Magdalena Gross is a famous artist who sculpts animals. You wouldn't think that would make her a target of the Nazis, but she is also Jewish, so she is totally a target, and her fame makes her even more of a target.
Magdalena takes courageous risks to maintain her independence. She lives as long as possible in open hiding of sorts and tries to go about her normal life. She refuses to flee Warsaw or move to the Ghetto until she is finally overwhelmed by danger.
A recurring motif of this book is how things normally considered sacred are destroyed without remorse by the Nazis: animals, animal lovers, innocence, childhood, art. Magdalena, with her bronze statues of animals, stands in connection with almost all of those categories. Her brave actions of resistance give us a taste of the attitudes that eventually turned the tide of the war.
Of course, we don't really get to know Magdalena closely. Instead of showing us her character, Ackerman instead tells us what Antonina thinks of Magdalena. For example: "Antonina saw Magdalena as a winning array of contradictions: emphatic yet vulnerable, daring but modest, zany yet highly disciplined, someone excited by life—which may be what appealed most to Antonina, who wasn't as stoic or solemn as Jan" (3.3).
And then Ackerman tells us, "The two women shared a passion for art and music, as well as a similar sense of humor, were close in age, and had friends in common—thus began what would become an important friendship" (3.3).
It's a friendship we never get to really see as readers, aside from one small scene when Magdalena sculpts croissant dough instead of clay. Why do you think her friendship was important to Antonina? Is it because Antonina appreciated having a kindred spirit in the house? Or would she have felt as close to any of her Guests? Or did she just really want croissants?