Looks like Ryohka's style (or Mikeou?) but not sure.

It's certainly different from Ryohka's usual style, but even more than Mikeou's. From the pictures I have seen, Mikeou has thinner eyelashes, less bright, and they are more "round". I don't know if it's an old picture from any of them, but this is my opinion on the matter anyways. What does it say on the original source, Ryohka?

PS: The highlight colors in the hair are also different from Ryohka's coloring.

Your analysis is reasonable, but I still think I've seen it before. When I go back to school I'll check some cg collections of Mikeou & Ryohka (of course they're old ones). If no luck, maybe I need to check some other eroge artists' as well...

p.s. the source said nothing about the artist.

Ok, I think so too, but I can't remember well. Maybe you can remember better...
I see, well, but even with the lack of info I think we can figure out who's the artist. (ok, you can. lol)
Good luck with that. I can search some too if you'd like.

I finally find it in Mikeou's doujin cg collection called Princess Story 妹達のラブラブ日記 (released in 2001).

Fireattack, your memory is incredible.