Harry, Hermione and Ron saw Professor Remus J. Lupin for the first time on the Hogwarts Express, wearing extremely shabby wizard's robes. Lupin has light brown hair flecked with gray, though he is quite young. He looked ill and exhausted; Ron observed, "He looks like one good hex would finish him off." But Lupin is a capable wizard. He drove one of the dementors of Azkaban from the compartment and earned Madam Pomfrey's approval for having given the students chocolate to help them recover from the shock.
Professor Lupin's classes gave the students their first true experience with Defense Against the Dark Arts and Dark Creatures. His teaching style, building confidence with knowledge with kindness and humor, made his classes the most popular. To assist Harry with the dementors, Remus Lupin taught him the Patronus Charm, working with Harry one-on-one to help him develop the skills necessary to use this difficult spell. He spent time with Harry, becoming a mentor and a friend. Perceptive and even-tempered, Lupin deflected Snape's hatred and gave poor Neville Longbottom his first moment of glory when facing the Boggart.
The truth of Professor Lupin's illness continued to elude all except Hermione, who, after Lupin disrupted the confrontation between Harry and Sirius Black in the Shrieking Shack, exposed Lupin as a werewolf. Thoughtful and introspective, Lupin revealed some of his own difficult life and the history of the group of friends which included James Potter and Sirius Black, and of the Marauder's Map. Harry learned that Professor Lupin is "Moony", one of the creators of the Marauder's Map. To prove the truth of their story, Remus and Sirius forced Peter Pettigrew (Scabbers) to return to human form and the sad story of Pettigrew's betrayal of Lily and James Potter unfolded. But racing to the rescue, when the map showed Sirius Black dragging Ron and Peter into the Whomping Willow with Hermione and Harry in pursuit, Remus had forgotten the Wolfsbane Potion, so that when he later transformed he was subject to uncontrolled dementia. Pettigrew escaped into the woods and Harry used the Patronus Charm to save Sirius, Hermione and himself from the dementors. Concerned for the safety of the students, Remus Lupin resigned and left Hogwarts.