Knowing Luna
If it happens at Hogwarts, Luna Lovegood knows about it.
She blends into the background. Everyone forgets she's there; they let their hair down and their secrets loose. She doesn't need an Invisibility Cloak to disappear. It's been her way since her mother died; she's convinced if no one else notices her, no one else will leave her.
Then Ron Weasley came barreling into her life, and for the first time she wishes that she was all the things she'll never be -- pretty, popular, visible. Oh, he sees her, even talks to her and doesn't tease her like the other kids do. He doesn't notice her, though -- all his attention is focused on Hermione and Harry. And she suddenly wants to be noticed.
Luna hears and sees and knows things that other people don't, and she's a Ravenclaw to the bone. She knows how to use her knowledge to get what she wants, and she's finally found something -- someone -- worth the effort.
Her father is surprised at the way her habitual vagueness falls away like a cloak she's finally outgrown, a chrysalis that harbored the bright butterfly she's about to become.
Everyone thinks they know Luna Lovegood.
They're about to learn they don't know anything at all.