She is not a crossdresser, cis woman or a trans man. They are a trans woman and there are so many evidences pointing to that. If you think she isn't tfem please go back to english class and review this magical thing called reading comprehension.
Mizuki would not need to hide their gender indentity if they were a crossdresser. Otokonoko (japanese term for a man who wears typically "feminine" clothing) has been normalized and accepted as a subculture in japan. It isn't considered "weird", unlike being trans is. Mizuki hid her secret from Ena because she thought Ena would be weirded out/not accept Mizuki.
Mizu5 confirmed mizuki was BORN A BOY. Cisgender means identifying as the gender you were born as. Headcanoning Mizuki as a cis woman/man is just transphobic judging by the amout of hints the story has about Mizuki's gender. And also, it would just NOT make sense with the storyline! If Mizuki was a cis girl from the start, what would their secret be? "I wore the boys uniform in middle school" ?
The Japanese pronoun "boku" (僕) is typically used by boys. It is considered masculine, but theres nothing stopping women from using it as well. It’s like a girl using he/him pronouns. They can still indentify as a woman because pronouns don't equate to gender. "Masculine" is different from "Strictly male". For example, tomboys are girls that act and dress stereotypically masculine. They are still girls, just like how Mizuki uses boku, and that doesn't affect their gender. Boku has a more "tomboyish" and "cool" vibe to it, and I feel like that fits Mizuki perfectly!
A point people use to defend tfem Mizuki is her reaction to being outed and misgendered by Student A. I think it is a very strong point because several crossdressers say that they WOULDNT react like how Mizuki did. If you say her response was because "Their secret was revealed from someone else other than them" then you're WRONG! Crossdressers don't hide the fact that they are male, they actually EMBRACE it! Theyre comfortable in their own body, all they do is wear feminine clothing. Trans women are the complete opposite. Thats why Mizuki was so horrified.
During middle school, the mirror in Mizukis room was covered by a cloth. This shows that Mizuki experienced gender dysphoria, something crossdressers NEVER have. They are comfortable enough in their own bodies to dress as the opposite gender because it is not a change in identity, it is simply a way one dresses.
The clothes Mizuki wears reflects their gender identity, which is female. It's not about how they dress, it's about their identity and how they express it. Crossdressing is not an identity, it's a dressing style and Mizuki's character goes way deeper than the clothes that she wears.
The meaning of IDSMILE is
Identity
Secret
Minority
Smile
Why would Mizuki's identity be such a big part of her character? If she really was a crossdresser, it would be about clothes and breaking the gender norms or something. And to add, Crossdressers/Otokonoko are not minorities, but trans people are.
Mizuki is definitly NOT cisgender in any way, that is what we know. However, Mizuki's gender IDENTITY is what we are unsure of. She could be a transfem nonbinary, or even just a trans girl. I choose to label her as a trans woman because that is what i am most comfortable with. As long as you aren't erasing representation, i will respect you.
if your question was answered here, dont comment it unless you need more reasoning.
"Why is this important?"
Mizuki's ENTIRE character isnt about gender. Her personality is nice and is the entire reason why i like nightcord25. However, her gender is kind of a "driver" for her story. If she didnt have a secret and was just a cisgirl her character would not be AS interesting in my opinion. In the idsmile story, Ena wouldnt have noticed anything odd about Mizuki and the story would literally just be the n25 members going to a haunted house and nothing else, so no arc starter. Mizuki would have no secret to tell to n25 so she and Ena would never go to the school rooftop to reveal her secret, so no Student A outing Mizuki. Ena5 would not happen.
+ i want trans representation and i hate people trying to erase it
"Why are you forcing your headcanon on us?"
I admit i AM forcing my headcanon on you but its for the sake of representation. Answer me, why do you want a crossdresser character in project sekai? Now, why do you NOT want a TRANS character in project sekai? Can't answer without seeming transphobic? EXACTLY! CROSSDRESSER MIZUKI HEADCANON *IS* TRANSPHOBIC! Representation for trans people and the rest of the queer community is important because of the oppression they face and for them to feel seen. Again, crossdressers are NOT an opressed minority so they dont need representation. "I DONT hc mizuki as a crossdresser because i am transphobic" So why? Even if you can find a reason you are STILL transphobic because erasing trans rep IS.
"But Mizukis gender is unknown?"
Half correct, its IMPLIED though. Implied means there are clues pointing to a certain possibility without it being directly stated, meaning unconfirmed. Uncomfirmed =/= canon because why would someone want to put clues in their story if theyre implying nothing? If Mizukis gender was just stated in her profile there would be less to no interest in her story/arc. There are so many hints about her being trans so that means its IMPLIED. So Mizukis gender IS unknown so people are trying to figure it out using clues in the story. They have been pointing towards Mizuki being transgender.
edit: transfem she/they mizuki canon if you disagree don't even TRY to change my opinion