" Harry and Ron are too convinced of Snape’s evil to figure out whose Patronus helped them in the forest. The reader, however, knows that Snape can cast a Patronus, which we have never seen, and that Rowling has repeatedly underscored the importance of the mother’s story while revealing almost nothing about Lily’s friendships. Snape must have been careful to avoid letting Hermione see the silver doe. She was the last to lose faith in him, and he has never been able to evade her understanding for long. "

― Lorrie Kim , Snape: A Definitive Reading