Remus Lupin and the White Wolf
Remus Lupin has always wanted friends, but with a dark secret to keep and an unexpected social anxiety, making friends may not come easy. Sirius Black, meanwhile begins noticing a few odd things going on, like students disappearing on occasion. Remus tries to help investigate, wondering whether he and a disappearing student may have something in common.
A canon-compliant Harry Potter prequel from Remus and Sirius' perspectives, mostly focussing on one of my headcanons.
Minor spoilers for the following:
Harry Potter and the Philosopher's Stone chapters 3 (sort of, only the fact that magic exists but if you didn't know that, why are you even reading a Harry Potter fanfiction), 6, 7, 8, 10, and 15 (VERY MINOR, all of it is merely the structure of a Hogwarts school year, the layout of the school, and the main wizarding sport)
Harry Potter and the Chamber of Secrets chapter 5
Foreshadowing throughout Harry Potter and the Prisoner of Azkaban, along with a few reveals from chapters 10, 12, 18, and 19
Harry Potter and the Order of the Phoenix chapters 6 and 28
Harry Potter and the Half-Blood Prince chapters 4, 7, and 8
and Harry Potter and the Deathly Hallows chapters 21, 33, and 35
If you haven't read the books, most of these won't even register as spoilers until you do
moving on from the Hary Potter series finally so this won't get a sequel