Most likely Naruto would be one of the main characters. Since it's in the title, he's probably the central figure in the story.
In addition to Naruto, there could be other characters from the Naruto universe. For example, his friends like Sasuke or Sakura might also play important roles. They could either be with Naruto in the abyss, helping him on his journey to become the king, or they could be back in the normal world and Naruto has to find a way to get back to them while dealing with the challenges in the abyss.
Most likely Naruto himself would be a main character since it's about him being the god of the abyss. Other characters from the Naruto series could also be involved, like Sasuke. Sasuke might be in a position where he has to either oppose or support Naruto's new role as the god of the abyss.
Harry Potter is the main character. However, depending on the direction of the fanfiction, it could include other important characters. Neville could be one of them, if the story is about how Hogwarts students support Harry in his new role. There could also be antagonists from the abyss itself, like a powerful dark entity that wants to overthrow Harry. And what about Luna? She might have some unique insights or abilities that are useful in the context of the abyss, so she could also be part of the story.
The main character is surely the dark king of the abyss. But there could be other characters like those who oppose him. Maybe there are brave adventurers who dare to enter the abyss to face the king. And perhaps there are some dark creatures that are either allies or enemies of the king within the abyss.
Obviously, the King of the Abyss is a main character. He is the central figure around which the story revolves. There might also be a hero or heroine who is either trying to defeat the king or help him, depending on the plot of the fan fiction.
Sasuke is often a main character in such fanfictions. He could be Naruto's right - hand man, using his skills and intelligence to support Naruto's rule. Their relationship might be more complex, with Sasuke having to come to terms with Naruto's new position of power.
Well, Naruto is likely to be one of the main characters since it's in the title. He would probably be involved in the interactions with the Arrancar King. Sasuke might also be there, as he is a major part of the Naruto story. And of course, the Arrancar King himself would be a central figure, driving the plot forward with his actions and motives.
Naruto is surely a main character. Additionally, there might be some original characters created specifically for this fanfiction. For example, there could be a powerful spirit or a guide in the world of the dead who helps Naruto understand his new powers. And characters like Kakashi could also be involved, perhaps providing advice or assistance to Naruto as he deals with the implications of being the king of the dead.
Since it's a Naruto - related fanfiction, Naruto is bound to be a main character. Regarding the Shadowkhan, there could be a powerful Shadowkhan leader or a group of Shadowkhan who play important roles. Maybe there's a Shadowkhan character who becomes Naruto's ally or rival in his journey to be the king. And of course, Naruto's sensei, Kakashi, might also make an appearance to guide Naruto in this new and strange situation.
Obviously, Naruto is one of the main characters as it's about him being the king of foxes. The nine - tailed fox, Kurama, is also likely to be a central figure as it's closely related to Naruto's new title.
Naruto is surely the central character in this fanfiction. Besides, characters like Kakashi could be involved. He might be Naruto's mentor in controlling and understanding his new 'king of fire' powers. And villains from the Naruto universe might also appear. They could be envious of Naruto's power and try to take it away or use it for their own evil purposes. So, a variety of characters from Naruto's world could be main characters in this fanfiction depending on the plot.