Obviously, the third devil would be a main character. But also, characters like Issei could still play a major role. They might interact with the third devil in various ways, such as training together or getting into conflicts. Another potential main character could be a new character that you introduce specifically for this fanfiction. Maybe a mysterious transfer student who has some connection to the third devil's past or powers.
One idea could be exploring a new power that the third devil discovers. For example, a unique ability to manipulate time on a small scale during battles, which would add a whole new dimension to the fight scenes. Another idea is to focus on the third devil's relationships with other characters in a different way. Maybe the third devil forms an unexpected alliance with a group that was previously an enemy, leading to some really interesting character development and plot twists.
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.