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 for his 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.

Professor Lupin first appeared in Harry Potter and the Prisoner of Azkaban, in which he taught Defence Against the Dark Arts at Hogwarts School of Witchcraft and Wizardry. At the end of the year, Severus Snape made public the fact that he was a werewolf, whereupon Lupin resigned in anticipation of the public outcry against a werewolf teaching at Hogwarts. During his tenure, he gave Harry private lessons in casting the Patronus Charm, the only known means of defence against Dementors. His students, excepting a few from Slytherin, held him in extremely high regard and loved the hands-on style of his classes. He was considered by Harry and his friends to have been their best Defence teacher, although his competition was hardly stiff in this regard: His predecessors were Quirinus Quirrell, who was possessed by Voldemort, and Gilderoy Lockhart, a fraudster; he was succeeded by Barty Crouch Jr., who pretended to be Alastor Moody, and a domineering and vicious Ministry of Magic official, Dolores Umbridge. His most recent successor was the intimidating and suspect former potions master, Severus Snape. Until the climax of Prisoner of Azkaban, Lupin believed that Sirius Black was guilty of betraying Lily and James Potter and killing Peter Pettigrew. He eventually finds out the truth, that Sirius is innocent and Peter is alive and is the traitor. He helps Sirius explain the truth to Harry, Ron and Hermione in the shrieking shack, and they all confront Peter. However, Remus is partly responsible for Peter's escape because he forgot to take his wolfsbane potion and transforms into a dangerous werewolf as they are leaving the shack.

Lupin reappeared as a member of the Order of the Phoenix in Harry Potter and the Order of the Phoenix and Half-Blood Prince, but his role in those books was smaller than in Prisoner of Azkaban.

In Prisoner of Azkaban, he is described as having "light brown hair" that is greying. In Order of the Phoenix, he has a pale face with premature lines. He has shabby, patched clothing. Few in the magical world want to employ a werewolf and almost all are prejudiced against them. With the addition of new anti-werewolf laws passed by the Ministry of Magic, finding employment became practically impossible and almost illegal. With Wolfsbane Potion provided regularly by Snape during his time at Hogwarts, he was able to retain his human mind during transformations. Since he is a werewolf and despises his transformations, boggarts take the form of a full moon when confronting him.

He is part of the new Order in the fifth book and the advanced guard who takes Harry away from the Dursleys in the beginning of the book. It is mentioned that Lupin lives in Grimmauld Place, headquarters of the Order of the Phoenix with Sirius Black, but doesn't stay there often as he is usually out completing secret tasks for the Order. Later he helps in the battle at the Department of Mysteries.

In Half-Blood Prince, he is hiding undercover as a spy amongst his fellow werewolves who are under the leadership of the very werewolf who originally bit him. Fenrir Greyback is now in the employ of Voldemort. Remus admits to Harry that he finds the werewolves' motives for siding with Voldemort hard to argue with, as he offers them more freedom than they are currently allowed.

At the end of the book it is revealed that Nymphadora Tonks is in love with Remus. He resisted becoming involved with her because of the risks from his being a werewolf, and he said he is too old, too poor, and too dangerous for her. However, the two are seen holding hands in one of the book's last scenes.

It is revealed in Deathly Hallows that he and Nymphadora got married (he calls her Dora). Remus is quite miserable for an extended period of time, though no one quites knows why. When he offers to help Hermione, Ron and Harry on their quest for Dumbledore, he reveals that Tonks is pregnant, and he's feeling immense regret for marrying her and burdening her with his lycanthropy. He's terrified their unborn child will be a werewolf as well, and thinks Tonks is best left with her parents while he leaves. Harry enrages him by scolding him for wanting to abandon his wife and child and calls him a coward.

Later, we learn Lupin has moved back in with Tonks and he is ecstatically happy about the birth of his son, Ted Remus Lupin. His fears and misery seem to have disappeared with the birth of his child, and he asks Harry to be Teddy's godfather.

During the Battle of Hogwarts, Remus fights alongside the Order and several students, leading a bunch of fighters into the grounds. and is later seen by Aberforth dueling Antonin Dolohov His body, along with the body of his wife, Tonks, is seen by Harry in the Great Hall. JK Rowling has revealed that it was Dolohov who killed Remus. Remus then appears to Harry, along with Lily, James and Sirius, to walk with Harry to face Voldemort.

Interesting Facts
Spouse: Nymphadora "Dora" Tonks
Children: Teddy Lupin
Occupation: Varies. (It's difficult for a man of Remus's condition to find work.) Was the Defense Against the Dark Arts Professor in 1993.
Hair: Light brown, greying prematurely
Eyes:
Blood Type: Half-blood/Werewolf
Hogwarts House: Gryffindor (Prefect)
Allegiance: Order of the Phoenix
Music:
Patronus:
Death: By Antonin Dolohov - May of 1998 Film Portrayer: David Thewlis & James Utechin (Teenage Remus)

Source - Wikipedia