In that panel there Fuu talks about the kiss he used "Aitsu" meaning "that girl". He was referring to the person who kissed him, whoever she is. If he was referring to Ichika he would be using "Koitsu".
So relax people it's not reveal yet.
Then again I'm pretty sure it's Ichika myself.
My bet is on Ichika because she seems to be getting desperate, and after the talk with Yotsuba on the roof and Miku in the sauna, she'd be all the more determined to catch up to Miku and Nino now.
Plus I think the only ones who makes that face at the end is Ichika and Itsuki.
I don't see how...