At Hogwarts, Lupin was sorted into Gryffindor, and soon made best friends with James Potter, Sirius Black and Peter Pettigrew. He kept his condition a secret from them, often providing them with excuses for his absence, such as his mother being ill. During their second year, however, his friends worked out the truth, and accepted who he was. Due to the strong bond of their friendship, and due to werewolves only being a danger to humans, Potter, Black and Pettigrew all decided to learn to become Animagi, to keep Lupin company during his transformations. By their fifth year, they had managed to do so, and used Potter’s invisibility cloak to sneak down the passage to the Shrieking Shack.

Lupin was a good student, but often got detention with Potter, Pettigrew and Black, for mischief-making. Somewhere between the years 1975 and 1978, they created the Marauder’s Map, which detailed every inch of Hogwarts, including secret passageways. There is some speculation as to whether it is fooled by animagi, Polyjuice Potion, or invisibility cloaks, but generally it can accurately identify every person within the grounds. However, it was confiscated by Argus Filch, only to be “nicked” by Fred and George Weasley during their tenure at Hogwarts. They then gave it to Harry Potter, James Potters' son, during his third year. To activate it you need to say the words “I solemnly swear I am up to no good” and to de-activate it, which makes it look like normal parchment, you need to say “Mischief Managed.”

Lupin’s disappearances cause curiosity in Severus Snape, a Slytherin in the same year as Lupin, who hated James and Sirius and therefore disliked Pettigrew and Lupin by association. In their sixth year, Black told Snape about the passageway beneath the Whomping Willow, hoping he would either be hurt by what he found at the other end, or too terrified to try to look. Potter managed to stop Snape, saving his life from Lupin as a werewolf, however Snape still uncovered Lupin’s secret and teased him about it, despite being sworn to secrecy by Dumbledore.

Lupin was a prefect at Hogwarts, and did not partake in the bullying of Snape, but he never stepped in to stop it, as he was so grateful for his best friends, something he came to regret in his later life.

First Wizarding War

During the first Wizarding War, Lupin joined the Order of the Phoenix along with Potter, Black, Pettigrew and Lily Evans, Potter’s wife and mother of Harry Potter. Eventually, Black began to distrust Lupin, suspecting him to be a spy for Voldemort. When a prophecy was made about a child who would defeat the Dark Lord, Dumbledore said it could be Harry, Potter and Evan’s son. To protect Harry, the Potters were locked in a secret place. Lupin was aware a Fidelius Charm was used to keep the Potter’s safe, but he was not involved in the plan. Because of this, Lupin was unaware of the Secret-Keeper switch from Black to Pettigrew.

Between Wizarding Wars

In 1993, Albus Dumbledore asked Lupin to fill the vacant Defense Against the Dark Arts post at Hogwarts. Dumbledore had Snape, now the Potions professor, provide Lupin with the recently discovered Wolfsbane Potion, which allowed him to retain his human mind during transformations.

Lupin was an adept teacher; the students learned how to identify hinkypunks and Red Caps, as well as to battle Boggarts. In one class, Lupin’s Boggart took on the form of the full moon, which the class, except for Hermione Granger, mistook for being a crystal ball. After a couple of events involving dementors, Lupin agreed to give Harry Potter private lessons in producing a Patronus, a spell which repels Dementors. 

The summer before Lupin started as the DADA professor, Black escaped Azkaban (more info in Lesson Three) and Lupin, like the rest of the Wizarding World, believed Black betrayed the Potters to Voldemort and murdered Pettigrew. However, by the end of the school year, Lupin knew the truth about the Potters' death, and that Pettigrew was in fact alive and hiding in his animagus form as Scabbers, the pet rat of Ron Weasley. Lupin was reunited with Black, and they decided to kill Pettigrew for the death of Lily and James Potter, only to be stopped by Harry Potter. That night was a full moon, and Lupin transformed into his werewolf form. Black turned into his animagus form of a dog to protect Harry Potter, Ron Weasley and Hermione Granger. At the end of the school year, Lupin resigned from his position after Snape let loose about his condition. Lupin was concerned parents of the other students, upon learning he was a werewolf, would protest, despite Harry Potter trying to convince him otherwise.

Second Wizarding War

When Voldemort returned, Lupin joined the renewed Order of the Phoenix, and participated in the Battle of the Department of Mysteries in 1996. Here he saw Black being pushed through a veil in the Death Chamber from a curse cast by Bellatrix Lestrange. Lupin held Harry Potter back from diving in after Black. Later that year, Lupin worked as a spy for the Order, living amongst other werewolves trying to convince them to join Dumbledore’s side.

Also in 1996, Lupin fought in the Battle of the Astronomy Tower, where he narrowly avoided a death curse from Death Eater Thorfinn Rowle. Upon hearing of Snape murdering Dumbledore, Lupin screamed in despair, which was the first time many people had seen him lose control. Afterwards, he finally accepted the love of Nymphadora Tonks, and soon had a quiet wedding.

In July 1997, Lupin took part in the Battle over Little Whinging, in which the Order tried to remove Harry Potter from his home in Privet Drive before Voldemort could get to him. He was a protector to George Weasley, who took Polyjuice Potion as an attempt to fool Voldemort and his Death Eaters, by having “seven Potters.” Whilst Lupin was flying, George Weasley was hit by Snape’s Sectumsempra spell. Once Lupin and George returned to the Burrow, it was discovered that George had lost an ear. That summer Lupin and Tonks also attended Bill Weasley and Fleur Delacour's wedding together, and both defended the Burrow when it came under attack from Death Eaters.

Soon, Lupin discovered Tonks was pregnant with his child, and he worried that his child would be born a werewolf like him, as it was not know that you cannot pass the werewolf gene on to a child. Only being bitten by someone in werewolf form can you become a werewolf. Being scratched by a werewolf in human form will result in wolf-like tendencies, such as a preference for rare meat, but no full transformation.

Whilst Harry Potter, Ron Weasley and Hermione Granger were in search of Voldemort’s Horcruxes, Lupin visited them a lot at Grimmauld Place, and offered to help them. Harry called him a coward for considering leaving Tonks alone whilst pregnant. Later, on Potterwatch, under a pseudonym of Romulus, when asked what he would say to Harry Potter if he was listening, he replied with: "I'd tell him we're all with him in spirit. And I'd tell him to follow his instincts, which are good and nearly always right." He then returned to Tonks to see his son being born, whom they named Teddy, after Tonk’s deceased Muggle-Born father. Lupin asked Harry Potter to be godfather.

Remus was killed by Antonin Dolohov and his wife Tonks was killed by her aunt, Bellatrix Lestrange during the battle at Hogwarts. Their son, Teddy, was raised by his maternal grandmother, Andromeda, with help from his godfather.

I Solemnly Swear That I Am Up To No Good

Sites:

http://harrypotter.wikia.com/wiki/Remus_Lupin

https://www.pottermore.com/writing-by-jk-rowling/remus-lupin

https://www.pottermore.com/writing-by-jk-rowling/werewolves