Well, aside from Anos Voldigoad, there are some significant characters. Misha Necron, for example. She has a special connection with Anos and her character development is quite interesting throughout the story. Sasha Necron is another one. She is strong - willed and her actions often influence the plot. They both interact with Anos in different ways, sometimes as allies, sometimes with a bit of conflict due to their different views, which makes the story more engaging.
The Anos Voldigoad light novel mainly focuses on the story of Anos Voldigoad, a powerful and unique character. It's set in a world full of magic and different races. Anos has to face various challenges, like dealing with political intrigues among the different magical groups and also proving his own power and identity in a world that has changed since his long slumber.
Issei, being the hot - blooded character he is, would probably be excited to meet someone as powerful as Naruto. He might challenge Naruto to a friendly spar at first, hoping to learn from him and test his own strength. And as he gets to know Naruto better, he could start to see him as a brother - in - arms, especially if they face common enemies together.
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.
Some characters might be intrigued. For instance, Issei might be curious about Fem Naruto's fixation on Ophis. He could see her as an interesting addition to their world and might try to understand her motives. Rias, on the other hand, might be more cautious. She would likely be concerned about how Fem Naruto's admiration for Ophis could affect the balance of power in their territory.
The storylines can be combined by creating a common threat. Suppose an enemy emerges that is interested in both the power of the Bijuu and the power sources in Highschool DxD. This would force the characters from both universes to team up. Also, the cultural differences between the Naruto world and the Highschool DxD world can be explored. Naruto could introduce concepts like ninja clans and shinobi training to the DxD characters, while they could introduce him to the complex social hierarchies and supernatural politics in their world.
Naruto would probably be very friendly and curious. He'd be excited to meet all the different types of beings in the Highschool DxD world. He might start by trying to make friends with Issei, sharing his stories about his own battles and adventures in his world.
The female characters from Highschool DxD, like Rias, would be intrigued by the unique personalities of the Naruto characters. Rias might see potential in some of them to join her peerage. And the Naruto girls, like Sakura, would be a bit shocked at first by the more revealing outfits and flirty nature of some of the Highschool DxD characters. But over time, they would bond over things like protecting their friends and the school. The Naruto boys would respect the power of the Highschool DxD devils and might even try to incorporate some of their fighting strategies into their own ninja combat styles.
One idea could be that Naruto is somehow transported to the world of Highschool DxD. He could use his ninja skills and chakra in the new world, maybe even teach some of the characters there a thing or two about combat. For example, he could show Issei how to better control his power using chakra control techniques.