Hermione's character could develop in terms of her understanding of the grey areas between good and evil. Saving Fenrir goes against the common perception of him as a monster. This could make her more open - minded and less judgmental. She might start to see that even the most feared creatures can have a chance at redemption.
There's a fanfiction called 'Greyback's Redemption Arc'. In it, Fenrir Greyback saves Hermione from a group of Death Eaters. The author might have written it to explore the idea of redemption for a character like Greyback. Maybe he saves Hermione because he sees her as a symbol of the good in the wizarding world that he has lost touch with, and this act of saving her could be the start of his journey towards redemption.
Their relationship could start with Hermione being a captive of Fenrir. But instead of harming her immediately, he becomes intrigued by her intelligence. As time passes, Hermione might start to see some humanity in Fenrir that others don't, and they could form an unlikely alliance against a common enemy.
One possible plot could be that Hermione stumbles upon Fenrir Greyback in a situation where he's about to be killed by some dark wizards who see him as a loose end. Hermione, being the empathetic and brave witch she is, decides to save him. Maybe she believes that every life is worth saving, even a werewolf like Fenrir. She could then try to rehabilitate him, using her vast knowledge of magic and her strong moral compass.
Hermione would likely react with her characteristic intelligence and courage. She would probably try to use her vast knowledge of magic to find a way to escape or at least resist Fenrir. For example, she might try to use spells that could incapacitate him temporarily. She would also be likely to try and outwit him in conversation, looking for any weaknesses or loopholes in his plans.
Their relationship could start with Hermione being cautious. She might initially see Fenrir as a threat despite his 'good' nature. But as she gets to know him better, she realizes his potential for good. They could start working together on small projects, like helping other werewolves in the community, which gradually builds trust between them.
At the start, Hermione would be appalled by Fenrir's cruelty. She would be enraged that he has caused so much harm to innocent people. But as she delves deeper into getting to know him, she would start to see the complex web of factors that contributed to his behavior. She might see the neglect and discrimination he faced as a werewolf, which could make her feel a bit of pity. However, she would still firmly oppose his cruel actions and try to find a way to either contain or reform him, all while being acutely aware of the danger he poses.
Fenrir could start as a threat to Hermione, but then as he observes her, he becomes intrigued by her knowledge. He might begin to learn from her, like how she uses magic in clever ways. This would change his view of non - werewolves and develop his character from a one - dimensional villain to a more complex figure.
I'm not sure if there is such a specific fanfiction. It seems like a rather dark and unusual pairing considering Fenrir Greyback is a werewolf known for his brutality. However, the world of fanfiction is vast, and there might be some out there that explore this concept, but it's not a common or mainstream idea in the Harry Potter fanfiction community.
Hermione could develop a more complex understanding of magic. Since saving Harry's parents would likely involve delving into some very rare and powerful magic, she would expand her knowledge greatly. This new knowledge could make her more respected among the wizarding community, and she might use her new status to advocate for better magical education and research, all while her character grows in wisdom and leadership.
In one fanfiction concept, Greyback saves Hermione when she is trapped in a magical maze created by dark wizards. Greyback, being a cunning and experienced dark creature, is able to navigate the maze and get to Hermione before the maze collapses or she is killed by the various traps inside. He might save her because he senses that she has a great potential that could be useful to him in the future.