What particular aspect of America is Chris meant to represent in The Reluctant Fundamentalist? It is meant in how America, similarly to Erica, is trying to regain something that is lost to them forever.
Chris represents that "fixed" notion of identity. Chris represents the nostalgic view of the past that comforts Erica, and creates a gulf that Changez can never really overcome. Erica comes to see Chris as a part of the past in which she was happy with herself. Chris is the portal into a past that makes Erica feel good about herself, as she feels more in control and more secure with her identity because it is not subject to the ever changing and mutable conditions of the present.
In terms of the historical representation of Chris, he becomes a part of that American identity where there is a sense of absolute understanding of its own power. Chris is representative of the America that existed in the World War II and Cold War paradigms. Chris' representation of nostalgia is the vision of America in its past. There is a confidence, and self assured swagger to how America operated in these settings. In contrast to the crippled and uncertain condition in the wake of the September 11 attacks, Hamid makes the claim that America, like Erica, reverted back to a vision of its own notion of "Chris" in dealing with the ever changing and mutable conditions of the present.
Changez would never be Chris. Even though he believes that he can overcome this gulf with Erica and with America, it becomes clear that the nostalgic cling to the past is too strong for him to endure. The romance with Chris is a wedge for him. It divides him. It creates him to be separate and marginalized from what he loves. Erica and America need their own "Chris" in times of crisis and this is where Changez finds himself on the outside looking in.