It could be that the OC simply doesn't like the character Rias or the general plot and themes in the 'High School DxD' series which the fanfic is based on. Perhaps they find the harem aspect, which is a big part of the story, unappealing or even offensive in some ways.
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.
The main thing is the new relationships. When an OC is paired with Rias, it creates fresh dynamics. We can see different sides of Rias's character through her interactions with an unknown element like the OC.
There could be the other members of Rias's peerage. In some fanfictions, they might be forced to choose sides between Rias and the OC. For example, Akeno might be torn between her loyalty to Rias and her growing respect for the OC's abilities.
One popular 'oc x rias dxd fanfic' is 'A New Dawn in Kuoh'. It focuses on an original character who forms a unique bond with Rias. The story has great character development and interesting plot twists.
One possible fanfic could be about a friendly rivalry between Rias and Riser that turns into a mutual respect. For example, they could start competing in various magical challenges within the Highschool DxD universe. This would show their growth as characters and how they overcome their initial differences.
One possible fanfic could be about their relationship as siblings in the context of the Highschool DxD universe. Maybe it could explore how Sirzechs' actions and decisions affect Rias. For example, his leadership in the Underworld might have an impact on Rias' development as a devil and her role in the school's Occult Research Club. It could also show how they support each other during difficult times, like when facing threats from other devil factions or powerful enemies.
A great fanfic idea could be that Rias and Riser are sent on a joint mission to the human world. They have to blend in as high school students. This would be a comical and interesting premise as they struggle to adapt to human life while also dealing with their own relationship. Rias might be more empathetic towards humans, while Riser could be more arrogant at first.
There's a fanfic called 'Rias and Issei: A New Adventure'. In this one, they go on a journey together that tests their relationship. The author does a wonderful job of portraying their feelings for each other and the challenges they face as a couple in the world of Highschool DxD.
The appeal lies in the potential for new and unique storylines. Since the OC is a creation of the author, it can bring fresh perspectives to the Highschool DxD world. For example, the OC could have abilities or a personality that no existing character has, which can lead to interesting interactions with Rias.
One great fanfiction in this category is 'A New Bond'. It delves into the relationship between an original character and Rias in a really interesting way. The story develops their connection gradually, starting from a chance encounter and evolving into a deep emotional bond. The author does a wonderful job of fitting the OC into the Highschool DxD universe, making it feel like they were always meant to be there.