Well, Naruto is the mainstay, no doubt. But there could be original characters created specifically for these fanfictions. These original characters might be related to the Darkness Dragon's origin or the history of the red eyes. They could be antagonists, trying to take the power from Naruto, or they could be allies, helping him master it. Additionally, characters like Jiraiya might be involved. He could be the one who initially discovers Naruto's red eyes and the connection to the Darkness Dragon and then sets out to help him understand and control this new and dangerous power.
Obviously, Naruto is a main character in these fanfictions. He's usually the one who gets the red eyes and the connection to the Darkness Dragon. Sasuke might also be involved, either as a rival who is intrigued by Naruto's new power or as a partner in exploring the secrets of the red eyes. Sakura could be there as well, perhaps trying to help Naruto control the power so it doesn't consume him. And sometimes, characters from other clans or villages might make an appearance, like Gaara, who might be interested in the power because of its potential threat or its possible use in protecting his own village.
Sasuke is another character who might be involved as a main character in such fanfictions. If the red - eyes concept is related to the Sharingan or some new form of eye power, Sasuke's connection to the Uchiha and their eye powers makes him a likely candidate to be part of the story. The red - eyed character might be his ally or rival.
Sasuke could also be involved. In many Naruto - related fanfictions, Sasuke is often part of the story. He might be initially skeptical or even jealous of Naruto's connection with the dragon soal. His rivalry with Naruto could take on new dimensions with the addition of the dragon element.
Sasuke is a very popular character in such fanfictions. Since he already has the Sharingan in the original Naruto series, fanfic writers often expand on his abilities with the Sharingan or give him new eye - related powers. They might explore how he further masters his Sharingan or what would happen if he obtained another type of eye power.
Saber might also be a part of it. Her loyalty to Shirou could lead her to stand by his side when dealing with the Book of Darkness. She can use her combat skills to protect Shirou from any threats that come their way while he is exploring the book's mysteries.
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.
Ruby Rose is often involved. Given her role as a protagonist in RWBY, she could be a key figure in the fanfiction related to the God of Light and Darkness. Maybe she is the one who has to find a way to restore the balance between the two forces.
Obviously, Harry Potter is the central character. But other characters like Hermione and Ron are often involved too. They might have different reactions to Harry's new 'King of Darkness' persona. Dumbledore could also be a major character, as he might try to guide Harry or figure out how to deal with this new development. Voldemort could also play a significant role, perhaps seeing Harry as a more formidable opponent or a potential ally in his own dark schemes.
Naruto is sure to be a main character. In addition to him, there could be a mysterious figure who is the keeper or the source of the dragon of darkness. This character could be both a threat and a potential ally. Naruto's sensei, Kakashi, might also play a role as he often guides Naruto. And there could be a group of shinobi from another village who are also interested in the dragon of darkness, either for good or bad reasons. Their interactions with Naruto and his friends would add more depth and complexity to the story.
Obviously Naruto himself is a main character. He is the one with the pacemaker so he is central to the story. Sasuke might also be involved. He could be initially skeptical of Naruto's new situation with the pacemaker but then come around to help. Sakura too, as a medical ninja, she could play a crucial role in understanding and perhaps even trying to modify the pacemaker.
Naruto is almost always a main character in 'naruto dice fanfiction' as the story is based on the Naruto universe. His friends like Sasuke and Sakura also often appear. Sasuke's cool and brooding nature can add an interesting dynamic when combined with Naruto's more upbeat and impulsive personality in stories related to the dice.