To develop the OC's personality, consider their goals and dreams. If the OC dreams of bringing peace between different supernatural factions, it would make them an idealist. And this would shape how they interact with Akeno. Akeno's personality development can be tied to how she responds to the OC's ideals. She might initially be skeptical but as she spends more time with the OC, she could start to believe in those ideals too, which would show a change in her personality from being more self - centered to more community - minded in the context of the Highschool DxD world.
For the OC, you can start by giving them a unique backstory. For example, if they had a tough childhood in the human world before being thrust into the supernatural world of Highschool DxD, it could make them more reserved yet determined. Akeno, on the other hand, is known for her alluring and mysterious nature. You could develop her personality further by showing her more vulnerable side when she's with the OC, like how she might open up about her own past traumas.
For the OC, you can start by giving them a unique backstory. For example, if they had a difficult past that made them more empathetic towards Koneko's own struggles. Koneko's personality is already somewhat reserved and strong - willed. The OC can gradually draw her out of her shell by being patient and kind. Maybe the OC has a sense of humor that Koneko slowly starts to appreciate, and this helps in developing her personality to be more open and less stoic.
One great fanfiction could be where the OC has a unique power that Akeno is initially intrigued by. For example, the OC might have the ability to control light in a different way than the devils in the Highschool DxD universe. This could lead to a series of adventures where Akeno and the OC team up to face various challenges. They could start off as just acquaintances but gradually develop a deeper bond as they fight side by side.
One way is to introduce a new character who causes some sort of rift between them at first. Maybe a new devil who tries to drive a wedge between Rias and Akeno by spreading false rumors. But then, as they figure out the truth together, they become even closer than before. It would be a journey of trust - building and forgiveness that would enhance their relationship.
Maybe the OC is an artist who can create magical art. Akeno is fascinated by this art and the emotions it evokes. They could start collaborating on projects that are not only artistic but also have a deeper meaning in the context of the Highschool DxD universe. For instance, creating art that can temporarily shield them from attacks or art that can influence the minds of their enemies. Their relationship could grow as they explore the potential of the OC's art together.
One idea could be that the OC is a new student at the school and immediately catches Akeno's eye. They start off as friends but slowly develop deeper feelings as they go through various supernatural battles together. Another plot could involve the OC having a special power that Akeno is intrigued by, and they bond over exploring and controlling that power.
Start with giving the OC Sitri a distinct personality trait. For example, if they are extremely curious, it can drive their actions and decisions. They could be constantly exploring the world of Highschool DxD and getting into all sorts of situations because of their curiosity.
For the OC, you can start by giving them a distinct backstory. If they had a tough childhood, it might make them more resilient and street - smart. This could contrast with Android 18's initially cold and detached personality. The OC could gradually break through Android 18's walls with their kindness and determination. For Android 18, her experiences with the OC could make her more open and empathetic. Maybe she starts to care more about the well - being of others, especially the OC.
First, you need to define their personality clearly. Are they slutty because of a past trauma or is it just their natural inclination? This backstory will help shape their character. For example, if it's due to a past betrayal, it can add depth to their promiscuous behavior.
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.
Another way is to focus on their relationships. Have them interact with different characters from Highschool DxD. If they form a friendship with Issei, they could learn about human values from him, which would contrast with their demon upbringing. Or if they have a rivalry with another demon, it could push them to become stronger. Also, let their relationships change over time, like starting as enemies and then becoming allies.