The devil is surely a main character. Maybe it's a high - ranking devil with a reputation in the underworld. The mortal, on the other hand, could be a young and innocent person. There might also be side characters like the devil's friends or the mortal's family members who play important roles in the story. For example, the mortal's best friend might be suspicious of the devil and try to keep them apart.
The main characters are the devil and the mortal. The devil could be a fallen angel - turned - devil with a tragic past that led it to fall in love with a mortal. The mortal might be a student or someone with a simple life. There could also be other characters such as a mentor figure for the mortal who warns them about getting involved with the devil. Or perhaps there are other devils who oppose the relationship and try to stop it for various reasons.
Another possible main character could be a demon friend of the devil. This demon might be skeptical of the devil's love for the mortal and try to convince him to break it off. The interactions between these characters would shape the story and the development of the devil - mortal relationship.
Well, perhaps a plot twist could be that the devil is actually being controlled by an even more powerful entity, and his love for the mortal is being used as a tool to manipulate events. Or, the mortal turns out to be related to a powerful figure in the DxD universe in a way that no one expected, which completely changes the dynamics of their relationship.
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.
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.
Maybe the devil loses his powers while falling in love with the mortal. This would turn the tables as he is now vulnerable and has to rely on the mortal for support. It could also lead to other supernatural beings coming after them, seeing the devil's weakened state as an opportunity.
Rias is also a popular choice. If she were to have a devil fruit power in fanfiction, it could completely change the power dynamics in the story. For example, if she had the Ope Ope no Mi (Operation - Operation Fruit), she could perform incredible surgical - like attacks and also heal her allies. Her leadership role in DxD combined with such a powerful devil fruit would make for very interesting storylines.
Rias Gremory is also a fan - favorite. Fanfictions could focus on her leadership skills within the devil community at Kuoh Academy. For instance, stories could be written about how she deals with new threats to her peerage or how she tries to strengthen her position among other devil clans.
One memorable character could be a half - angel - half - devil protagonist named Alex. Alex is constantly in a battle with his inner selves, the angel and the devil. His devil side might make him a bit mischievous and quick - tempered, while his angel side gives him a strong sense of justice. Another could be a female character, Lily, who uses her dual nature to her advantage. She can be both seductive like a devil and pure - hearted like an angel in different situations.
Issei Hyoudou is often still a very popular character. His journey as a devil and his growth in power are central to the High School DxD series, and in the Devil Reborn Redux fanfiction, fans usually enjoy seeing his further development. He might face new challenges and relationships that keep his character engaging.
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.