To develop the OC Son of Shiva in the Highschool DxD fanfic, consider his relationships. He might have a complex relationship with the existing characters. For example, he could be in awe of some of the powerful devils or angels in the story. At the same time, his Shiva heritage could make some characters either fear or respect him. His growth could also come from facing challenges related to his identity, like trying to fit in while being different from everyone else.
Well, the character could be developed by first exploring his Shiva - given powers. Maybe he has incredible strength or some unique divine abilities. He could start as a bit of an outcast in the highschool setting of DxD due to his otherworldly nature. Then, as the story progresses, he gradually makes friends and allies, learning more about the world of DxD and how to use his powers in this new context.
He could be a game - changer. His powers from being the son of Shiva might be so great that he can influence the balance of power between the different factions in the Highschool DxD world. Maybe he could be a mediator between the devils and the angels, using his unique position as an outsider with divine powers.
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.
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.
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.
The OC, as the Son of Sparda, could fit in by being a new and powerful entity. He might be initially seen as an outsider, but his powers would quickly draw attention. He could become involved in the power struggles between the different factions in Highschool DxD. For example, the devils might try to recruit him, while the angels might be wary of his presence. His unique abilities would also make him a valuable ally or a fearsome opponent. And in the high school setting, he could be a bit of an enigma to the other students, both human and supernatural.
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.
Sora's character can be developed by giving him relationships. For example, he could form a deep friendship with Issei. Through their interactions, Sora could learn about loyalty and friendship in the context of the Highschool DxD world. This would also allow his own values to be challenged and refined.
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.
Fanfiction could also develop Gasper by exploring his relationships in more detail. His friendships, his crushes, or his mentorships. For example, if he has a mentor figure who teaches him how to better control his powers, we can see how that relationship evolves over time and how it impacts Gasper's growth as a character. We can also see how his friendships are tested in tough situations, and how he responds, which would show his loyalty and the values he holds dear.
I haven't read this particular fanfiction yet, but generally, a fanfiction about the son of Sirzechs in the Highschool DxD universe could be really interesting. It might explore new powers, relationships, and storylines that the original series didn't cover. For example, how the OC would interact with the existing characters like Issei and Rias, and how his heritage as the son of Sirzechs would shape his destiny in the Devil society.