Remus Lupin

Remus John Lupin, nicknamed Moony, first appears in Prisoner of Azkaban as the new Defence Against the Dark Arts professor. His students, except those belonging to the Slytherin house, hold him in extremely high regard and love his hands-on teaching style. During his tenure, he gives Harry private lessons in casting the Patronus Charm, to aid him in fighting off the Dementors flanking the Hogwarts grounds.

As a young boy, Lupin was bitten by the werewolf Fenrir Greyback; as a result, he transforms into a vicious werewolf every full moon, only retaining his human memories by drinking the Wolfsbane Potion. Albus Dumbledore, one of the few wizards who was sympathic towards his condition, made arrangements for Lupin to attend Hogwarts as a child, where he became close friends with James Potter, Sirius Black, and Peter Pettigrew. Until the climax of Prisoner of Azkaban, Lupin believes Sirius to be guilty of betraying James and Lily and killing Pettigrew. Upon discovering that Sirius is innocent, and that the very much alive Pettigrew is the traitor, Lupin helps Sirius explain the truth to Harry, Ron, and Hermione in the Shrieking Shack. However, Lupin had forgotten to take the Wolfsbane Potion that night and so he reverts disastrously to beast form, retaining no control over his actions. Sirius, in dog form, drives Lupin safely away from the others into the Forbidden Forest, where he wakes the next morning restored to his human body and mind. Snape, however, furious over Sirius's eventual escape and his resultant loss of the Order of Merlin promised to him by Cornelius Fudge, publicly reveals the nature of Lupin's lycanthropy. Anticipating a public outcry in response to the threat he himself agrees he poses for students, Lupin resigns from his post.

Long after resigning as a teacher, Lupin remains as a friend to Harry. In Harry Potter and the Order of the Phoenix, Lupin joins the newly reformed Order of the Phoenix and is part of the advance guard which escorts Harry from the Dursley family home in the book's opening chapters. Lupin is rarely seen in Grimmauld Place as he is often away performing secret tasks for the Order. Later in the book, Harry sees his father and Sirius bully a school aged Snape through Snape's memory. Remus does not participate in the bullying and gives several small indications that he disapproves of his friends' behaviour. When Harry confronts Remus about the scene he witnessed in the Pensieve, Lupin expresses regret at never having had the courage to tell his friends when they were "out of order". Later, Lupin participates in the battle at the Department of Mysteries where he duels and overpowers Lucius Malfoy. In Harry Potter and the Half-Blood Prince, Lupin reveals that it was Fenrir Greyback who bit him when he was a child. Towards the climax of the book, Lupin is part of the defenders of Hogwarts when Death Eaters penetrate the school, and after Dumbledore's death, it is revealed that Nymphadora Tonks has fallen in love with him, despite a thirteen-year age gap. He resists becoming involved with her because of the many risks posed by his lycanthropy, and insists that she deserves someone "young and whole".

However, by the opening of Harry Potter and the Deathly Hallows, it is revealed that Lupin and Tonks have married. Later in the book, Lupin informs Harry, Ron and Hermione that Tonks is pregnant. However, feeling ashamed that he has turned Tonks into an outcast and afraid that his unborn child would have a high chance of inheriting lycanthropy, Lupin plans to leave Tonks and their unborn child. This results in an intense argument between him and Harry, who insists that Lupin's actions are cowardly. Eventually, Lupin recognises the truth in Harry's words and returns to his wife in time for the birth of their son, Teddy Lupin. Lupin remains active in the Order throughout the year, and he provides the casualty reports section on the pirate radio station Potterwatch under the pseudonym of Romulus. Lupin commands a group of defenders on the school grounds during the Battle of Hogwarts. Both Lupin and Tonks die in combat, killed by Antonin Dolohov and Bellatrix Lestrange, respectively. At the time of his death, Lupin was the last surviving member of his group of friends. Rowling has since stated that she originally intended for both Lupin and Tonks to survive, but finally killed them off to compensate for the last-minute reprieve she gave to Arthur Weasley in Harry Potter and the Order of the Phoenix. Lupin, along with Sirius, James, and Lily, makes one last appearance in the series when Harry activates the Resurrection Stone to bring them forth to comfort him. Lupin expresses his regret that Teddy will never know his father, but says that he will know why he died: trying to make a world in which his son could live a better life.

In the film series, Lupin is portrayed by David Thewlis as an adult, and by James Utechin as a teenager.