My thoughts on who that “Itsuki” is among the quint.
Ichika seem favourable to be the “Itsuki” that kiss Fuutarou. If not for one detail that disqualified her from being that “Itsuki”. Her leg injury, specifically the one on her left ankle. While I’m no expert, I’m pretty sure her leg injury still hasn’t heal by the last day of the trip. What this ‘Rena’ manage to do was RUN towards him and then run back afterwards without any sort of difficulty what so ever, is what lead me to believe it’s not Ichika.
If it was Nino, she will take the opportunity to absolutely let him know she stole his first kiss, the fact that this “Itsuki” didn't even reveal herself after the kiss make me doubt it was her. Plus, she wouldn't risk having him think that he just kissed the actual Itsuki if it were the case.
Miku, my most likely suspect. Her hesitation does seem to align with this ‘Itsuki if she ever did try to kiss Fuutarou, and the unhappy face afterward could be attributed to her kissing Fuutarou because of the fall and not because she mustered up her “courage” to do the deed herself. As for why she didn’t reveal herself? She probably assumes that Fuutarou knows it’s her, given what happen last chapter she may even believe that Fuutarou can now immediately distinguish her even with an disguise on.
Yotsuba… it could be her but she never pursuit Fuutarou with any romantic intent before, so that leads me to doubt it’s her. If it’s really her what change? A counterpoint for Yotsuba being that “Itsuki” is that we also see a bird eye view of the order which they descend from, with Yotsuba the among the front and talking with Fuutarou’s dad, if she were to try and make any move on Fuutarou she would have to run pass the other quint, arousing suspicion.
Unlikely to be Itsuki. So far, she considers and confirm her relationship with him to be friends in this arc. Not only that, she now knows that Miku is in love with Fuutarou so it’s very unlikely that she will make any move on him.
Another thing that I want to bring up is that “Future Fuutarou” didn’t put a ring on the bride and instead skip for the kiss itself. After all the build-up for that ring if he really did put it on the bride, it’ll solidify the bride being Yotsuba, but he didn’t, which open up the possibility for the other quint to be the bride and put a huge damper on the theory that the bride is Yotsuba because of her holding his ring finger during the last day of camp trip. Speaking of which, if the campfire legend is true, then when all the quint hold his finger shouldn’t the legend’s most logical choice to bound be whoever has the most love for Fuutarou at that time?
But back to the current mystery, what I do know of this mysterious “Itsuki” is: She is able to run, is still disguise as “Itsuki”, she specifically chooses this moment to try and kiss Fuutarou, hesitated when it come to the actual kiss itself, is unhappy or upset with the outcome, and apparently has no need for words. The timeframe for when the rest are out of view and the kiss is very short so whoever did it must be near the back with the sole intention of kissing him in mind. Also, the possibility that Fuutarou is close/near the “Bell of Vows” could be coincidental and just Fuutarou being alone is enough of a reason.
There, those are my thoughts and speculation. What do you guys think?