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.
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 interesting plot idea could be Issei and Le Fay teaming up to face a new, powerful enemy that neither of their usual allies can handle. They could combine Issei's power of the Boosted Gear with Le Fay's magic in unique ways. Another could be a story where they accidentally end up in an alternate dimension and have to find their way back while also dealing with the strange rules and creatures there.
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.
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.
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.
The relationship between Issei and Rias would evolve in a beautiful way. They would be each other's support system. Issei, who has always been passionate, would become more refined in his love for Rias. Rias, on the other hand, would become more lenient towards Issei's sometimes - rash decisions. They would have a relationship based on mutual understanding, respect and love. Their engagement would be the starting point of a long and fulfilling journey together, filled with love, laughter and some tears as they face the challenges of their world.
Their relationship could develop uniquely if Issei starts to teach Ophis about human emotions more deeply. Since Ophis is a powerful being, she may not fully understand all the nuances of love. Issei could show her through simple acts like sharing his favorite food or taking her to a place that has special meaning to him.
In many fanfictions, Issei's character develops in terms of power. He starts as a relatively weak character and gradually becomes stronger through training and various challenges. For example, he might learn new techniques from a mysterious mentor or gain new powers from a magical artifact.
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.
Well, it would be quite an interesting relationship. Naruto's strong will and combat skills from his own world could mesh well with Rias' leadership in the Highschool DxD universe. He would likely bring a new sense of loyalty and determination to her peerage. Also, his ability to make friends easily might help him fit into the complex social structure of the DxD world.