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.
In many fanfics, the main character with the red rinnegan might be a self - made ninja. They could be someone who has had to struggle to understand and control their new power. It could also be a character who is on a quest to find out more about the red rinnegan's origin. They might interact with other Naruto characters like Naruto himself, Sasuke, or Kakashi, and these interactions would shape their journey.
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.
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.
Obviously Naruto would be one of the main characters. He is the central figure around whom the'seeing red' concept is likely based. There may also be his friends like Sasuke and Sakura, as they are often involved in Naruto's storylines and could be part of the events that make him see red.
Obviously, Naruto is likely to be one of the main characters. There might also be some original characters created for this fanfic who are native to the area of the red sand. And perhaps some of Naruto's friends from the original series like Sasuke or Sakura could also make appearances depending on the plot.
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.
Naruto could also be the main character in such a fanfic. Even though he doesn't have the Rinnegan in the original, in a fan - created story, he could be involved in a plot where he has to deal with the secrets of the Rinnegan. For example, he might be trying to help a friend who has the Rinnegan and is in danger because of its secrets.
Sasuke is also frequently a main character in these fanfics. Imagine if Sasuke had red eyes instead of his Sharingan. It could completely change his character arc. He might be even more of an outcast or have a different set of powers that make him both feared and respected. And then there are the Akatsuki members. Some fanfics might make one of them have red eyes and explore how that changes their role within the organization and in the overall Naruto story.
Maybe the red hair is a result of a genetic experiment or a curse that is linked to the Rinnegan. In the fanfic, it could be explained that an ancient ninja tried to combine the power of the Rinnegan with a powerful fire - type jutsu. The experiment went wrong in a way that gave the character red hair and also altered the Rinnegan's abilities. For example, the Rinnegan might be able to absorb and redirect fire - based attacks more effectively because of this connection.
One interesting concept could be the unique powers associated with the red rinnegan. Maybe it has powers that are different from the regular rinnegan in terms of combat abilities, like being able to manipulate a new type of energy or having a special form of genjutsu. Another concept might be how the character who possesses it interacts with other characters in the Naruto universe. They could either be feared or revered, and this would change the power dynamics among different ninja clans.
Sasuke could also be a main character. Given his own powerful eyes and his connection to Naruto, he could be involved in a story about the black rinnegan. Maybe he has to team up with Naruto again to deal with a powerful enemy with the black rinnegan.