Hmm... I know I've argued different points on this topic but...
I think Yotsuba could have been the first "Rena" to meet Fuutaro in Chapter 42 and not Itsuki!! lol 😀
I had been going on the idea that first Rena was Itsuki because she saw Fuu's picture in his handbook in the Chapter 2.
But now I'm thinking... Itsuki did find the picture of Fuu and Yotsuba from 5 years ago... but afterwards she confronted Yotsuba and told her that she saw the pic of her and the boy in Fuu's handbook. It really makes more sense that if a sister finds a picture of a boy and her sister from 5 years ago, that she'd want to tell her (Yotsuba) of such a crazy find.
In Chapter 42 "Rena" shows Fuu the charm to show him that she is the girl from 5 years ago in Kyoto... Yotsuba knows the meaning of the charms! https://mangadex.org/chapter/381143/2
I got thrown off thinking only Itsuki knew the pic of them was in the handbook so Rena had to be Ituski... But if Itsuki told Yotsuba where the pic was when she saw it, then she would know too.
And even though Yots is notoriously known for being bad at pretending to be another sister... she's not pretending to be another sister... just in disguise.
In Chapter 34, Itsuki shows him the charm she keeps with her when trying to show him that she's determined to change herself too. When Fuu asked where she got it, she said she didn't remember if she bought it or given so doesn't know the whole story of how Yotuba bought them or that he was with her when she did.
Chapter 41 page 5.... I'm thinking yotsuba was the person hiding while Fuu and Itsuki was talking and Fuu told Itsuki that because of the girl in Kyoto that he had decided to become a person that would be necessary to someone. https://mangadex.org/chapter/363853/5
And in Chapter 42, Rena basically repeated and made the comment that he has become a person "necessary" to the sisters. https://mangadex.org/chapter/381143/9
Edit: Chapter 79, it was Itsuki posing as Rena since she revealed that after their second meeting but I think it helps the author throw off the readers in figuring out Yotsuba was the first Rena at the lake.
Anyways, that's just my current theory and tossing it out for fun lol.