It could be other Death Eaters. They might suspect that Fenrir is up to something with Hermione and try to stop him. For example, Bellatrix Lestrange could be a major antagonist. She's known for her cruelty and loyalty to Voldemort, and she might not trust Fenrir's actions regarding Hermione.
Well, in this kind of fanfiction, Fenrir might be protective of Harry perhaps because he sees something special in Harry. Maybe he has his own reasons that are different from the normal perception of their relationship in the original works. It could be that Fenrir has a change of heart or there are some hidden plot elements that make him want to safeguard Harry.
In Norse mythology, Fenrir is a gigantic and terrifying wolf. He was born from the union of Loki and the giantess Angerboda. The gods were afraid of Fenrir's growing strength and decided to bind him. They made chains to hold him but Fenrir broke them easily. Eventually, they tricked him into being bound by Gleipnir, a special chain made with the help of dwarves. Fenrir is prophesied to play a major role in Ragnarok, the end of the world in Norse mythology.
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.
One of the great Danmachi fanfics is 'A Different Perspective'. It offers a fresh take on the characters and their relationships within the Danmachi universe. The story delves deeper into the backstories of some minor characters and makes them more significant. Another good one is 'The Unseen Adventures'. It takes the characters on new quests that are not in the original story, adding more excitement and mystery.