Well, Naruto is the key character for sure. The neglected god is also essential. There could be some antagonists as well. Maybe there are some old ninja clans who know about the god and want to keep it neglected because they fear its power. So the leaders of these clans could be considered main characters in the sense that they oppose Naruto and his connection with the god. Also, there might be some wise old hermits or sages in the fanfiction who know about the god's true nature and try to guide Naruto in dealing with it.
Obviously, Naruto is one of the main characters. Since it's related to him being involved with a neglected god. The neglected god itself is also a main character, as the whole story revolves around its relationship with Naruto. There might also be other characters like Sasuke or Sakura who could be involved in different ways. For example, Sasuke might be suspicious of Naruto's newfound connection with this god and it could cause some tension between them.
I'm not sure specifically as it's a fanfiction and can vary greatly. But it could potentially have Naruto characters like Naruto himself or Sasuke, combined with some Tokyo Ghoul characters like Kaneki. Maybe there are also some original characters created for this fanfiction.
It could be that some of the neglected characters from Naruto, like Rock Lee for instance, are made main characters in this fanfiction and are paired with a Gremory character. Maybe a Gremory character who also has been somewhat overlooked in their own series. This would give a new perspective and new storylines to explore.
Another option for the main characters could be Kaiba from Yugioh. His pride and determination would make for an interesting dynamic when faced with the neglect from the Naruto world. And from Naruto, Sasuke could be involved. He might initially see the Yugioh characters as a distraction from his own goals, but later on, things could change.
The jailer and an old, wise shinobi could be the main characters. The old shinobi used to be a great hero but now is retired. He sees the same potential in the jailer that he once had. He mentors the jailer, teaching him old techniques that were forgotten. Together, they face a new threat to the village that the other shinobi are ill - equipped to handle.
A potential main character could be a long - lost relative of Hashirama who has the mokuton ability but has been neglected in the main Naruto story. This character could then embark on a journey to discover their true power and place in the ninja world.
Obviously Naruto himself would be a main character. He's the focus of the 'neglected' aspect and the 'king' idea probably revolves around him. Sasuke might also be a significant character. His relationship with Naruto could be very different from the original. Since Naruto is neglected, Sasuke might be in a more powerful position within the village, which could create a lot of tension between them.
Sasuke could be one of the main characters. He might be initially suspicious of the neglected emperor, thinking he could be a threat to the balance of power. But as the story progresses, he could come to understand the emperor's situation. Another character could be Kakashi. He would likely be the voice of reason, trying to analyze the situation objectively and guide Naruto and the others in dealing with the emperor.
The main character has to be Naruto. But we can't forget about the Hokage, like Tsunade. She could be involved in changing Naruto's situation. Maybe she gives Naruto a special mission or training that helps him break free from being neglected. Then there are Naruto's friends from Team 7. Sasuke's relationship with Naruto is complex, and in this fanfic, it could be explored in the context of Naruto's new standing. Sakura, too, could be part of the story, perhaps by finally seeing Naruto in a new light and helping him gain more acceptance in the village.
Obviously Naruto would be one of the main characters as it's about him being the neglected child. Other characters from the original Naruto series like Iruka, Sasuke, or Sakura could also be main characters, depending on how they interact with Naruto in this fanfic.
Often, an original character that has Saiyan traits and is neglected in the Naruto world can be a main character. This character might be an outcast, like a young boy or girl with Saiyan - like strength but is shunned by the ninja community.