Final answer:
Keiko's secret is that she got one thousand dollars to try to get her and her family out of the relocation camp.
Step-by-step explanation:
Keiko's secret that she tells Henry is that she got one thousand dollars to try to get her and her family out of the relocation camp.
She explains that they can't stay in the valley for much longer and they need money to escape the country and start a new life.
This secret reveals Keiko's desire to leave and her willingness to go with Henry or at least try.