Perhaps there are new characters created specifically for this fanfiction. These original characters could be introduced to add new dynamics to the story, like a mysterious mentor figure who guides the protagonist through the new challenges in the DxD - inspired world of this fanfiction.
Well, one interesting aspect could be the unique interpretations of the DxD universe. Fanfictions often explore different character relationships that might not be fully explored in the original work. For example, they could develop a deeper bond between characters that were only briefly associated in the official story.
There could be a group of characters. Maybe a new team of benders who are on a quest with the Avatar. For example, a water bender with a special ability that is crucial to the story, a fire bender with a mysterious past that ties into the 'flame in the dark' aspect, and an earth bender who provides stability and wisdom to the group.
It's hard to say for sure without reading it, but it could be Naruto himself. Since it's a Naruto - related fanfiction, he might be the central figure dealing with the frozen flame concept.
Often in such fanfictions, the main character, the gamer, is at the center. He might be joined by various female characters from DxD to form the harem. For example, Akeno Himejima could be one of the main characters in the harem. She has her own charm and powers, and her relationship with the gamer would be an important part of the story. And of course, there could be other characters like Koneko Toujou who might also play significant roles, either in the harem or in the overall plot.
Well, aside from Naruto and Issei, there could be other important characters. Rias Gremory, who is a devil princess in DxD, might play a significant role. She could be curious about Naruto and his powers, or she might even see him as a potential ally or threat. Also, characters like Akeno Himejima, Kiba Yuuto, and others from the DxD series could be part of the story and interact with the reborn Naruto in different ways.
I don't know for sure since I haven't read it. But it could feature the ancestors or predecessors of the main characters from 'High School DxD'. For example, if Issei is a main character in the original, it might have someone related to his family line in the past.
It's likely that the main characters from the original 'Highschool DxD' series would still play important roles. Maybe Issei Hyodo would be there, adapting to the new quetzalcoatl - influenced environment.
In addition to Gohan and Issei, Rias could be a main character in this fanfiction. She is a powerful devil and a key figure in the Highschool DxD universe. Her relationship with Gohan could be complex, especially when it comes to power dynamics and her own goals for her peerage. She might be intrigued by Gohan's power and want to recruit him or at least form an alliance with him.
Since I haven't read it, it's hard to say for sure. But it could be Naruto himself as he is the main character in the original series. There might also be an original character created for this fanfiction who has a connection to the dragon of the darkness flame.
In most shinso vampire Highschool DxD fanfiction, aside from the shinso vampire itself, characters like Akeno Himejima could be main characters. She has her own powers and personality that would interact interestingly with a shinso vampire. Also, Koneko Toujou could be important. Her relationship with the shinso vampire might involve her cat - like nature and her role in the devil society. And of course, there could be original characters created specifically for the fanfiction that become main characters as well.