I feel the author probably had difficulty writing Mitani considering their experience with sexuality and gender is so linked to being born a male, so writing a female character and what she would go through in regards to sexuality would of course be difficult.
In addition to that, as most of us readers on here aren't Japanese, the way we view womanhood and sex are different than that in Japan, where women are constantly being put down, assaulted, obligated to marry by 28 and retire to raise kids, etc.
I personally saw her character--regardless of how the author intended it--to be just a much a victim of what it means to be a woman as Yo was a victim of what it means to be man. While men are pressured to have sex, women are pressured to be what makes them want to have sex. When you have the obligation of marriage and inevitably becoming a dedicated house wife imprinted on you since basically day one, having the one guy you like as a kid basically use you to confirm he could never be into you then leave almost immediately afterwards to come back years later as a woman (or x-gender, wakashu, whatever it may be) and be more attractive to men than you? Basically being "more than a woman"? She was desperate to prove that she still *was* a woman--an identity that was so heavily linked to sexual appeal to her.
She did a fucked up thing, and her manipulation of Yo was disgusting, but she's ultimately also a victim of the world the author is trying to communicate to us. But while Yo and Kei are much more empathetic due to being personifications of the author's identity, Mitani literally can't be due to the author's limited experiences in regards to not being born female. That's why she feels unsatisfactory, I think, and feels more one-note than the others. She's a foil, but also one-and-the-same, just not enough.
You’re literally awesome!! Writing this whole thing that doesn’t just explain mitani’s character, but how women are treated and feel themselves. ilysm I hope your pillow is whatever temperature you want it to be tonight
This is so sweet omg right back at you Boba
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Ok but tell me why I don’t feel satisfied with Mitani she literally could not let go of her feelings. She actually was a character that I did not feel bad for in the end. She literally forced herself on Yohei ☹