I don't really know for sure as I haven't read it. But it could be the 'dark gamer' themselves as the main character. There might also be some of the original PJO characters like Percy Jackson or Annabeth Chase who interact with this new 'dark gamer' character.
Since it's a 'pjo fanfic' with 'the dark gamer' in it, it could be about a gamer who has a rather dark or edgy personality. In the context of the Percy Jackson universe, this gamer might enter the world through some sort of magic or accident. Once there, they could start to change the dynamics of the story. For example, they might use their gaming knowledge in unexpected ways to battle monsters or solve puzzles. They could also form alliances with some of the more unconventional characters in the PJO world, all while their 'dark' side adds an interesting twist to the plot.
Obviously, Dark Naruto would be one of the main characters. He is the center of this fanfiction concept. Other than him, characters like Sasuke would likely still play important roles. Sasuke might be either an ally or an adversary to Dark Naruto, depending on how the story is written. Also, characters like Kakashi Sensei could be there to try and guide or stop Dark Naruto, given his new, more dangerous state.
Often, the main character is a self - insert type of gamer who gets transported into the DBZ world. They usually have the ability to use gamer - like mechanics to interact with the DBZ universe.
Probably Ben 10 is the main character. And depending on the plot of the fanfic, Gwen could also be involved. She has her own powers and could either help or hinder Ben in his gamer - style quests. Also, Grandpa Max might make an appearance to offer advice or some sort of guidance as he usually does in the Ben 10 stories.
There could also be original characters created specifically for this fanfic. Maybe a game master who controls all the virtual worlds Goku enters. This character could be a bit of a mystery, with their own motives and backstory, and they interact with Goku in ways that drive the plot forward.
Most likely, the main characters would be Ruby, Weiss, Blake, and Yang, as they are the central characters in RWBY. In the 'gamer fanfic' version, they would be the ones experiencing the game - like elements.
Another main character could be the game master or the final villain in the game world. This character would constantly create obstacles for Ron and his friends, making their journey through the game full of challenges and excitement.
The main character is Naruto, no doubt. But don't forget about the other ninja in the village. In this fanfic, they could all be part of a gaming - style ranking system. Hinata might be another character involved, perhaps with a special role in the game - like story. And Jiraiya could also make an appearance, maybe leaving Naruto some kind of in - game legacy or quest that Naruto has to complete. So it's not just Naruto alone, but a whole cast of Naruto - universe characters that are likely to be in the 'naruto the gamer fanfic'.
The main character is, of course, the original character (oc). But there are often other supporting characters as well. These could be friends or allies of the oc who help them on their adventures in the world of the fanfic. They might have different skills or personalities that complement the oc.
There could also be antagonists in the fanfic. They might be other forces in the sekirei world who oppose the main characters' goals. These could be rival sekirei groups or those who want to disrupt the new 'gamer' order in the sekirei world. Their actions drive the conflict and make the story more engaging.