Remus Lupin and the White Wolf
17 parts Complete 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
Tbh I like AO3's tagging system better than Wattpad's now that I've read some fics on there. Might cross-post this there one day.