It can develop realistically if they face challenges together. Let's say they have to deal with opposition from other devils who don't approve of their relationship. They stand by each other through this, which strengthens their bond. Also, if they participate in devil training together, they can see each other's growth and be attracted to the evolving versions of themselves.
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.
Realistically, their relationship might start with friendship. Issei could befriend Kuroka first, and they hang out casually. Over time, they notice that they miss each other when apart. They start to feel jealous when the other is around someone else. These are natural steps in a relationship that could lead Issei and Kuroka to fall in love.
As their relationship progresses, introduce shared interests. Maybe they both like a certain type of magic or have a passion for a particular kind of food. They can bond over these things. Also, don't rush the romantic aspect. Let them become friends first. For instance, they could study together for a magic exam and slowly start to see each other in a different light.
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.
It could start with them being forced to work together on a mission. At first, they might clash due to their different personalities. Issei is more impulsive and Yubelluna is more reserved. But as they face difficulties during the mission, they start to understand each other better. For instance, when Issei rushes into a fight without a proper plan, Yubelluna scolds him but also helps him out. This back - and - forth gradually turns into a more affectionate relationship.
One possible fanfiction idea could be that Issei and Kuroka's relationship develops in a way that they decide to start a family. Kuroka, with her feline nature, might be initially hesitant about motherhood. But Issei's determination and love for her could change her mind. They could face challenges from the supernatural world around them, like other factions being against their union due to Kuroka's past. However, their love for each other and their unborn child could be the driving force to overcome these obstacles.
Their relationship could start as comrades in arms. As they face challenges together, they begin to respect each other's skills. Then, gradually, they might develop a friendship based on mutual trust. Eventually, it could turn into a romantic relationship as they discover more about each other's personalities and what they truly care about.
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.
Their relationship could develop further through shared challenges. For instance, if they face a really tough opponent together, they'll learn more about each other's strengths and weaknesses, and that can deepen their bond. Another way is through communication. If Issei starts to open up more about his insecurities and Rias does the same, they can become closer on an emotional 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.
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.