Remus John Lupin is a supporting character in the Harry Potter book and film series, making his first appearance in the third book Harry Potter and the Prisoner of Azkaban.
He was born in 1960 as the half-blood wizard son of wizard Lyall Lupin and his Muggle wife Hope. When he was around 5 years old in 1965, Remus was attacked by a werewolf named Fenrir Greyback in retaliation against Lyall after he insulted Greyback by saying that werewolves deserved nothing but death. Remus survived the attack (after Lyall chased Greyback away by firing spells at him) but ended up a werewolf himself. Lyall and Hope tried to find ways to treat Remus of his lycanthropy, but unfortunately there was none that were effective. For the first few years, Lyall was able to keep Remus under control whenever he was in his werewolf form by keeping him in a locked room and placed several silencing charms. However by the time Remus was ten, he could tear down doors. This resulted in Lyall having to use more powerful charms.
Remus and his parents would constantly move house in order to avoid their neighbours becoming suspicious of Remus. Because of Remus' condition, Lyall and Hope believed that he would not be accepted into Hogwarts School of Witchcraft and Wizardry. However, the school's headmaster Albus Dumbledore visited them and assured them that Remus could still attend Hogwarts as he would make precautions.
Remus began attending Hogwarts in 1971 and was sorted into Gryffindor House where he befriended three fellow Gryffindors: James Potter, Sirius Black and Peter Pettigrew. Together, the quartet named themselves The Marauders.
Whenever the time came for Remus to transform, he would claim to his friends that he was going to visit his mother, who he claimed to be unwell. Afterwards, Remus be escorted to a small building guarded by a Whomping Willow, where he could transform safely without causing harm to anyone. The noises of Remus transforming led rumours being made that the building was haunted. Dumbledore encouraged these rumours in order to protect Remus from being discovered as a werewolf. This led to the building being known as the Shrieking Shack.
During their second year at Hogwarts, James, Sirius and Peter figured out that Remus was a werewolf, but continued to be friends with him. They later secretly learnt to become Animagi (which are wizards and witches that can transform into animals) in order to accompany Remus during his transform (as werewolves were only a threat to humans not animals). After learning how become Animagi in their fifth year at Hogwarts in 1975; James took the form of a stag, Sirius took the form of a large black dog whilst Peter took the form of a rat. Being in the company of his friends in their Animagus forms tamed a transformed Remus. The Marauders gave themselves nicknames in reference to their alternative forms; Remus' was "Moony", James' was "Prongs", Sirius' was "Padfoot" and Peter's was "Wormtail". Together on nights Remus would transform, the friends would leave the Shrieking Shack and travel around the grounds of Hogwarts, where they would find secret areas. This inspired the quartet to create a map of the school including its secret areas, which they titled The Marauders Map. That same year, Remus became a Prefect, but had difficulty disciplining his friends, who would bully a Slytherin student in their year called Severus Snape.
After graduating from Hogwarts in 1978, Remus along with his friends joined The Order Of The Phoenix and helped them fight a Dark Wizard called Lord Voldemort and his followers The Death Eaters. Due to being a werewolf, Remus was unable to find empty but received financial support from James. It later became clear that there was someone spying for Voldemort in the Order. Remus and Sirius began to suspended each other of being the spy. In 1980, James and his wife Lily went into hiding in order to protect their infant son Harry (who was Sirius' godson) as Voldemort wished to kill him as he had heard a prophecy that a wizard born in late July would defeat him. James and Lily had intended to have Sirius serve as their Secret Keeper in order to protect their hideaway, but Sirius believed that choosing him would be too obvious and instead suggested Peter to be Secret Keeper instead. Remus was unaware of this switch, due to Sirius believing him to be Voldemort's spy. In autumn 1981, whilst on a business trip for the Order, Remus discovered that James and Lily had been killed by Voldemort after supposedly being betrayed by Sirius, who also apparently killed Peter, who had attempted to confront him. Sirius was sent to the wizard prison Azkaban for killing Peter and twelve Muggles (a wizard term for non-magical people) during the killing. Harry survived the attack and was taken to live with his maternal Muggle aunt Petunia Dursley and her husband Vernon and son Dudley.
In 1993, Remus was given the job of Defence Against The Dark Arts professor at Hogwarts and was told by Dumbledore that he would be supplied with Wolfsbane Potion (a potion that would help werewolves keep their human consciousness after transforming). On 1 September that year, Remus travelled to Hogwarts on the Hogwarts Express. During the journey, Remus rested inside the same carriage as a now 13-year-old Harry and his friends Ron Weasley and Hermoine Granger. Remus defended Harry from a Dementor (a creature that guards Azkaban) was searching the train for Sirius, who had fled Azkaban.