She thinks nobody knows. Loads of people know.
At least, loads of people have suspected. She gave them good reason to, ever since our first year, always mooning over her. Professor McGonagall says this, Professor McGonagall did that. Really now, it was quite intolerable sharing a living space with her.
No, I'm being harsh. It wasn't all that noticeable after our second year. I think she figured that people might be getting wise to her little crush, and nothing embarrassed her so much as the thought that her One True Love might find out. So she hushed up some. It's not as if I ever heard her wonder about the Professor's relationship status even in her talkative days, but now more than ever she kept her musings strictly to quips from class - and she made sure to talk about other Professors as well.
People pestered Lav and me. Wanted to know if our roommate really was a Lesbian. How horrible for us, having to live with the gay one. Can't they just put her in the boys' dormitory?, they sympathised. Really, though, even if she did fancy girls, Lav and I would never worry about changing in front of her. We were much too young for her taste.
We wanted to investigate the rumor, because we were right sick of the questioning by the fifth year. So we started staying up at night; she was a notorious sleep-talker. Well, really notorious only to Lav and me. She mumbled to herself most nights, but once a week, thereabouts, she would sit bolt upright and ask, loudly, "What time is it?" or some other inane question. Effectively waking us up. Then she'd fall back down on her bed and kip some more. The only times we'd really gotten sleep were when she was out of the room for an extended period, like that time second year when she got Petrified. We hoped that her little habit would reveal the inner workings of Little Miss Perfection, and maybe some deep secrets as well, but mostly it was just nonsense syllables.
Then, quite suddenly, our luck changed. One night, as she made her usual rounds of "mmmf" and "bleg," a word came out. Just one. "Minerva."
We could have heard wrong. It could have meant anything. But it didn't. It meant her, Minerva McGonagall, the One True Love of one Hermione Granger. Or Miss Granger, as she undoubtedly prefers to be called. Probably calls herself that when she's alone in her knickers.
Certainly this little phenomenon required further investigation.