There could be an original character created specifically for this fanfic. This character might be the one who actually has the eternal rinnegan and is central to the story. Their backstory, motives, and relationships with other Naruto characters would drive the plot forward.
Madara Uchiha is another character who could be a main focus. Given his connection to the Uchiha clan and his own powerful Sharingan - related abilities, fanfiction could expand on his story by giving him the eternal mangekyou rinnegan and exploring what he would do with this even greater power.
Sasuke Uchiha is often a popular choice. Given his natural Uchiha abilities and his connection to the Rinnegan in the original Naruto story, he's a likely candidate in fanfiction. His brooding nature and quest for power make him a great character to explore further with the added power of the Eternal Rinnegan.
It's hard to say without actually reading the fanfic, but it could be a new character who has the red rinnegan. Maybe this character is an outcast from a well - known ninja clan because of their unique eye power. Or it could be an existing character from the Naruto series who somehow acquires the red rinnegan and then becomes the focus of the story.
Given that it's a Naruto - based fanfic, Naruto Uzumaki might be one of the main characters. He could be the one who discovers the existence of the eternal guardians. Sasuke could also be involved, perhaps in a conflict or alliance with these guardians. And if the fanfic focuses on the shinobi world as a whole, characters like Kakashi or Tsunade might also play important roles. As for the eternal guardians themselves, they could be unique characters with their own personalities and powers.
In the 'naruto the eternal king fanfic', Naruto is the central character. However, his friends from Team 7, including Sasuke and Sakura, are probably going to be main characters too. Additionally, characters from other villages such as Gaara from Sunagakure might be involved. They could be important in terms of international relations in the story where Naruto is the eternal king. Also, there might be new characters introduced who are specifically related to the concept of Naruto being an eternal king, like advisors or enemies who only exist because of his new status.
Another possibility is that it could involve Naruto as a main character even though he doesn't have the Sharingan. His relationship with the character having the Eternal Sharingan could be explored. Maybe Naruto helps the character come to terms with their power or they team up to face a common enemy. This would add an interesting dynamic to the fanfic as Naruto's ideals and the power of the Eternal Sharingan collide and combine in different ways.
Since it's related to Naruto, Naruto himself is most likely a main character. He is the one becoming the eternal emperor.
Perhaps a plot where the character with the eternal rinnegan has to face an extremely powerful enemy that can only be defeated by mastering all the abilities of the eternal rinnegan. This could involve a series of intense battles and strategic planning. For example, they might need to figure out how to combine different rinnegan - related techniques to counter the enemy's attacks.
The main character is Naruto for sure. But in addition to his regular comrades, there could be new characters introduced specifically related to the gamer aspect. For example, there might be a character who acts as a sort of 'game master' in the Naruto world, guiding Naruto in understanding and using his Rinnegan in a game - context. And his friends would still be important, with Sakura perhaps being the one to heal him during his more dangerous 'game - like' battles with the Rinnegan powers, and Kakashi could be a mentor figure in how to combine the Rinnegan and gamer strategies.
There can also be original characters created specifically for the fanfic. These characters might be gamers who somehow get transported into a world where the rinnegan exists and they have to figure out how to use it within the game mechanics. They bring a fresh perspective to the story.