Perhaps the fanfiction focuses on Hermione's research into some dark magic spells and she finds out that Rowle has some knowledge about them. So, she has to get close to him to learn more. In the process, they might develop an unlikely friendship. Rowle could start to see the value in the light side of magic through Hermione's influence.
One possible plot could be that Hermione, in her quest for knowledge about the dark arts practitioners, comes across Thorfinn Rowle. She might initially be studying him as part of her research on Death Eaters. And as she tries to understand his motives and past, she gradually gets to know him on a more personal level.
At first, Hermione could be very guarded around Rowle. But as she delves deeper into understanding him, she realizes he has some unique perspectives on magic. They could start to exchange ideas, which gradually turns into a more friendly relationship. He might even start to protect Hermione from other threats in the wizarding world, seeing her as a beacon of hope for his own redemption.
You can start by looking on major fanfiction platforms like Fanfiction.net or Archive of Our Own (AO3). On Fanfiction.net, use the search bar and type in 'Hermione gets to know Thorfinn Rowle'. AO3 also has a great search function. Just be sure to check the tags and summaries to find the ones that match what you're interested in.
She could save him by appealing to his better nature. Hermione is known for her intelligence and ability to reason. So, she might talk to Thorfinn Rowle, convince him to turn away from whatever dark path he was on that put him in danger in the first place. By showing him an alternative way, she could in a sense'save' him from his own self - destructive actions.
Maybe he's attracted to the unique interpretations of Hermione's character in the fanfiction. Different fanfictions might show different sides of her, like her intelligence in new and exciting ways.
One common theme is the idea of redemption. Often, Rowle is depicted as a character in need of redemption, and Hermione plays a role in that process. For example, she might help him see the error of his ways.
He could be into the ones that focus on Hermione's academic achievements. Maybe stories where she discovers new spells or makes great contributions to the wizarding world's knowledge.
One great fanfiction could be 'A Meeting of Minds' where Hermione and Remus meet in the Hogwarts library. Hermione is researching for a complex spell and Remus, being the knowledgeable wizard he is, offers his help. This leads to a series of conversations where they start to understand each other's perspectives on magic, life at Hogwarts, and their respective backgrounds. It's a slow - burn story that beautifully develops their relationship.
Obviously, Harry Potter could be the one to save her. He has a strong bond with Hermione and is very brave. He'd stop at nothing to rescue his friend. He might use some of his unique magical abilities, like his connection to Voldemort, to find her.