@SprinKah Look at Chapter 14, page 18... the picture Fuutaro had in his student book next to the group pictures the girls were looking at of their day in Kyoto... There is no coincidence there of a 6th girl looking like the other 5 in the city on the same day. If Fuutaro had only paid attention when they were looking at the album he/they would have been shocked. Chapter 34 where Fuutaro tells Itsuki the story of the girl and she shows him the charm she carries... the same charm she remembers Rena buy a boat load of them that day they met. And then Rena just happens to show up in his life in Chapter 42 and knows he's teaching the girls and she gives him back his student book but keeps the picture of them from 5 years ago (so that he doesn't piece it together so quickly. Yeah I don't like how they made Fuutaro seem so dense in that chapter. Ichika is a great actress eh with the wig and disguise to not look like the quints
).
I don't know who the girl is... I think they're all cool so I wouldn't mind any of them. But to me, it looks like it's narrowing to Mika, Ichika and Nino. Mika's smitten by him but the feelings not returned. Same with Nino. But Ichika has much more development through the entire story from the festival and her hiding her acting career and getting her to the girls, to the trip where they are stuck in the shed, to the cafe where she's acting and finds her studying between scenes. He has a deeper relation with her than any of the others and maybe a common connection with his desire to work hard initially to provide for her sister and Ichika taking the mother role since their mother died. And now Ichika is thinking it's ok to maybe go with her feelings over Mika's and Nino's and go on the attack.
Anyways, one part it clear in the above chapters and in the pictures... Rena was one of the five girls that day.