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.
To me the antagonist is the American Government. I have a relative who wrote the book The Years of Infamy. They (the family) were incarcerated in the camps. They had everything that they had worked for taken away and given to the state of California. While in the internment camp, their land was sold out from under them.
To be Japanese in the United States during World War II meant that the government automatically identified you as a suspect of fear. Many Japanese lived in this country for generations and had even fought in World War I. It was a difficult time for people who believed they were under protection in America.