Yeah... Besides Riona that people said it's clearly stated, all the others will be left "up in the air." I think it's part of Mr. DPC's strategy to cater to the largest audience possible, and, since many guys like Oriental girls, having a lot that are ambiguously Oriental help with that, so, if you think they're Asian, awesome; if not, awesome too -- whatever makes you happy. (And I'm not complaining about that; I think he's quite smart). So, in my imagination:
- Riona is Japanese
- Jill is Chinese (Cantonese)
- Zoey is Filipina (too be honest, I didn't think she was Oriental until I saw her family, which is also kinda ambiguous)
- Sally is Vietnamese
All descendant, of course (and Jill is mixed because her father is obviously white). And this is my imagination, of course, nothing canon. And there are also weird things, like, Jill have blue eyes, which could be explained by her father being white, but her mother also does! This could cross her from the Oriental pool, but Riona -- the only one supposedly to be canonically Oriental -- also has blue eyes (well, she could be mixed too, but she's the one who look less ambigous, and even her
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is described as being Japanese: "Hyuna is a beautiful pale skin beauty from Japan")!
But, whatever -- what matters is everyone being happy.