His character can be developed by giving him challenges. For example, he could be put in situations where he has to choose between loyalty to his family and what he believes is right. This would force him to grow morally. Also, having him face opponents stronger than him in battles would make him train harder and develop his combat skills.
Another important aspect of his character development could be his discovery of his own identity. He might start to question whether he wants to be just known as Rias's little brother or carve out his own name. This self - discovery journey could involve him exploring his past, his family's secrets, and his own unique abilities. He could then use this new - found understanding to redefine himself and his place in the Highschool DxD universe.
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.
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.
Their relationship could be developed uniquely by having them swap bodies for a while. This would force them to see things from each other's perspective. Issei would understand the pressures Rias faces as a devil princess, and Rias would get a taste of Issei's more carefree but determined human side.
Another plot could revolve around him getting involved in a school-wide mystery. Since he's related to Rias, he has access to certain areas and people. He starts to notice strange happenings at school that seem to be related to an ancient devilish artifact. As he delves deeper into the mystery, he forms unexpected alliances with other students, both devil and non - devil, and has to use his wits to solve the mystery before it's too late.
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.
In some fanfictions, Issei might develop a more serious personality earlier on. Instead of his usual comical and pervy self from the start, he could be shown as someone with a greater sense of purpose, perhaps due to a different backstory or a unique encounter. For example, if he had a more tragic event in his past related to the supernatural, it could shape him into a more brooding and focused character.
One way is to explore her family background more. For example, showing how her upbringing in the devil society influenced her leadership style in the student council.
One way to develop her character is to have her learn new skills. For instance, she could start training in martial arts or some form of magic that is accessible to humans in the Highschool DxD world. As she learns, she could face setbacks and challenges that force her to grow both as a fighter and as a person.
To develop Mordred in a Mordred x Highschool DxD fanfiction, her backstory can be explored further. Maybe she has some unresolved issues from her original story that resurface in the Highschool DxD setting. For instance, her complex relationship with her father figure. As she tries to come to terms with these in the new world, she could change and grow. She might also start to question her own values and beliefs as she experiences the different cultures and power dynamics in Highschool DxD, which would be an interesting way to see her character evolve over the course of the fanfiction.
Tiamat could be developed further by exploring her relationships with other characters. For instance, if she starts to mentor a young character, it would show a different side of her. She could teach them about her powers and the history of her kind.