(Note: This definition is mainly about the Marauder's Era Remus, not the modern day Harry Potter Remus. Most of this information was provided by Jo Rowling herself through interviews and transcripts.)
Remus Lupin was born on March 10th, 1959. Lupin was bitten by the vicious werewolf Fenrir Greyback as a young child (some believe his age to have been around 6), thus becoming a werewolf himself. His parents tried everything but there was no cure. He and his parents were justifiably afraid he would not be able to attend school, but headmaster Albus Dumbledore allowed him to enroll at Hogwarts - as long as certain measures were taken. A house was built in Hogsmeade with a secret passage leading to it from under the Whomping Willow. Lupin was smuggled into this house for his monthly transformations to ensure his safety and the safety of others. The transformation from human to werewolf is difficult and painful, and when isolated from humans or the company of other animals, a werewolf will harm itself with its own teeth and claws out of frustration from not being able to attack. The villagers mistook Lupin's screaming as that of extremely violent ghosts. The house was dubbed "the Shrieking Shack" and became known as the most haunted building in Britain. Although the house was not really haunted at all, Dumbledore promoted this rumour to discourage curious villagers from exploring.
Remus kept his transformations a secret, but his friends James Potter
, Sirius Black
, and Peter Pettigrew
deduced the truth in their third year. By their fifth year they had learnt how to become Animagi themselves to keep him company during his transformations. (A werewolf is only a danger to humans.) He was given the nickname "Moony" by his friends because of his condition. In their sixth year, Sirius Black played a prank on Severus Snape
(though Sirius Black, James Potter and Peter Pettigrew hated Snape, Lupin claimed he had no reason to hate him until the 6th book). He told Snape where Remus went every month, knowing full well that Snape could be killed if he approached Lupin in his werewolf state. Snape, being determined to get them in trouble followed Sirius' directions. James stopped Snape part way down the tunnel, saving his life, but Snape saw Remus in werewolf form and was sworn to secrecy by Dumbledore. Snape was never able to forgive Sirius (also James and Remus) for their prank and believed that James' motive in foiling the plan was nothing more than self-interest, to avoid expulsion from Hogwarts. Even though Remus did not know anything of Sirius' prank, Snape loathed and blamed him as well.
Remus was a prefect, though he had trouble exercising discipline over his friends. In an interview, J. K. Rowling linked this to Remus' desire to be liked, "because he's been disliked so often." Remus' main fault is that "he's always so pleased to have friends, so he cuts them an awful lot of slack." Sirius said Remus was the "good boy" in Order of the Phoenix, and Rowling has said he was the "mature" one. Sirius said that Lupin did not participate in his and James' bullying (of Snape, in particular), but Lupin regrets never having told them to stop. He is also noted for being one of the co-creators of the Marauder's Map, which later fell into the ownership of Harry Potter.
He was in the original Order of the Phoenix.
Remus Lupin is a fictional character in the Harry Potter series.