>>111631Well if Mahiro was transformed into a girl by his imouto, we should assume she wanted the best for him, right? So if we go off the basis of this assumption then Mihari would want Mahiro to be the best girl of all time. There we can see that whatever form Mahiro takes when turned into a girl should be the most optimal form so he can live the best re:life possible. So, the Mahiro we see is the ideal girl, and that ideal looks exactly like a younger version of Shinki.
Therefore, Shinki is the ideal woman. I'm sure you understand now.