I'm Hirata Koichi, and I used to have a sister some time ago, her name was Emi. We were quite close, and others would say we were a little too close, but we didn't care about that. We were happy.

But then our parents got divorced, and as we have no way to contact each other, we've become estranged. I lived my life the only way I know of, until on my first day of high school, I met her again as my new seatmate.

Emi: “Hey… it’s been a while”
Hirata: “Yeah, a really long while”