I'm kind of enamored with the concept of Eve being a cyclops, which would explain why her eyes aren't seen in the storybook segment in S1 as well as the exorcists crossed out eyes. Though I'm convinced her and Roo are one and the same, and the latter had her design shown with regular eyes, so it's unlikely to be the case.
Slightly unrelated, but with Abel being blonde and having golden eyes, I can only assume Cain will end up being a brunette with whatever Eve's eye color is (red, perhaps?).