Apparently in manga and anime regular characters are often drawn as if they were European-ish (so, some of them are going to have blue eyes or blond hair, not common in Japan). This convention is in part historical (matching American comic books that inspired manga), and in part to make the characters more characteristic and easier to distinguish from each other. But in HnG this applies only to Japanese characters – people from abroad are drawn in a more naturalistic and stereotypical way. Koreans and Chinese will look actually like Asians, and Americans and Europeans will be an even more exaggerated version of themselves. I guess it’s a very different sensitivity than what’s common in the US right now.

IIRC that was mostly the adults, or older teenagers, who don't get a lot of screen time so their designs are simplified?

It kind of follows a general trope in manga/anime where characters' eyes are smaller or thinner to indicate age/seriousness/maturity/intellectualism. The Korean kids tended to have the same kinds of eyes as the Japanese kids, like Hikaru's main Korean rival (forget his name) having almost exactly the same trapezoidal design as Akira. Hikaru's eyes also change into this later in the series, changing from his original carefree wide-eyed design.