@Orpheus99 No, they've always been 5 siblings since early childhood. 5 years ago, everywhere they go, they always dressed up in a very similar manner, for top to toe. And then in Kyoto, one of the sister met with the small-rebellious Futarou and inspired him to be a study freak. The girl who met with Futarou didn't introduce her name to Futarou, so Futarou didn't know her name until they met again, 5 years later, under different circumstance. That's why, when Futarou asked her name, she's panicked and pick a name "Rena", meaning Zero.
Now, they're back to Kyoto, they want to settle everything there with Futarou.
Itsuki who wants to revealed to Futarou that she's Rena.
Nino who wants to spend most of her time with Fuu-kun (and make him look at her more).
Miku who wants to showed him how she get better in cooking.
Ichika with her stealthy snake-style who wants to keep him for herself.
Yostsuba.......she just simply wants everyone to be happy.