'Wit beyond measure is man's greatest treisure', said Luna in a sing-song voice
Harry Potter and the Order of the Phoenix, J.K. Rowling
The character Luna Lovegood, known to a lot of her fellow students as "Loony", makes her appearance in Harry Potter and the Order of the Phoenix. Harry's first encounter with her is on the Hogwarts Express, where Ginny Weasley, who is in the fourth year with Luna, asks if they can share her compartment as all the others are full.
At first sight, Harry thinks Luna gives off "an aura of complete dottiness". She is described as having "straggly, waist-length, dirty blonde hair, very pale eyebrows and protuberant eyes that gave her a permanently surprised look". She has her wand behind one ear, as would others safekeep a pencil, and she carries a necklace of Butterbeer corks.
Luna's father is the editor of the rather dubious magazine The Quibbler, which comes to play a rather significant part in this book. Luna herself has a preference for reading the magazine upside-down, which is what she is doing as the others enter the compartment, and she also seems to believe every mad story written in it, including the one about Sirius Black really being the lost pop star Stubby Boardman, leadsinger of the Hobgoblins.
Luna is in Ravenclaw, the Hogwarts house known for taking the brightest students. Without any serious spoilers of book number five, it can be disclosed that Luna also has her less loony sides, that she seems to take a special liking to Ron Weasley and that she has seen death at a young age.