One common theme is redemption. Often, the fallen angel character is seeking redemption, and Issei may play a part in that process. Another theme is the power of love that can overcome differences, as they are from different species in the story's context.
One popular issei x fallen fanfic is 'A New Bond'. It explores the relationship between Issei and a fallen angel in a unique way, with interesting character development and a plot full of twists. Another one could be 'Fallen's Redemption' which delves into how the relationship changes the destinies of both characters.
I'm not entirely sure specifically as it could be about a variety of things. It might be a fan - made story involving characters named Issei and Yasaka. It could be set in a fictional universe where they have an adventure together, or perhaps a story exploring their relationship in a unique way.
Their relationship could develop through shared experiences. For example, if they train together, they'll learn about each other's fighting styles and limits. This mutual understanding can be the foundation for a deeper emotional connection.
One plot idea could be that Akeno and Issei have to go on a secret mission together. During this mission, they face various challenges that test their relationship and their individual strengths. This could lead to some romantic moments as they rely on each other for survival.
Another common theme is adventure. They often embark on various quests or face challenges together in the fanfic. For example, they might be fighting against common enemies in the supernatural realm. This not only tests their abilities but also strengthens their bond.
A major element is the way their relationship affects the people around them. Their friends and allies may have different reactions to their relationship, which can create sub - plots. Moreover, the cultural and social context within their fictional world also shapes their relationship. Their different backgrounds within that world can lead to both challenges and opportunities for their relationship to grow.
There's a fanfic called 'Rias and Issei: A New Adventure'. In this one, they go on a journey together that tests their relationship. The author does a wonderful job of portraying their feelings for each other and the challenges they face as a couple in the world of Highschool DxD.
A significant element is the role - reversal scenarios. Sometimes, Rias may take a more submissive role while Issei becomes the dominant one in certain situations, and vice versa. This adds an interesting layer to their relationship. Additionally, the fanfic may explore their cultural differences, as Issei comes from a human background and Rias from a demon one. This can lead to some humorous and also some deeply emotional moments as they try to understand each other better.
Love and acceptance is also a very common theme. Given their different natures, the fanfics often explore how they come to love and accept each other for who they are. This can involve Issei accepting Ophis' more otherworldly aspects and Ophis learning to value Issei's human qualities like his compassion and determination.