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 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.
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.
The human OC can be developed by giving them unique abilities. For instance, they could have a special power that Rias doesn't have or understand. This would make them stand out and be a threat to Rias. Another way is through their backstory. A tragic or interesting backstory can make the readers sympathize with the OC and understand their motives for bashing Rias.
One common theme could be power dynamics. Since it's a Rias - bashing fanfiction, it might explore how a human OC challenges Rias' authority or power within the Highschool DxD universe. Another theme could be the exploration of different character relationships. The human OC may interact with other characters in ways that contrast with how Rias does, leading to new and interesting story developments. It could also deal with the concept of loyalty, as the OC might sway the loyalty of characters who were previously on Rias' side.
Often, there's a theme of challenging the status quo that Rias represents. In the original, she has a certain position and set of relationships. Fanfictions might bash her to create a new narrative where the OC can break free from the typical Rias - centric storylines. For example, if Rias is seen as too controlling in the original, the fanfic could have the OC resist her authority and form new alliances or ways of doing things.
For the male OC, you can start by giving him a distinct personality trait. For example, if he's really shy, it can create interesting interactions with Rias who is often more confident. This shyness could be due to his insecurities about his newfound powers or his place in the supernatural world.
One great fanfiction idea could be that Issei discovers a new power within himself that is directly linked to Rias's heritage. Maybe he can tap into an ancient magic that only works in harmony with Rias's own powers. As they train together to master this new ability, their relationship deepens beyond just the typical master - servant bond in the Highschool DxD world.
A common theme is power - up. Rias may gain new powers or improve her existing ones. This could be related to her devil heritage or through some special training or encounter.
In 'Highschool DxD' fanfiction about Rias, her character often retains her strong - willed and noble nature. She is a powerful devil, and in fanfictions, her leadership skills and loyalty to her friends are usually emphasized.