It's hard to say if it's extremely popular. There are so many different themes in the fanfiction world. Some people might be really into it, but it might not be as mainstream as some other Harry Potter - related fanfiction themes.
Well, a lot of fanfiction is out there but for this particular one, you might need to use detailed search terms on the fanfiction platforms. Look for tags related to Rowle, Hermione, and 'infatuated' to narrow down your search. Some platforms also have user - created collections that could potentially have what you're looking for.
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.
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.
One great Hermione and Rowle fanfiction is 'The Unlikely Alliance'. It explores an alternative universe where Hermione and Rowle are forced to work together on a dangerous mission. Their initial distrust gradually turns into mutual respect as they face numerous challenges. Another interesting one is 'Rowle's Redemption'. In this story, Rowle has a change of heart due to Hermione's influence and they end up on a path of changing the wizarding world for the better.
There are probably some Thorfinn Rowle and Hermione romance fanfictions, but they may be hard to find. Since Thorfinn Rowle is a relatively minor character in the original series, not many authors might be inspired to write about this pairing. But if you search specifically for it on major fanfic platforms, you might get lucky.
Rowle is another main character. He's usually a dark wizard, so in this fanfiction, he could be a complex character with his own motives and backstory that Hermione has to deal with.