1) Hmm, hard one. I think Minami's. I never go for this type of character in VNs, I mean Aiden is more my type but man, Minami's great and I love his voice and I really want to see how things develop (and who's gonna make the first real move; either one would be fun).
I plan to play all of them, of course. Aiden is my type and I already really like him from the demo - I want to know more about him and see how he'll confess to Rei or the other way around. I want to see the twins (obviously), because hell yeah threesome and I want to see if they actually fall in love with Rei or they'll just have fun with him. And I want to see Jun because it's his ex and Rei seems to still have feelings for him - besides, he gave him Kii so that's a plus.
4) I don't think naked sprites are needed. I prefer sticking to CGs for erotic scenes - would rather have an extra CG than naked sprites.