Rachel's mother, Kerry, has one of the most interesting stories in the novel—and we don't even know any of it until the final chapter. Initially, we are led to believe that Kerry moved from China when Rachel was six months old because her husband died in an industrial accident. Hoo boy, do we find out differently!
When Kerry was a teenager growing up in a small village, Xiamen, she fell in love with an older man, Zhou Fang Min, who was from a bigger city. She ran away with him, got married, and was horribly treated by his parents because she was just some "village girl" (3.20.6). To avoid the poor treatment, she convinced Fang Min to move away, where he then became drunk and violent and she had to hide for safety.
She ended up falling in love with Kao Wei, a young man who would continually protect her. HE is Rachel's father. But no one knows this. When Rachel is born a girl, the Zhous want to pour acid in her eye so she is disabled and Kerry can have another child (preferably a boy). Understandably, Kerry scoops up Rachel and disappears, eventually to America.
Is Kerry some peasant from China? Yes. Is she an amazing mother who sacrificed nearly everything to save her daughter? Yes. Is it wonderful that she and Rachel have a close and special relationship? Absolutely yes.
Kerry essentially functions as a foil for all the "traditional" customs of the wealthy in Singapore. While Rachel is exploring this world, Kerry gives her suggested behaviors based on her experiences in Mainland China. In some ways she helps us question the behaviors of the Young family in a way that we may not know to question.
In the end, Kerry's secret past catches up with Rachel, causing Rachel a lot of heartache. However, this deepens Rachels appreciation for family and brings Kerry and her daughter closer.