Bad ending to a bad manga. Whether this manga came from somewhere personal or just a whim of the author to touch on complicated gender identity issues, it was executed poorly, bordering on offensively so. The bizarre theme of connecting childhood play and sex was just fucked up and I'm amazed the editor didn't tell him to rethink that whole angle. The resolution we get for Mitani is... she's happy? Somewhere, doing something? Her sexuality and coming to terms with womanhood was one of the most compelling plotlines, and it was resolved by Kei giving her a smooch and sending her on her way. If you gave me a $1000 I couldn't tell you what Yo's arc was supposed to be. He was coming to terms with Kei's identity, or questioning his own, or both, he was abandoning manhood but then appears at the end looking totally conventionally male and seemingly entirely unchanged except he's older and happy.
I just don't know what the point of any of this was. I hope someone got something fulfilling out of this manga cuz I certainly didn't.