In terms of what the novel says about American identity, there are a few threads you could pick up – one is Nick’s comment in Chapter 9 about the novel really being a story about (mid)westerners trying (and failing) to go East : “I see now that this has been a story of the West, after all--Tom and Gatsby, Daisy and Jordan and I, were all Westerners, and perhaps we possessed some deficiency in common which made us subtly unadaptable to Eastern life” (). This observation suggests an American identity that is determined by birthplace, and that within the American identity there are smaller, inescapable points of identification.
Wei-Chen Sun is actually the Monkey King's eldest son, sent to earth in human form as an emissary for Tze-Yo-Tzuh. His test of virtue is to spend forty years in the mortal world while remaining free of human vice. When he initially arrives to Jin Wang's school he is "presented as a nerdy but fearless recent immigrant from Taiwan."  After Jin kisses Suzy Nakamura and Jin and Wei-Chen have a falling out, Wei-Chen transforms into an "angry and despondent Asian American hipster"  and gives up his mission for Tze-Yo-Tzuh. Despite Wei-Chen's multiple transformations, hints of his identity as a monkey are subtly and blatantly represented throughout the story. During the scene with Amelia in the biology lab, Wei-Chen has an affinity to the teacher's lipstick-wearing monkey, who will not leave him alone. Wei-Chen can easily recognize that the monkey is actually a male, just as the monkey can actually recognize that Wei-Chen is a monkey himself.  When Jin tells Wei-Chen that he has spoken to Wei-Chen's father (the Monkey King), a panel depicting Wei-Chen's true identity as a monkey juxtaposes his hipster human form in the next panel. No matter what transformation Wei-Chen takes, he cannot dismiss his true identity as a monkey.