It's funny how some people think that Eiko doesn't love Kamu as much as she love Kiyose. The only reason we see Eiko embarrassed when flirting with Kiyose is because she's not used to being attacked aggressively. Not to mention you can see how Eiko treat Kamu different than the others because Kamu is special to Eiko. But, I guess it's no use to discuss about this since it's all up to author in the end.
I still ship Eiko x Kamu x Kiyose though.