I don't know if we even played the same game. It is not clear if Mizuki would like to use she/her pronouns or otherwise, but one thing that is sure is that they are deeply, deeply ashamed of being AMAB. They go into a full-blown panic attack in Mizu5 over a classmate telling Ena about it. If they were just a cis guy, why would they feel such an incredible sense of shame over it? Why would they put so much stock into nightcord seeing them as just one of the girls and be incredibly upset at the idea of their birth sex being discovered by them?
They could be a transwoman, they could be non-binary, we don't know their exact pronouns for sure yet, but to say that they are just a cis dude is completely baffling when the most traumatic point of their story up to this point has been about how traumatic the mere idea of Ena finding out what their birth sex is.
And now I just noticed the 1488 on your username so I guess this whole thing is pointless.