My guess is they aren't quintupletes but sextupletes instead just Rena or Zero was not much for the tightness the other girls have and so she and their parents decided that instead of six sisters it was five by her leaving the house with some other family of theirs. She meets fuutarou, talks about the five girls and then disappears. And now reappears asking about them so as to possibly take on their persona when she so desires to fool him in the future.
Or maybe it's Itsuki since she is the only other one apart from Miku and Nino that has long hair. Yeah I bet it's her.