Honestly, I'm kinda happy that there didn't end up being any relationships that fulfilled the whole trope of a guy turning into a girl and ending up with his former best friend and/or rival. One of my pet peeves is when genderbend stories change sexual orientation too - like, that's not how it works?? I am somewhat disappointed with the resolution though, because the story was played so insanely safe when it had such a great set-up.
Okay, I see a lot of people criticizing Mizuha for being a terrible person - but isn't that the point of her character? Unlike Hayase, who was proud and would go to any lengths to have Fushi, fully understanding how terrible they may be; or even Kahaku, who wanted to believe he was doing the right thing but ultimately sacrificed himself, Mizuha is much more naive. She never saw the nokkers in action, she didn't know the true history of her family, she was instead fed lies that Hayase was a protector of Fushi, and that she helped him immensely in the past. She sees no reason as to why Fushi shouldn't care for her deeply. You also have to remember her upbringing.
Mizuha was great at picking up almost anything, to the point of realizing that people valued her accomplishments over her as a person. When she finds out about Fushi and meets him, she suddenly finds what she believes to be the one person who should care about her for who she is, as well as the one thing she couldn't just obtain all on her own. She wants him to express concern and love for her, to show her how important she, by extension of her family, is to him. It's almost pitiful that she doesn't know the truth of Hayase's past with Fushi. She's just making a fool of herself, and Fushi is deciding to play along because he wants to like her and be friends with her, similarly to Kahaku. But this arc, for these two, can only end in a shattered relationship.