The big difference between Mizu and Mulan is that Mulan is very clear that her identity is as a woman, she is playing a masculine role and concealing her womanhood. Mizu on the other hand never does any if this, and infact seems incredibly uncomfortable when required to conform or deal with femaleness; often reacting violently to people trying to gender him as female. Like I don't want to spoiler stuff here but the midway flashback arc is basically a trans coming out story in miniature.