We all know how much Wakaki likes to throw us off, so Chihiro is just as likely. After all, Chihiro's lack of being unique is what makes her unique (if that makes sense). IIRC, Keima had trouble with her capture because her actions and ideals were outside of Keima's experience. Anyway, I'm rambling.
I have heard the Gemini theory going around. Another one I have heard is neither having Mercury, but a third party. That sounds like a more plausible idea, but this is Wakaki, so I could be way off the mark.