From Chibi Manga:

She was her best friend and he was her first love...she didn't plan to change that. Ayu and Kaho were friends since kindergarten. They were the exact opposite; and even if both them were cute, they hated men. But one day, Kaho found someone she loved. "why? I was the one in your heart before..."
"Let's play at football" said a boy that Ayu never saw before. But, she felt her heart beating so fast and hurting so hard. It was the beginning of her first love...

Includes oneshots:
...Suki
It happened by accident... my first kiss, with Naohara-kun... 6 years passed, but that awkward feeling didn't go away.

Zutto Zutto Daisuki (I Will Forever Love You)
In the winter of my second year of high school, I had an unforgettable love. I really, really like you very much...but at that time I couldn't protect you. I'm sorry Ema.