@elefantine97 on the 2nd last page, the author says:
Kaoru has somehow been hiding the fact that he's a guy... naturally I would think that there are questions about this, but I believe you can expect those things to be addressed gradually over time
Basically, "how does Kaoru continue to pass flawlessly well into his college ages?" is the question the author doesn't want to answer right now.
The obvious answer is puberty blockers. If you're a transfem, then you'll know just how hard testosterone FUCKS UP your body irreversibly - passing isn't a "guaranteed" thing, even for those who've been on hormones for ages. And yet the fact that Kaoru doesn't get clocked constantly (voice, figures, face, etc) without taking hormones (remember, he had a flat chest in ch. 1) means somehow he's "naturally" remained feminine, that he didn't get masculinized by puberty. Thus, obviously puberty blockers (or any sort of anti-androgens).
@JaneJeon yeah, I know. I know too well haha Thankfully I got my some rare asian genes so I don't have facial hair nor masculine features
except my jaws for some reason :-S I wish I had puberty blockers 4 years ago ;-;
oh and thanks for the spoiler, that's too bad, but ohwell, i like these kinds of stories regardless
bokura no hentai is probably my favorite