In fanfiction, the sky's the limit. It could be that an ancient prophecy in the 'Highschool DxD' universe foretells of a foreign hero (Naruto) coming to sire a special child (Kunou). To fulfill this prophecy, some magical forces manipulate events so that Naruto and Kunou's mother cross paths. Naruto's kind heart and powerful abilities might attract Kunou's mother, and they end up having a relationship that leads to Naruto being the father. Also, the author might be playing with the idea of mixing different power systems and cultures, with Naruto representing the ninja culture and 'Highschool DxD' representing the devil and supernatural culture, and this union results in Kunou.
One possibility is that a writer might create a backstory where Naruto's unique chakra or abilities are needed to save Kunou's mother from a great danger. In the process, a bond forms between them and through some unorthodox means, Naruto becomes the father. Another aspect could be that in the fanfiction world, there are new rules and the author wants to explore the combination of Naruto's ninja world elements with the devil - based world of 'Highschool DxD' to make Naruto Kunou's father.
If Naruto is Kunou's father, it would mean a whole new set of powers for Kunou. Combining Naruto's chakra - based powers with the devil - like abilities in 'Highschool DxD', Kunou could be extremely powerful. For example, she might be able to use Naruto's Rasengan technique along with her own devil powers.
If we consider Naruto and Kunou in a Highschool DxD fanfiction, Naruto could potentially be seen as a rival to some of the more powerful male characters in DxD. His determination and never - give - up attitude would be a contrast to the often more laid - back or self - interested characters in that world. Kunou, on the other hand, might form alliances with some of the female characters, sharing her knowledge of yokai with them and learning about the devils and angels in return.
There could be some really interesting interactions. Naruto might befriend Issei, and they could exchange training techniques. Naruto's rasengan could be a very unique move for Issei to learn about. Kunou could interact with Rias, perhaps having a bit of a rivalry at first due to their different leadership styles. Kunou is more reserved in her kitsune ways while Rias is bold as a devil leader.
One possible fanfiction could be about Naruto somehow ending up in the world of Highschool DxD and meeting Moka. Maybe Naruto's skills and his never - give - up attitude would impress Moka and the other characters. He could teach them some of his ninja techniques, like the Shadow Clone Jutsu, which could be useful in battles against the devils and other supernatural beings in the Highschool DxD world.
In a Sharingan Naruto Highschool DxD fanfiction, the Sharingan could also play a role in terms of illusions. Imagine a scenario where the character uses Sharingan - based illusions to deceive the enemies from both worlds. They could make a Highschool DxD devil believe they are in a completely different environment like a dangerous Naruto - style forest filled with traps. And for Naruto characters, they could be made to see illusions of their worst nightmares or false threats from the Highschool DxD world, which would make for some really intense and engaging storylines.
His speed would likely be enhanced. In the DxD universe, there are fast - moving opponents like the devils and angels. Naruto, already known for his incredible speed in his own world, would need to adapt and become even faster. His rasengan, for instance, might be modified to be more effective against the magical defenses of DxD characters.
A great fanfiction could involve Naruto's overpowered (op) abilities being introduced into the Highschool DxD universe. For example, Naruto's shadow clone jutsu could be used in the battles at the high school in DxD. His ability to harness chakra might be a new form of power in that world.
He would be a very protective father. Given Naruto's own experiences growing up without parents and his strong will to protect his loved ones, he would be extremely vigilant in safeguarding his family in the Highschool DxD fanfiction.
Naruto would likely be more rebellious. In the original Naruto series, he was an outcast but still had a strong sense of justice. In this crossover, as a delinquent in the Highschool DxD world, he might question the existing power structures more. His sense of humor could also take on a more edgy tone.
One possible antagonist could be some of the traditionalist shinobi in the village who oppose Naruto's new family situation. They might see Kunou as a threat due to her heritage.