You're assuming the OP is wanting to write about a trans girl, but the secret doesn't say that. It's possible for a boy to want to be a girl, knowing he isn't one. I think that's what the OP meant, and is afraid of reactions like yours that think the character is supposed to be trans.

It's also possible that some trans girls think of themselves as "boys who want to be girls" at some point because that's the closest way they can describe it concretely for the time being. And that's OK.