Margaret, Maddie, Kittyhawk, Käthe Habicht… we'll go with Maddie.
Early in their friendship, Maddie "played Watson to her friend's giddy Sherlock Holmes" (1.11.XI.43.199), and the comparison to Holmes and Watson is actually a pretty accurate description of the way Julie and Maddie function in the novel.
Though there might appear to be two main characters at first, especially because each friend narrates one half of the novel and most of Julie's half is focused on Maddie, Julie is really the main character, the primary player the whole way through. It's Julie's story, but the only way we get to know Julie is through Maddie—similarly, we get to know Holmes through his friendship/partnership with Watson. Also, Maddie is the stable character, the cautious one who throws Julie's mad bravery into relief.
In Part 1, we get to know Julie through Maddie. She tells us her own story by telling Maddie's story because she can't bear to talk about herself as she was before her interrogation by the Ormaie Gestapo—and in the process, we can't help but learn a few key things about Maddie, too.
The main thing we find out is that Maddie will do anything to fly. As long as she can be in the air, she's willing to serve in any capacity, for any branch of the military. We also learn that she really hates gunfire, which of course turns out to be ironic since her most memorable act in the narrative is to shoot Julie in the head. But back to the flying:
Once the electric undercarriage failed, and she had to earn her keep by taking her turn at the hand pump so they didn't have to crash land. Once they let her ride in the gun turret. She loved that, like being a goldfish alone in an empty sky.
Once they had to lift her out of the plane after landing because she was shaking so badly she couldn't climb down herself. (1.9.XI.43.85-86)
Not technically supposed to be aboard on these trips, Maddie is willing to do anything and everything just to be in the sky. Unlike Julie, who actually enjoys many elements of her training and actions in the SOE, Maddie bucks up and does what needs to be done without getting any real joy out of anything—except, of course, for flying.
When we finally see Maddie through her own eyes in Part 2, we discover that she's much braver than she thinks she is. As Kittyhawk, she has her passenger (Julie) parachute out so hers will be the only life at risk when she lands her damaged plane. She tells us:
Funny—it seemed the most heroic thing in the world when he told me about his friend, dead amazing that anyone could be that brave and selfless. But I didn't feel heroic when I did it—just too scared to jump. (2.25.39)
Whether she feels brave or not, Maddie is a model captain (or pilot, as the case may be)—willing to do everything she can to save her passengers, and willing to go down with the ship (okay, plane) in the process. Adding points to the bravery column, when she's stuck in France, Maddie assists the Resistance, taking on the identity of Käthe Habicht instead of thinking only of her own safety. Plus, when push comes to shove, she loves Julie enough to kill her rather than let her suffer.
In short: actions speak louder than words, and time and again Maddie's actions show bravery, even if she doesn't feel it in her heart.
Because Maddie is the only one left at the end, she completes Julie's story, telling us what happens after Julie's death, where Julie is buried, and where Julie's compiled "confession" ends up. As the title indicates, Maddie is not the main character, but she is the way we get to know the main character, the way we understand Julie beyond what Julie herself could tell us—and based on how much she's willing to do for her friend, we don't think she'd mind playing this part at all.