At first, it would become very strained. Harry might avoid Lupin at all costs. He would no longer trust Lupin and would be suspicious of his every move.
As the story progresses, it could either get worse or start to heal. If the truth behind the reason for Harry's hatred is gradually revealed, there could be a chance for reconciliation. However, if more misunderstandings pile up, the relationship could become irreparable. Lupin, on his part, would probably be very confused and hurt by Harry's sudden hatred, which could also affect his own actions towards Harry.
One possible reason could be a misunderstanding. Maybe Lupin did something that Harry misinterpreted as betrayal or negligence. For example, if Lupin was forced to keep a secret from Harry for his own safety, but Harry found out in a way that made him feel deceived.
In many such fanfictions, Harry might start off with a basic admiration for Lupin as his teacher. But as the story progresses, he could come to see Lupin as a true father figure. Maybe Lupin shows him kindness and understanding that Harry lacks from other adults in his life. This could make Harry's view of Lupin deepen from simple respect to a more profound emotional connection.
One possible event could be that Lupin lied to Harry about something important. In the world of Harry Potter, trust is a huge factor. If Lupin deceived Harry, it could easily turn Harry against him. Say, Lupin lied about his whereabouts during a crucial time when something bad happened, and Harry suspected Lupin's involvement.
Maybe in that fanfic, Lupin did something that Harry considered a betrayal. For example, he might have made a decision that put Harry or his friends in danger without consulting them first.
Lupin might use his own experience as a werewolf to relate to Harry. He could teach Harry how to control his thirst for blood, much like he controls his lycanthropy. For example, by using potions or certain meditation techniques.
Dumbledore would be very concerned about Harry's future. He would try to guide Harry in controlling his new abilities. Their relationship might shift to a more teacher - student dynamic focused on this specific issue. Dumbledore could also use Harry's situation to teach the wizarding community a lesson about acceptance. He might encourage Harry to be open about his condition and show that being bitten doesn't make him any less of a hero. This would be a big change from their previous relationship which was more about the overall fight against Voldemort.
One great 'Harry Potter - Arsene Lupin' fanfic is 'Lupin's Legacy'. It delves deep into Lupin's past and his relationship with the other characters. It shows how his werewolf nature affected his life at Hogwarts and beyond in a really interesting way.
At first, it might be a bit rocky as Teddy is a young boy dealing with the loss of his parents and Harry is still learning to be a father figure. But as time passes, they start to bond over simple things like Quidditch. Harry teaches Teddy how to fly a broomstick, and this shared activity brings them closer.
The fanfic could explore it by showing their different perspectives. We get to see Albus's point of view of why he hates Harry and also how Harry reacts to this hatred. This gives a more in - depth understanding of their relationship.
Character development makes it interesting. In a good fanfic, Lupin's complex character is explored further. His struggle with his werewolf identity, his loyalty to his friends, and his role as a teacher all can be delved into more deeply than in the original books.