Though Lupin mentions this in the books, the Harry Potter encyclopedia Wizarding World confirms, in a piece about the horrifying werewolf Fenrir Greyback (the late Dave Legeno), that Greyback is responsible for biting a young Remus Lupin at just four years old and condemning him to a life with lycanthropy. Even worse, Greyback bit young Remus just to get back at Remus' father, Lyall, who had publicly insulted werewolves, and while Lyall never forgave himself, Greyback proudly bragged about his "accomplishment."
As time wore on, Lupin struggled to hold down jobs — and without help and kindness from Hogwarts headmaster Albus Dumbledore (Michael Gambon), he might not have even been able to attend Hogwarts at all. Knowing Lupin's struggle makes Greyback's lack of remorse even worse, as Wizarding World points out: "[...] according to Remus, Greyback 'regards it as his mission in life to bite and to contaminate as many people as possible.' The sheer number of Greyback's victims proved that the werewolf had not one scrap of regret for his wrongdoings, or the many lives he affected along the way. Seeing what Remus went through at Hogwarts illustrated just how far one bite could go in terms of both physical and emotional issues that lasted a lifetime."
Just to make matters more tragic, Lupin ultimately loses his life alongside his wife, Tonks, during the Battle of Hogwarts. Thankfully, his son Teddy, who doesn't share his father's affliction, is cared for by Harry himself, but it's still disheartening that one of Harry Potter's most lovable characters suffered so much throughout his life.
The Harry Potter films are available to rent or buy on most major platforms.
