The main focus is really on Peter, this British boy evacuated to the countryside during WWII, and Kimi, a German pilot who crash-lands near Peter's village. Their unlikely bond is the whole engine of the story.
You've also got Peter's mum, who's struggling with the war and his dad being away, and his friend Lizzie, who brings this fiery, suspicious energy because her brother is fighting. Kimi himself is fascinating—not a cartoon villain, just a scared, injured kid far from home. The local Home Guard guys add pressure, constantly searching for the 'enemy' hiding right under their noses.
What stuck with me was how the book makes you question who the real enemy is through these two boys. It's less a huge cast and more a tight, tense character study.