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Remus Lupin is a member of the Order of the Phoenix and Sirius's best (surviving) friend. We don't see much of him during Book 5, though he often stays at Number Twelve, Grimmauld Place when Harry visits Sirius. Remus has a rough time of things because he is a werewolf, which makes him a social outcast in the wizarding world. What's more, Professor Umbridge has recently drafted anti-werewolf legislation that makes it even harder for Remus to hold a job. But he continues to be a steadfast and loyal member of the Order, despite his struggles in the larger wizarding world.
Remus was always the most responsible of the Marauders, the circle of Hogwarts friends that included Sirius, James Potter, Remus, and Peter Pettigrew. Remus was a prefect in his Hogwarts days, responsible for keeping track of his fellow students' behavior. In Professor Snape's memory of James and Sirius's bullying, Remus is present but does not participate. Still, Remus doesn't try to stop James and Sirius from picking on Snape. He feels guilty about not stopping his friends' more mean-spirited pranks to this day. Remus has always been calmer and mellower than Sirius, and he provides some balance for Sirius's quick temper for much of Book 5.
Remus's struggles with his outsider status in wizarding society have prematurely aged him, leaving him gray-haired and worn. But he has also experienced more adult life than Sirius has, which has made him a more balanced authority figure. He is the one who holds Harry back from starting after Sirius when Sirius falls through the archway in the Department of Mysteries. By the end of the novel, Remus and Peter Pettigrew – a.k.a. Wormtail, a.k.a. the traitor who sold out Harry's parents – are the only surviving Marauders.