It could be that the OC creator simply doesn't like the typical tropes or storylines often found in Rias fanfiction. For example, if the fanfictions tend to be very formulaic or over - use certain clichés, the OC might be a symbol of a break from that. Also, perhaps the OC has a specific backstory or purpose that is at odds with the general narrative of Rias fanfiction.
Maybe the OC has a completely different personality or set of values from what is often portrayed in Rias fanfic. It could be that the OC has had bad experiences related to the typical tropes or themes in those fanfics.
Well, it could be due to their different personalities. Naruto is all about hard work and loyalty to his friends in his own universe. Rias Gremory has a more complex and sometimes manipulative personality in her story. In fanfiction, the writer might want to create a conflict by making Naruto dislike her, perhaps because he sees her methods as being at odds with his own values.
One option is to simply avoid reading or engaging with Rias fanfiction. If the OC hates it, there's no need to subject it to that content.
If the oc hates it but still wants to give it a chance, they could try to find a different take on the rias fanfic. Maybe look for ones that are more in line with their own values or interests. For example, if the oc is interested in action, find a rias fanfic that has more action elements rather than just the typical romance. Another thing to do is to try and write their own version of a rias - related story that they would like, incorporating the elements they enjoy and leaving out the ones they don't.
Maybe the OC has different values from what is presented in the 'Rias DxD' fanfic. For example, if the fanfic has a lot of over - the - top or unrealistic elements that go against the OC's idea of a good story, it could lead to dislike.
The OC can simply avoid reading any more of that fanfic. There are plenty of other stories out there in different fandoms or even within the 'High School DxD' fandom that might be more to their taste.
Maybe Naruto has his own set of values and the concepts in Rias fanfiction go against them. He's from a different fictional universe with its own rules and ethics, and the ideas in Rias fanfiction might seem out of place or even wrong to him.
Maybe it's because of the different power systems. Naruto has chakra and his ninja abilities, and Rias has her devil powers. If in the fanfiction these power systems are not properly integrated or if one overshadows the other in an unappealing way, Naruto could be depicted as hating Rias. Say, if Rias' devil powers are made to seem too overpowering and it goes against Naruto's sense of fairness in combat, it could lead to such a scenario.
Maybe he hates it because it often distorts the real ninja world he knows. Fanfictions might create false or out - of - character scenarios that he can't stand.
It could be that the fanfiction writer took a different approach to the story. In some fanfictions, the Cullens could be involved in situations that go against what Bella stands for. Perhaps they are written to be more secretive and untrustworthy, leading to Bella's hatred.