Even though everyone else sees Keiko as a Japanese girl and the enemy, she's insistent on the fact that she's American—just like all her white peers. She doesn't deserve to be treated differently just because she happens to have Japanese ancestors. When Henry says that he's Chinese, she tells him that he can say that, but that she'll always identify as American:
"That's fine. Be who you are," she said, turning away, a look of disappointment in her eyes. "But I'm an American." (12.101)
Keiko has to fight for her status as an American citizen, even though she was born in the United States and doesn't so much as speak Japanese. Still, the U.S. government—and her peers—see her as part of the problem and pick on her for it. It's definitely not easy for her to grow up as a Japanese American girl during World War II, but Keiko never stops holding her head high. Nobody breaks Keiko's stride.
Although Keiko is up against a lot of obstacles and meanness from people (even strangers), she never stops persevering or working to make things better for the people that she loves. She's always there for Henry, and even helps her parents preserve their family memories when they have to burn their photo albums, sneaking out past curfew—even though she could get into trouble or get hurt—to meet up with Henry and give him the photo albums:
"These belong to my family. My mother told me to take them to the alley and burn them. She couldn't bring herself to do it. Her father was in the Japanese navy. She wanted me to burn all her old photos from Japan." Keiko looked at Henry with sad eyes. "I can't do it, Henry. I was hoping you might hide them for us. Just for a while. Can you do that for me?" (21.45)
Even though it could get her in a lot of trouble, Keiko works hard to save those photo albums because she knows how much they mean to her parents. Even if she doesn't feel as culturally connected to the photos of Japan, she cares enough about her parents to save the things that they hold dear, making sure their connection to their past doesn't all go up in flames.
And Keiko is true to Henry, too. She remains loyal to him even when kids at school pick on him for being a "Jap lover" and for wearing his "I Am Chinese" pin. She never wavers in her defense of him, which is great—if you've read up on Henry elsewhere in this section, then you know he's one loyal dude. In Keiko, he has his perfect match.
Despite their ferocious loyalty to each other, Keiko still ends up being the one who gets away for Henry. After she's moved to a Japanese American internment camp, it becomes harder and harder for them to keep in touch—and eventually, Henry gives up on his first love and writes her a letter to say goodbye:
The letter wasn't just a good-bye—it was a farewell. He was wishing her a happy life, and letting her know that he'd be leaving for China in a few months, that if she might be returning soon, he'd meet her, one last time. In front of the Panama Hotel. (46.20)
Keiko never shows up, and she and Henry both move on and end up marrying other people and starting their own families. They don't speak for decades, but at the very end of the book, Henry's son hears his father's story and is compelled to seek out Keiko. Henry and Keiko's spouses have both died, so they can finally meet again without feeling guilty about disrespecting their spouses. And with that, they pick up just where they left off. Aw.