The theme of redemption can also be found. Sometimes Issei has made mistakes in the past and the angel helps him to redeem himself, to become a better person in the story.
One popular 'angel issei fanfic' is 'Angelic Encounter'. It has an interesting plot where Issei meets an angel in a very unexpected situation. The story is full of action and the character development of Issei is quite engaging.
Another theme is the idea of family. In Supernatural, Sam and Dean have a strong brotherly bond, and in Dark Angel, Max has her own 'family' among the transgenic group. In crossover fanfic, these family concepts often collide and merge. For example, the transgenic might be seen as a new kind of 'family' by the Winchesters or vice versa. It's about how different forms of family support and protect each other in the face of danger.
In some issei angel fanfiction, the story could revolve around Issei falling in love with an angel, which is a big taboo in their world. This love story would be filled with challenges and obstacles they have to overcome.
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.
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.
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.
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.
A common theme could be the development of their relationship. Since it's a fan - created pairing, many fanfics might focus on how they get to know each other better, overcome differences, and grow closer.
One common theme is the exploration of their relationship dynamics. Given that Ophis is a very powerful being and Issei has his own growth journey in the series, fanfics often delve into how they interact and influence each other's development.
Love against all odds is also a frequent theme. Given their different positions and the nature of the High School DxD world, their love might be seen as something that goes against the norm. So, many fanfics will center around how they fight for their love in the face of various obstacles, whether it's social hierarchies or threats from other characters. It makes for a very engaging read as the story unfolds.