To write their character development well, give them common goals. Let's say they both want to improve the relations between different factions in the DxD world. As they strive for this goal, they learn from each other's strengths and weaknesses. Issei can learn Kuroka's stealth and cunning, while Kuroka can learn Issei's determination and ability to inspire others. This mutual learning process will enhance their character development.
For Issei, you can start by showing his initial reaction to Bennia. Maybe he's a bit intimidated at first by her unique qualities. Then, as they interact more, he starts to see her in a new light and his character develops as he learns to respect and appreciate her differences. For Bennia, her development could be about opening up more. She might be initially reserved, but through her experiences with Issei, she becomes more confident and outgoing.
Issei's character development in Highschool DxD fanfiction can be quite diverse. In many fanfictions, he may become more powerful at a faster rate than in the original series. This could be due to different plot devices or new mentors introduced in the fan - created stories.
Writers often portray their relationship as a slow - burn romance. At first, there might be attraction due to their physical appearances and powers. Then, as they interact more, they start to understand each other on a deeper level.
One possible plot could be that they decide to have a traditional wedding in the Underworld. All their friends and allies from the different factions in Highschool DxD come together. Koneko might be initially hesitant but then fully supports her sister Kuroka. Issei would be nervous but excited, and there could be some comical moments as he tries to follow all the wedding customs.
Some popular themes in Issei and Kuroka fanfiction could be adventure. They might go on quests together in the supernatural world of Highschool DxD. Another theme could be relationship development, like how they gradually fall in love and overcome the differences between a human - turned - devil like Issei and a nekomata like Kuroka.
Rias would definitely be a guest. As Issei's former master and a close friend, she would be there to support him. Akeno would also come, being Rias' queen. Koneko, Kuroka's sister, would be there of course. Xenovia and Irina might also be invited as they are friends with Issei. From the devil side, Sirzechs and Grayfia could be present as important figures in the devil society.
Their relationship could develop realistically through communication. For example, they start by sharing their dreams and fears. Issei might talk about his goal to become a powerful devil, and Kuroka could open up about her past traumas. As they become more comfortable with each other, small gestures like holding hands or a gentle touch on the shoulder could gradually lead to deeper feelings.
One interesting plot idea could be that Kuroka and Issei have to team up to solve a mystery in the Underworld. Maybe there are strange disappearances of some low - level demons, and they suspect a powerful, hidden force is behind it. As they work together, their relationship deepens from just comrades to something more romantic.
One plot idea could be that Issei and Kuroka are forced to team up for a school project. During this time, they start to really understand each other's personalities beyond their initial impressions. Issei discovers Kuroka's softer side that she usually hides, and Kuroka sees Issei's determination and loyalty. This could lead to the start of their romantic feelings.
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.