It could be that Percy gets involved in a conflict where he sides against Artemis' hunters. His actions in this conflict might make Artemis view him as an enemy. For example, if he accidentally harms one of her hunters while trying to protect someone else, she may grow to hate him.
One possible scenario is when Artemis criticizes Percy's fighting style. Percy has his own way of battling monsters, which is effective but might not be as refined as the way the hunters of Artemis do it. If Artemis were to publicly criticize his style in front of others, it would surely wound Percy's feelings. He might feel like he is not good enough in her eyes.
One possible reason could be a misunderstanding. Maybe Percy accidentally disrupted one of Artemis's hunts or sacred rituals in the fanfic. Another reason might be that some prophecy or fate in the fanfic's made - up world has pitted them against each other. For example, if there's a prophecy that Percy's actions will somehow limit Artemis's power or freedom in the future.
One possible reason could be a misunderstanding. Maybe there was some miscommunication or false information spread that made Artemis think Percy had done something unforgivable. For example, if Percy was wrongly accused of harming one of Artemis' beloved creatures or desecrating her sacred places.
One possible reason could be differences in their divine natures. Percy is a son of Poseidon, a god associated with the sea and a more fluid, emotional element. Artemis is a virgin goddess of the hunt, very strict and unyielding in her ways. This contrast in their essences might lead to clashes and Percy's dislike.
Perhaps in the fanfiction, Scott falls in love with someone that Stiles also has feelings for. This love triangle situation could turn Stiles' feelings into hatred as he sees Scott as a rival for the person he cares about. Also, if Scott accidentally reveals one of Stiles' secrets that he promised to keep, Stiles would likely be very angry and could start to hate Scott.
One possible scenario is that Minato was resurrected in a wrong way in the fanfiction. His behavior after resurrection was not what Naruto expected. Maybe he was more cold - hearted or seemed to have different priorities that didn't include Naruto, so Naruto started to hate him.
Perhaps in the fanfiction, Sirius gets involved with the wrong crowd. And Harry, who has a strong sense of right and wrong, can't stand being associated with Sirius any longer. Or maybe Sirius has a falling out with one of Harry's other important people, like Hermione or Ron, and Harry takes their side and starts to hate Sirius as a result.
One possible scenario is if Naruto is constantly blamed for the team's failures. For example, if every mission that goes wrong is pinned on Naruto, even when it's not his fault, he might start to hate the team. This could be due to a misperception by the other members or a malicious plot within the fanfiction world.
One scenario could be that Edward accidentally endangers Bella's family in the fanfiction. Since Bella is very protective of her family, this could be a major turning point for her feelings towards him. For instance, if his vampire nature attracts some dangerous vampires to Forks and they target her family.
One possible scenario is that Jacob reveals a secret that Bella doesn't like. This could make her feel betrayed and start to hate him.