Well, two of them are just straight up mixed white Americans, on accounts of the fact their story takes place in the US. Well, technically one of them is just from "the city" which, although intended to be vaguely New Yorkish in nature, is not committed to being any particular place.
From my first novel the MC is half mixed-Asian (mostly Japanese) and half vaguely Italian. Given that the only country on Earth where such a mix is likely to occur is the US, I suppose he'd have to be an American too. I guess he could also be Japanese though? He's kind of an expat in his own story, so maybe he would be a second generation Japanese American?
For the story I'm working on right now, I'm not sure. It's set in the far future, and the ML is from a space colony which intentionally pulled from a wide population in its founding. Culturally, I'd say they're a little "neo-Israelite formed by a group of people only vaguely familiar with Israel," but mostly they have their own culture. I should probably make him a little middle eastern looking, now that I think about it, though I'm still in the planning stages, and I usually worry about character appearances last.
The FL in the story is explicitly a racial mix, but I'd say she's most like a Chinese expat, given where she came from.
You might mean ethnicity~.
