Professor Lupin was a werewolf. A fact that Hermione had known for some time. He missed classes once a month, and frequently had new scars appear on his face and hands. Harry had mentioned first term that Professor Snape brewed a potion for the professor, and after Snape set the essay, she deduced that it must have been Wolfsbane. His boggart took the shape of the moon for Merlin’s sake.