The main characters are, of course, Captain N and the Game Master. Captain N is often portrayed as a heroic figure, coming from our world but having the ability to enter and interact with various game worlds. He uses his wits and some special powers to solve problems in those worlds.
Maybe they get trapped in a game world where the rules keep changing randomly. They then have to figure out how to adapt and find a way out while also dealing with the strange inhabitants of that world. This could lead to some really interesting character development and interactions between them.
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.
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.
Well, obviously Captain Darko is a main character. He's the central figure around which the stories revolve. There might also be his trusty first mate, someone who has been with him through thick and thin. This character could provide support and sometimes comic relief.
Obviously, Percy is the main character as he is the captain. But depending on the type of fanfiction, there could be other important characters. For example, if he's a sports captain, his teammates would be significant. They could have different personalities and skills that interact with Percy's leadership. If he's a ship captain, his crewmates, like the navigator or the first mate, would play important roles in the story.
Well, 'n' and Ash are the main characters. Ash, with his passion for Pokémon and his goal of becoming a Pokémon master, and 'n' who has his own connection to Pokémon. In the fanfiction, their travels bring out different aspects of their personalities. For example, Ash's determination might inspire 'n' at times, and 'n's unique perspective on Pokémon could make Ash see things in a new way.
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.
A mysterious figure from the captain's past could be a key character. Maybe this person was involved in the events that led to the promise in the first place. They could reappear and try to either help or hinder the captain in keeping the promise. Also, a young crew member who looks up to the captain could be important. Their naivete and admiration could add an interesting dynamic as they try to understand the weight of the captain's promise.
An otherworldly being who has a special connection to the concept of infinity. This being might be trying to either prevent or accelerate the 'n meets infinity' situation, and their actions drive the story forward. Maybe they are from a dimension where infinity is a tangible and manipulable force.