Loyalty is also a prevalent theme. They are shown to be loyal to each other no matter what. In many fanfictions, this loyalty is tested, but they always come through for one another, whether it's against external enemies or internal doubts within the group or their own minds.
One common theme is friendship. Their relationship is often depicted as a strong bond that helps them through various difficulties in the Highschool DxD universe.
One common theme is the exploration of their relationship in a supernatural setting. Since Highschool DxD is full of demons, angels, and other mythical beings, their relationship might be affected by these elements. For example, they might have to fight together against a common enemy, which would strengthen their bond.
One common theme is power. Since Tiamat is a very powerful being in the 'Highschool DxD' world, fanfictions often explore how Issei's power grows or is affected by his relationship with her.
Another common theme is the exploration of the different arcs from the series but with a focus on Issei and Rias. This could include their adventures in the Underworld, dealing with rival devil groups, or facing new threats together. Fanfictions may expand on these storylines while highlighting their relationship.
A common theme is Kiba's relationships. This can be his romantic relationships, like with a female character from the series. His friendships within the group are also explored a lot. The theme of redemption is also there. Kiba might be trying to atone for something he did in the past. And his role in battles, especially his strategic thinking and combat skills, is another common theme in Highschool DxD Kiba fanfiction.
The theme of self - discovery is also common. Issei in many fanfictions is on a journey to find out more about himself, his place in the world, and his true potential. And of course, the theme of the supernatural world and all its elements like magic, different species, and ancient secrets is often a big part of these fanfictions as it was in the original Highschool DxD.
Power and growth are common themes. Since Great Red is such a powerful entity, Issei often experiences significant power - ups in these fanfictions. For example, he might gain new abilities related to Great Red's power.
Protection is also a big theme. Asia is often depicted as a kind - hearted and somewhat naive character. So, Issei protecting her from various threats, whether it's other devils or supernatural beings, is quite common. This also serves to show his loyalty and love for her in the fanfictions.
One common theme is power escalation. Since Issei is OP in these fanfictions, the story often focuses on how he continues to gain and control his ever - increasing power. Another theme is harem - related, as in the original Highschool DxD, Issei has a harem. In the fanfictions, his OP status might attract even more characters to his harem, and the dynamics within the harem change because of his new power level.
Well, often in Highschool DxD Issei bashing fanfiction, a common theme is power redistribution. Since Issei is a central figure in the original in terms of power - up and his role in battles, these fanfictions might shift the focus to other characters getting stronger or more important in different ways. It could also be about changing the overall tone of the story. Instead of Issei's more comical and pervy attitude driving parts of the plot, the fanfiction might have a more serious or different comical approach by removing or reducing his presence.
Power - up themes are also quite common. Issei often gets new powers or abilities, and Rias may play a role in that. For example, in some fanfictions, Rias helps Issei to master a new form of his Sacred Gear. This not only strengthens Issei but also deepens their bond as they train together. Another aspect is the exploration of the supernatural world in the context of their relationship. Since Highschool DxD has a rich supernatural setting, fanfictions may expand on this, like how Issei and Rias face new threats from the underworld or other dimensions together, which further cements their relationship and showcases their individual growth.