Sorted into Gryffindor house, Remus Lupin was swiftly befriended by two cheerful,confident and rebellious boys, James Potter and Sirius Black. They were attracted by Remus’s quiet sense of humour and a kindness that they valued, even if they did not always possess it themselves. Remus, always the underdog’s friend, was kind to short and rather slow Peter Pettigrew, a fellow Gryffindor, whom James and Sirius might not have thought worthy of their attention without Remus’s persuasion. Soon, these four became inseparable.

Remus functioned as the conscience of this group, but it was an occasionally faulty conscience. He did not approve of their relentless bullying of Sevens Snape, but he loved James and Sirius so much, and was so grateful for their acceptance, that he did not always stand up to them as much as he knew he should.

Inevitably, his three best friends soon became curious as to why Remus had to vanish once a month. Convinced by his lonely childhood that his friends would desert him if they knew that he was a werewolf, Remus made up ever more elaborate lies to account for his absences. James and Sirius guessed the truth in their second year. To Remus’s astonished gratitude, they not only remained his friends but thought up an ingenious method of easing his monthly isolation. They also gave him a nickname that would follow him all through school: ‘Moony’. Remus finished his school career as a Prefect. (source)