The general consensus is that it's Ichika, from fitting the "big sister" personality, to having the ability to act, to not having an alibi that day.
Nino was a suspect, but that was until it was shown she still believed in the Kintarou charade. If she knew Fuutarou in the past, and had apparently realized it was him, she would have realized the photo (which Rena took) was of Futarou. That his "cousin" story was bullshit from the outset, that Kintarou's existence (at least initially) was predicated on a lie.
Miku could still be Rena, and honestly is the front runner behind Ichika especially considering her ability to act. But I feel like she would have capitalized on the realization that Fuutarou wad her childhood sweetheart.
Yotsuba is right out. She's a terrible liar and was doing laps with the running club. She can't be Rena.
Itsuki, of the quints, has had the most background information regarding this relationship dynamic between Fuutarou and Rena for awhile now. Yet she never acted on it if she was Rena. Instead Negi gives us a scene where Fuutarou spills the beans on the significance of Rena to Ichika during a midnight stroll through the park. A scene which culminates in the bushes rustling and Fuutarou saying that he thought he saw someone there before they clearly fled. We immediately get Rena's appearance the next chapter, which implies Rena (and whoever she is) was the third party overlistening Futarou and Itsuki that night. However this could still be admittedly a red herring.
My personal ranking for who might be Rena is:>Ichika>Miku>Itsuki>Nino>Yotsuba