One reason could be competition. If Chloe is presented as a rival in the fanfiction, Ash might hate her for trying to outdo him in Pokemon battles or other aspects. For instance, if she constantly shows off her skills or Pokemon, it can make Ash feel annoyed.
There could be a backstory in the fanfiction where Chloe has wronged Ash in the past. Maybe she stole an opportunity from him, like a chance to participate in a big Pokemon event. This would give Ash a valid reason to hate her.
One reason could be a conflict of interests. If they are competing for the same thing, like a magical artifact or a position in a magical society, and Susan uses underhanded tactics, Harry would hate her.
One reason could be that he was treated unfairly by other shinobi. If he was constantly looked down upon or discriminated against by his fellow shinobi, it might make him develop a hatred for them.
One possible reason is that Naruto is so focused on his ninja training and his goals in the shinobi world that he views Christmas as a distraction. He might think it takes people's attention away from more important things like self - improvement and protecting the village.
It might be due to the way their goals are presented in fanfiction. Naruto is often about protecting his village and friends. If Artemis's goals in the fanfic are in conflict with Naruto's, like if she's more focused on her own self - interest in the story, Naruto would hate her.
It's possible that the writer of the fanfiction has given Akeno a backstory or personality trait that directly clashes with Naruto's. Maybe she is extremely arrogant and Naruto, who is humble and believes in everyone's potential, would find that detestable.
One reason could be that he doesn't like how some fanfiction writers misinterpret his character. They might make him act in ways that are completely out of character in relation to Renesmee. He has a certain image of himself and his relationship with her, and fanfiction can mess that up.
In some fanfictions, Peter could have a secret that he's keeping from the Avengers. When it's eventually revealed, they might feel betrayed and turn against him. It could be something related to his powers, or his past associations that the Avengers find unacceptable, thus resulting in their hatred.
The cultural differences between Naruto's world and Rias's world could play a part. Naruto's world has a strong sense of community and loyalty within the ninja clans. Rias's world of devils has a more hierarchical and self - serving structure. These differences could cause Naruto to have a negative view of her in fanfiction.
One reason could be the pressure. Harry is constantly under the spotlight during the tournament, and he may not like that kind of public scrutiny. In fanfiction, this pressure could be amplified to an unbearable level for him.
One reason could be a misunderstanding. The fanfic might have set up a situation where Sakura misinterprets Itachi's actions. For instance, she could think he is plotting against her when in fact he has good intentions.