Luna is a delightfully eccentric Ravenclaw student who doesn't really seem to fit in anywhere. She also happens to be the exact friend that Harry needs this year.
Hermione accidentally introduces her as "Loony Lovegood" because that's how a lot of other students see her. We can't blame them. She's got those weird radish earrings and she's always going on about Nargles.
But Luna actually has a lot of wisdom for someone so flighty:
LUNA LOVEGOOD: We believe you, by the way. That He Who Must Not Be Named is back, and you fought him, and the Ministry and The Prophet are conspiring against you and Dumbledore.
HARRY POTTER: Thanks. Seems you're about the only ones that do.
LUNA LOVEGOOD: I don't think that's true. But I suppose that's how he wants you to feel.
HARRY POTTER: What do you mean?
LUNA LOVEGOOD: Well if I were You Know Who, I'd want you to feel cut off from everyone else. Because if it's just you alone you're not as much of a threat.
Luna's the one who tells Harry about the Thestrals. She diffuses the tension in The Hog's Head to ask if he can really conjure a Patronus. She's also the only one who says she's sorry when Sirius dies. And she drops that wisdom on him about how "the things we lose have a way of coming back to us in the end." This girl is a truth teller.
Harry also seems to connect with her over their shared grief. They've both lost parents and experienced death. That's not something their classmates can really relate to. It's not even something Harry can even talk to Ron and Hermione about. But Luna just gets it.
Sure she wears a butterbeer cork necklace and skips through the halls, but this girl is on to something.