Which Harry Potter book was the first in J.K. Rowling’s series to feature the character Luna Lovegood?

Harry Potter and the Order of the Phoenix was the first J.K. Rowling book to feature Luna Lovegood.

Luna is introduced in the fifth novel, published in 2003. She is a Ravenclaw student in Harry’s year and is known for her unusual beliefs, calm manner, and distinctive view of the wizarding world. Her father, Xenophilius Lovegood, publishes The Quibbler, a magazine that challenges the official account of Voldemort’s return.

Luna becomes an important ally to Harry, Ron, Hermione, and Neville. She joins Dumbledore’s Army and later takes part in major events of the series. Readers sometimes place her debut in the fourth novel because several later central characters are introduced there, but Luna does not appear until Order of the Phoenix.