Luna Lovegood is a real "love her or hate her" character, and that debate still goes on within Harry Potter fandom to this day. Whatever you thought of her, Luna was very different as a character, managing to stand out as strange even in a film franchise packed full of strange people. We can credit Evanna Lynch with bringing that to the role with her performance.
Lynch was a huge Harry Potter fan long before the films were even made, and credits J.K. Rowling with helping to save her life. Hospitalized with anorexia at just eleven years old, Lynch wrote to the author as a fan, and received a response back that was handwritten from Rowling, in which she described the disease as "destructive, not creative", which she credits with kick-starting her recovery. After that, securing an acting role within the Harry Potter franchise and working with Rowling in the future must have been her destiny.