well the backside of chapter 84 confirms that Ichika was the Rena of the past.
i think that Rena is Ichika because to me she is the only one that makes sense and has the most hints pointing towards her. first to provided evidence that it its Ichika i will eliminate the other 4 sisters. the main point of evidence towards the my theory that Ichika is Rena is that the other 4 sisters did not say anything or recognize MC when Nino saw the picture or during the courage test when all four of them saw the MC with the wig on and didn't notice except for Ichika who walked up to him and said " so you died your hair" or something along the lines of that. then there is also that time where Rena appeared and Yotsuba appeared in a track jersey right when he went up to the bank which crosses her of of the list. I will also cross of Miku because to me she doesn't really have the personality to confront him like that ( my opinion) and she is also not very athletic so i find it hard for her to escape that fast. i'm also going to cross off Nino due to her personality and the fact that she got fooled by MC when he dressed up as the blonde haired version of him. finally i will cross of Itsuki the hardest one to get rid of in my opinion but i don't think its her due to a few reasons. the first is like the rest which is the fact that she didn't recognize him during the courage test; the second is that she she showed the charm she didn't remember who got the charm and she didn't say anything about her getting one with a boy. there you have it, my reasons as to why Ichika is Rena and why the others are not Rena.