Issei could also develop morally. As an exorcist, he would be exposed to different ideas of good and evil. He might start to question some of the actions of the devils he's associated with and strive to find a middle ground where he can still be true to his friends but also uphold the values of an exorcist. This internal struggle would make him a more complex and interesting character.
His relationships would change and help in his development. He might become more respected by some in the DxD world who initially looked down on him, as being an exorcist is seen as a noble profession in that universe. At the same time, he would face new challenges in his relationships with his devil friends who might be suspicious of his new role.
Issei could develop by learning the discipline and self - control that comes with being an exorcist. This would contrast with his more impulsive devil - side nature. For example, he might have to resist the urge to use brute force and instead use the more strategic and spiritual techniques of an exorcist.
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.
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.
In some fanfictions, Issei's character develops by becoming more mature. He realizes the consequences of his actions more deeply. For example, when he uses his powers, he starts to consider the collateral damage and tries to minimize it, which is a big step from his more impulsive self in the original story.
Perhaps Issei discovers that there are exorcist families with ties to the DxD world in ways that no one suspected. He gets involved in their internal power struggles and has to choose sides while still maintaining his loyalty to his original friends. It could lead to some intense battles and moral dilemmas.
Issei could become more mature. He might be less impulsive and more strategic in his actions. For example, in battles, instead of just charging in, he would plan his attacks better.
Issei's character can be developed by showing his growth in handling his new powers. For example, he could become more strategic in battles rather than relying solely on brute force. Yubelluna's character could be developed by exploring her past. Maybe she has some unresolved traumas that affect her current actions. By delving into these, her personality can become more complex.
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.
A smart fem Issei in fanfiction can also bring new story arcs. Instead of just following the typical 'get stronger and fight bad guys' plot, she could be more interested in things like researching ancient magic or finding ways to improve the devil society through her intelligence. This would lead to different types of conflicts and resolutions. Maybe she discovers a way to bridge the gap between different supernatural factions using her knowledge, which would be a very different story from the original where Issei is mainly focused on his own growth and his relationships with his harem.
I'm not entirely sure as the description is rather unclear. It could potentially be about a character named Issei in the 'DxD' universe and some sort of action related to a fanfiction about him. Maybe it's about exploring or creating a fan - made story where Issei is involved in a stroking action, though without more context it's hard to be specific.
One possible plot idea could be that Issei discovers his singing talent during a school event at Kuoh Academy. His singing is so enchanting that it even has an effect on the devils and angels present. For example, it might temporarily calm the hostilities between different factions. Another idea could be that Issei uses his singing as a form of magic, perhaps it's a new power he discovers that is related to his Boosted Gear. He could sing to heal his comrades or to break through magical barriers. And yet another plot could involve Issei entering a singing competition in the supernatural world, where his performance attracts the attention of powerful beings who want to use his talent for their own purposes.