Akiyama Mizuki is not a girl. He is a biological guy, with an officially unknown gender. The comments are not negative, just to let the author know.

Upd: Please also do not start an argument about this.

[deleted]

user318461063 said:

Akiyama Mizuki is not a girl. He is a biological guy, with an officially unknown gender. The comments are not negative, just to let the author know.

Upd: Please also do not start an argument about this.

How is it a biological male if the gender is unknown?

Glancing over the fan wiki it seems like it might be implied, but not openly stated.

As for what is open:

Trivia

• On the Global server of Project SEKAI, Mizuki is referred to with both "she/her"and "they/them" pronouns, though it is unknown which Mizuki personally uses. When possible, Mizuki's name is used in place of pronouns entirely.
• On the Japanese server of Project SEKAI, Mizuki typically uses the masculine first-person pronoun Boku (ボク), which is most often used by immature or young boys, though it is also used by tomboyish girls.