Harry Potter and the Half-Blood Prince
House: Ravenclaw, 5th Year
Quidditch: temporary Commentator
Luna has "a knack for embarrassing honesty" (7.107), and she likes to spout the conspiracy theories that she's garnered from The Quibbler, a magazine at which her father is an editor. A little offbeat, a little odd, Luna is not popular, but she doesn't bend to peer pressure or try to be someone she's not. She wears her radish earrings with pride and isn't afraid to voice her opinions. Harry counts her as one of his good friends.
In an online interview at lunafans.net, Rowling expresses one of the reasons why Book 6 was so fun for her to write:
"You know what I've enjoyed writing – you know Luna's commentary during the Quidditch match? [Laughs] It was that. I really enjoyed doing that. Actually I really enjoyed doing that. You know, that was the last Quidditch match. I knew as I wrote it that it was the last time I was going to be doing a Quidditch match. To be honest with you, Quidditch matches have been the bane of my life in the Harry Potter books. They are necessary in that people expect Harry to play Quidditch, but there is a limit to how many ways you can have them play Quidditch together and for something new to happen. And then I had this moment of blinding inspiration. I thought, Luna's going to commentate, and that was just a gift. It's the kind of commentary I'd do on a sports match because I'm — [laughs]. Anyway yeah, it was that." (source)