We can sympathize with pretty much every character in this book. They're all likeable, tough, and determined. Then why do we feel so deeply for Peter? Well, the narrator lays it out for us pretty early on:
Redheaded Peter, her sister's fiancé, had not married anyone in the years since Lise's death. He had changed a great deal. Once he had been like a fun-loving older brother to Annemarie and Kirsti, teasing and tickling, always a source of foolishness and pranks. Now he still stopped by the apartment often, and his greetings to the girls were warm and smiling, but he was usually in a hurry, talking quickly to Mama and Papa about things Annemarie didn't understand. He no longer sang the nonsense songs that had once made Annemarie and Kirsti shriek with laughter. And he never lingered anymore. (2.44)
We feel for this guy because his entire life has been turned upside down to the point that he's not really himself anymore. It seems like the only thing about him that hasn't changed is the color of his hair. That, and his love of the Johansens.
"Peter is in the Resistance? Of course! I should have known! He brings Mama and Papa the secret newspaper, De Frie Danske. And he always seems to be on the move. I should have figured it out myself!" (16.23)
Let's back up a second: Resistance? De Frie Danske? What are these things?
We're glad we asked. History time!
The Resistance was a movement where people, well, resisted the Nazis—and saved lives. Different countries had their own resistances, and Denmark definitely joined in: "After the German occupation of Denmark in April 1940, a resistance movement began operations there. Its activities included killing informers, raiding German military facilities, and sabotaging rail lines" (source).
In fact, the Danish resistance was pretty vocal and incredibly successful during its early years, or so say the experts:
For a number of years, the Danish Jews lived in relative tranquility. Whenever the specter of anti-Jewish legislation and persecution was raised by the Nazis, the public would express its vocal opposition. The steadfast stand on this issue by the Danish people and their government, combined with the relatively small number of Danish Jews, persuaded the Germans to defer the "Jewish question" in Denmark until after the war was won. (Source.)
Okay, now that we have the background, what's up with De Frie Danske?Well, folks, this is a real Danish newspaper that people used to keep up on all things Resistance. In a locked-down town like Nazi-controlled Copenhagen, it was incredibly hard to get any news the Nazis didn't want you to have. People like Peter, though, made it possible by passing these papers along.
Peter was part of a larger movement to help the Jews in Denmark. But he's also an individual, and we can't forget that each individual member of the Resistance sacrificed a great amount to help others. Peter, in fact, lost the love of his life (Lise) to the cause.
Up until the very end, Peter stands up for what he believes in and tries to keep other people safe in an otherwise unjust world. Even right before he dies, he's still thinking about the Johansens:
He had written a letter to them from prison the night before he was shot. It had said simply that he loved them, that he was not afraid, and that he was proud to have done what he could for his country and for the sake of all free people. (17.8)
All of our main characters make it through the war—except Peter. He's found out by the Nazis and killed as a criminal, rather than being recognized as the hero that he is. And without Peter's heroism, Annemarie wouldn't have been able to be a hero herself.