I always thought it was Itsuki, but something bother me... And it was that I couldn't believe "Rena" grew up to be someone like Itsuki. She's gotten pretty boring. Now it's been revealed it was actually Yotsuba, it really makes sense. I expected Rena to be cheerful, but Itsuki was not it...
So, Itsuki first met Fuutaro and then Itsuki started talking to the other sisters about him. And then Ichika was curious about him and decided to visit him?
@Shyri
But the thing is, if they do become friends instead of becoming a couple, then she won't keep up the "Ai" act because she does not need to. While if they were dating, then she would try to keep up the act as Aqua loves this.
@loliqot
He didn't want to continue living that life, so when the time came where they were fated to die, he decided to sacrifice his own life to save his.