In my opinion, the antagonist in this book is society in general -- perhaps this can be split into American society and Japanese American society.
I say this because the protagonist is, quite clearly, Jeanne Wakatsuki Houston herself. So then we need to ask what her conflict is and who is stopping her from doing what she wants to.
To me, the conflict is between Jeanne, who wants to be herself (part Japanese, part American) and the two societies. They both reject her in some ways because she is trying to be in between they two of them.