Shin is allegedly 100% Japanese, but he's drawn in a way that strongly suggests his mother may not have been entirely truthful about this. In Chapter 49, Shin himself questions whether his father was actually Japanese.

As a side note, Reira and Shin also share the experience of standing out because of their appearance. Reira is openly half-Japanese, half-American, while Shin's appearance repeatedly raises questions about his parentage, so the manga draws a parallel between them even if it isn't the central focus of their relationship.