A plot could involve a conflict within the Templar Order regarding how to deal with the existence of the supernatural beings in Highschool DxD. Some might want to ally with them, while others see them as a threat to be eradicated. This could lead to an internal power struggle within the Order.
Start with a backstory. A character from the Templar Order might have a family history of serving the Order, which shapes their motives and actions. In Highschool DxD, a character could have a past event that led to their current position or power. Develop relationships. A Templar might form an unlikely friendship with a devil in Highschool DxD, which challenges their beliefs. Through these relationships, the character can grow and become more believable.
The Highschool DxD manga is finished. It completed its journey, leaving behind a memorable story. Although it's over, its influence and popularity among manga lovers remain strong.
The Highschool DxD manga has indeed finished. It wrapped up after completing all the planned story arcs and providing a satisfying conclusion for the readers.
If Spawn were to interact with the main characters, he would bring a whole new level of seriousness. He has a dark past and a lot of baggage. When it comes to Issei's goal of becoming a harem king, Spawn might find it absurd but also interesting in a way. He could offer his own perspective on power and how to use it responsibly, which would be a contrast to the more carefree attitude of some of the Highschool DxD characters. And with the other characters like Akeno and Koneko, he'd be sizing them up, trying to figure out if they're threats or allies.
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.