Towards the end, you quickly realize why the MC 'male' was always cast as a shadow. What would have been even more tricky is if the doctor referred to the MC by their surname, but have the name be ambiguous as well. The MC friend's behavior at being confessed to by someone whom they likely never considered being of an alternate persuasion was pretty accurate.
One other clue I had considered to the MC's true identity is how her senpai towered over her--in her delusion thinking she was a man, her delusion looked quite tall, which would make the senpai freakishly tall in comparison.
Well-put together story though. Even as brief as it was, I was fooled up until almost the end.