Another way to develop their characters is to put them in a situation where they have to make a difficult choice. For instance, they might have to choose between loyalty to their friends or following their hearts. This would force them to grow as individuals and as a pair. Their actions during this time would reveal more about their personalities and values.
One interesting plot idea could be that Kiba and Tsubaki have to team up for a special mission in the Underworld. They face various challenges and in the process, they discover new things about each other's pasts and their own powers. This could lead to a deeper bond between them.
Kiba could be developed by exploring his emotional side more. For example, showing his insecurities about his place in the group. This would make him more relatable.
For Asia, her character could be developed by exploring her past more deeply. Maybe she could have flashbacks to her life before joining the supernatural world, which would help explain her kindness and her eagerness to help others. As for Gasper, he could be made to overcome his extreme shyness. For example, by being put in situations where he has to speak up to protect Asia or their friends. This would show his growth and development.
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.
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.
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.
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.
One common theme is character development. Writers often explore Tsubaki's growth, her relationships with other characters, and how she evolves within the Highschool DxD universe. For example, her relationship with the main character or her progress in her own powers.
For Issei, in fanfiction, he could be developed by giving him new powers. For example, he could gain some of Ophis' power in a special encounter. This would not only make him stronger but also force him to learn how to control and use it, which could lead to a more mature Issei. His relationships could also be developed further. With Ophis, he could start to understand her better and become more than just an acquaintance.