He might react with hostility. Dark Naruto has a certain edge to him, and if he sees Rias as a threat or someone he doesn't like, he'll likely oppose her. He could use his combat skills to challenge her directly.
In fanfiction, such a betrayal could occur due to power struggles. Maybe Rias sees Naruto as a threat to her position in the Highschool DxD world. She might fear his growing power and influence, which could overshadow her own. So, in order to maintain her status, she betrays him.
One great fanfiction idea could be that Issei discovers a new power within himself that is directly linked to Rias's heritage. Maybe he can tap into an ancient magic that only works in harmony with Rias's own powers. As they train together to master this new ability, their relationship deepens beyond just the typical master - servant bond in the Highschool DxD world.
A common theme is power - up. Rias may gain new powers or improve her existing ones. This could be related to her devil heritage or through some special training or encounter.
In 'Highschool DxD' fanfiction about Rias, her character often retains her strong - willed and noble nature. She is a powerful devil, and in fanfictions, her leadership skills and loyalty to her friends are usually emphasized.
One possible fanfiction could be about Rias and Sona teaming up for a school event. They might put aside their usual competitiveness and work together to organize a devil - related festival at the school. They could combine their unique skills and powers to create an amazing event that not only impresses the other students but also strengthens their bond as friends.
A possible way for their worlds to merge in fanfiction is through a reincarnation plot. What if some of the Naruto characters were reincarnated in the Highschool DxD world? Naruto himself could be reborn as a devil or an angel. This would completely change the power balance and relationships in the Highschool DxD world. Rias would have to deal with this new Naruto, who still has his ninja spirit but now has to adapt to the rules and powers of the new world.
Well, in such a fanfiction, Dark Naruto might use his powerful and often shadowy abilities to outwit Rias. Maybe he'd expose some of her less - than - noble actions in the Highschool DxD world. He could easily overpower her in battles, using his ninja skills and the dark powers he has. For example, his shadow clones could be used to confuse Rias and her peerage while he strikes from the shadows.
There's a fanfiction where Naruto has godlike powers and teams up with Rias in Highschool DxD. It shows how Naruto's power and his different mindset from the DxD characters can create some really cool and unexpected storylines. For example, his way of handling the devils and other supernatural beings is quite different, which makes for an exciting read.
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.