Voldemort could also be a main character in a different way. Instead of just being the villain, he could be a complex figure who is also in the 'game' for the master of death's power, but with his own motives and backstory that are explored more deeply in the fanfiction.
Perhaps it could be a character like Bran Stark. He has the power of the Three - Eyed Raven, which gives him access to a vast amount of knowledge and some control over time and events. It's not a far stretch to think that he could develop into a Master of Death in a fanfiction.
In this fanfiction, Naruto is the central figure. However, other important characters could include Kakashi. Kakashi, being Naruto's former teacher, would be interested in Naruto's new power and would try to guide him on how to use it responsibly. There might also be new characters introduced, like some sort of death spirit or deity that has a connection to Naruto's power. These new characters could play a role in either helping or hindering Naruto as he tries to master his role as the master of death.
It could be an ancient shinobi who has mastered some kind of death - related jutsu and has ascended to a higher state of being. This shinobi might have been forgotten in the history of the ninja world.
Obviously, the 9th Doctor is one of the main characters. He's a Time Lord with his own set of values and a strong sense of justice. He travels through time and space in the TARDIS.
In this 'Game of Thrones' fanfiction about the Master of Death, the story might revolve around how this character's power affects the political landscape of Westeros. Since death is a major factor in the power struggles in the original series, having someone who can manipulate it could change everything. Allies would seek this person's favor, and enemies would fear them. They could use their power to revive important leaders or soldiers, tipping the balance of power in battles and wars.
One possibility could be Doctor Strange. With his vast knowledge of the mystic arts, he might stumble upon the power of the 'Master of Death'. His already existing powers over magic and the multiverse could be a stepping stone to this even greater power.
Typically, Naruto and his friends are main characters. However, the game master takes on a significant role. They could be a mysterious figure that the characters are initially wary of. Characters like Kakashi might also be prominent as they are important mentors in the Naruto world. The game master might disrupt the normal flow of things, making these characters react in different ways and creating new story arcs.
There could be a clever and shrewd character who is very good at manipulating the rules of the game. Maybe a mentor - like figure who helps the main character understand the nuances of the game. And of course, there would likely be rivals, those who are equally competitive and aiming for the top position in the game.
Since I don't know the 'game master story' in detail, I can't say for sure who the main characters are. But it could be the game master themselves and the players in the game they are running.
Obviously, Harry Potter himself. He's the one who has become the Master of Death. Hermione and Ron would also be important. They'd be there to support Harry, and also to question his new power and the moral implications. Dumbledore might also play a role, perhaps as a guide from beyond the grave, since he was always so interested in the Deathly Hallows and what it means to be the Master of Death.