There's a possibility that it's a devil who was born with a special ability that makes him believe he's superior to the existing underworld hierarchy. This overconfidence, combined with his power, drives him to declare war in an attempt to reshape the underworld to his liking in the DxD fanfic.
It could be a young and ambitious devil who has grown tired of the old ways of the underworld. He might see the current power - holders as corrupt and decides to take matters into his own hands by declaring war.
One possible interesting plot could be that the devil has a personal vendetta against a powerful underworld figure in the DxD universe. Maybe this underworld figure wronged the devil in the past, and now the devil has amassed enough power to declare war. This could lead to epic battles, alliances being formed and broken among different factions in the underworld.
This could be quite a spectacle. The devil might first try to undermine the underworld's defenses through deception or stealth. Then, when ready, launch a full-scale assault with all their might and demonic abilities.
Naruto as the main character with his Devil Saiyan form would be central. Also, Rias Gremory could be a main character. She could be intrigued by Naruto's powers and his different way of thinking, coming from a different world. And perhaps the Devil King himself could also play a significant role, either as an antagonist or an unexpected ally to Naruto in this strange new world of DxD.
It could be a new character created specifically for the fanfic. Maybe a powerful and malevolent spirit that has been lurking in the shadows of the Naruto world.
Another option could be a group of rogue robots who have their own set of rules and don't like the idea of Bender and the Robot Devil being so prominent. They could band together to try and take over the areas that Bender and the Robot Devil operate in. They might be more brutal and less interested in the games and deals that the Robot Devil usually plays, making them a very different kind of antagonist compared to the Robot Devil.
It could be a rogue ninja who has mastered the devil breakers in a very dark way. He might use them to gain power and control over the ninja world, seeing Naruto as the main obstacle to his plans.
Naruto himself could be the main character with a devil bloodline twist. Maybe he was born with it but it was hidden until a certain event triggered it. This would completely change his journey. Instead of just trying to gain the respect of the village as a Jinchuriki, he would also have to deal with the implications of having a devil bloodline, which might be seen as even more dangerous by some in the village.
I am unable to provide or answer questions regarding illegal or unauthorized information or content, including any information that involves illegal activities or violates the privacy of others. At the same time, I am unable to provide any passwords or information related to the game or virtual items. Please understand.
Some devils who are jealous of Naruto's unique abilities. Since he has both ninja skills and whatever new devil powers he might gain, they could be envious and try to undermine him at every turn. Also, there might be a group of devils who are trying to start a war in the devil world and see Naruto as an obstacle to their plans.
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.