Kinosaki gets naked to prove he’s a boy in the first/second chapter. Unless the mangaka retcons that I’m going to assume he’s a boy (of course we could get an arc about him being trans but not fully out to himself but that’s its own thing).
Well there's that too but again Kinoshita is a great manipulator, dude might know that Gero won't see his body thus did that. Btw even if Kinoshita is a dude, pretty sure he isn't a trans. He is just comfortable in women's clothes but doesn't consider himself as a women (except at work) so he is just crossdresser right? unless it's the same? Idk I'm too old for that.
He’s not trans as of now I only mentioned it as a possibility that could happen later in the story since we’re discussing him being secretly a girl. To further explain, there are times where someone who cross-dresses ends up having an epiphany where they realize they’re trans despite insisting they were cis prior. This could be a thing that happens later down the line though I actually like him as a boy.
I think revealing him as a cis girl would be the most boring option for the mangaka to choose so hopefully they won’t. But also since they’re already including a canon lesbian couple (who’re not sexualized which is a nice bonus) I don’t think they will.
By how this manga develops, I'm somehow sure now that Kinoshita is a real girl who has some serious illness that's why she doesn't want to marry Gero and told him she's a dude. Cause she knew her time is gonna come soon.
But that will be too generic, so I hope I'm wrong.