Love and friendship are common themes. Their relationship often starts as a friendship that gradually evolves into something more. For example, in many fanfics, they support each other through tough times, which deepens their bond.
There's also 'Issei and Yumaa: The Adventure Begins'. In this fanfic, they start on an exciting journey filled with magic and mystery. It's a relatively short but sweet story that focuses on their initial connection and how they start to rely on each other.
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.
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.
One popular trope is the 'protector' trope. Issei often takes on the role of protecting Asia from various threats in the fanfics, which plays on his character development in the original series as someone who wants to be strong and protect those he cares about.
I really like the fanfic 'Koneko's Heart for Issei'. In this one, it delves deep into Koneko's emotions towards Issei. It starts with her being a bit reserved but as the story progresses, her true feelings start to surface. Another interesting fanfic is 'Issei and Koneko: United'. This fanfic shows how they work together to face challenges in the Highschool DxD world, and their relationship blossoms as they rely on each other.