Hey, I think Fuutarou already knew which one is "Rena", or at least, he able to guess who might be it...
That word, "I'm one of the people you've helped, too.", it somehow implied that Fuutarou realized which one is "Rena" because the one who made Fuutarou as he is now, was Rena.
Additional Edit :
You know, seeing Itsuki had not been involved too deeply with Fuutarou, what if the last chapter of the Festival Arc was from the childhood friend's perspective? 😂