who views the assistance as a form of divine intervention of kindness.

Both Mandelbaum and Abraham are eventually murdered and cremated. Vladek does not know the details, but he believes that a guard took Mandelbaum's cap and threw it several feet away and then commanded Mandelbaum to retrieve it. When Mandelbaum went to retrieve the cap the guard shot him and claimed he was trying to escape—thus earning a few days on leave as a reward. As a final ironic judgment, Vladek notes that the Polish smugglers are also interned in Auschwitz where they die—once their usefulness to the Nazis ends they are consigned to death as common criminals.