Mysterious girls. (Nazojo?) The less they want me to know about them, the more interested I become.
Take a character like Suwa Setsuna &, as a comparison, Asuho Minahoshi, both from "Hoshizora no Memoria".
Setsuna on first impression is odd, rude, secretive & cold.
Meanwhile, Asuho is (in my opinion, obnoxiously) cheerful, friendly, outgoing & has an obvious crush on thee from the start. -_- zzz.
It's so much better when the game keeps them as hidden as possible throughout the game, too. (Kagari & Yume)
And, of course, there's the alien heroin sub-catagory, which includes my fav gal of all, Nagonago.