Which J.K. Rowling novel contains the first appearance of the character Luna Lovegood?

Harry Potter and the Order of the Phoenix contains Luna Lovegood’s first appearance in J.K. Rowling’s novels.

Luna is introduced when Harry, Ron, Hermione, Ginny, and Neville travel on the Hogwarts Express at the start of Harry’s fifth year. She is a Ravenclaw student known for her unusual beliefs, her calm manner, and her loyalty to Harry’s cause.

Luna becomes a member of Dumbledore’s Army, the student group Harry teaches to defend themselves against the growing threat of Voldemort. She also joins Harry, Ron, Hermione, Neville, and Ginny in the Department of Mysteries battle at the end of the book.

She appears in the remaining two main Harry Potter novels and becomes an important ally. Readers sometimes associate her debut with Harry Potter and the Half-Blood Prince because her role expands there, but her first appearance is in Order of the Phoenix.