Often, there might be an antagonist, perhaps a powerful and corrupt ninja from a large and influential clan. This antagonist might be after the same things as the Ronnin, like a powerful artifact or a secret technique, but for more malicious reasons. And then there could be a wise old sage - like character who gives the Ronnin advice from time to time.
Well, of course, the Ronnin himself is a main character. He's likely to be a highly skilled and experienced ninja. There could also be a young, aspiring ninja who looks up to the Ronnin and follows him on his adventures. This young ninja could be from a small, unassuming village.
Another possible plotline is that the Ronnin discovers a secret connection between different ninja villages. He uncovers a conspiracy that certain powerful individuals in these villages are trying to start a war for their own gain. The Ronnin then has to use his wits and combat skills to expose the conspiracy and prevent the war, all while being hunted by those who want to keep the secret.
Naruto himself could be one. Imagine him getting a hold of a legendary weapon that enhances his already powerful Rasengan. Sasuke is also a likely candidate as his quest for power could be intertwined with a legendary weapon.
The main characters are usually Orochimaru, Jiraiya and Tsunade. Orochimaru is known for his dark and ambitious nature, always seeking more power. Jiraiya is the pervy sage but also a powerful ninja with great knowledge. Tsunade is the strong - willed female Sannin with amazing medical and combat skills.
Since it's a Naruto - related fanfic, Naruto is likely to be one of the main characters. There may also be characters related to the Shinigami, though without reading it, it's hard to say exactly who they could be. Maybe the Shinigami itself could be a major character, or other characters from the Naruto world who get involved in Naruto's encounter with the Shinigami.
Naruto himself is often a main character. Given his adventurous nature and his own experiences with summoning, it's easy to imagine him getting involved with a legendary summoning contract in fanfiction. He might discover it during one of his quests or it could be passed down to him in a mysterious way.
It could be a rogue samurai who has turned against his own clan and wants to destroy all ninja - samurai relations. He might have a powerful sword that can nullify ninjutsu.
It could be Team Rocket - style villains who want to capture Naruto because they think he has some special power that can help them control Legendary Pokemon. They might try to trick him or set traps for him.
Well, obviously Ash is the main character. But in many such fanfictions, his Pokemon like Pikachu are also very important. They often play a crucial role in helping Ash handle his godlike or legendary powers. Sometimes, other trainers like Misty or Brock might also be part of the story, providing support or adding some comedic relief.
Probably the children of various legendaries such as Articuno, Zapdos, and Moltres. They each might have distinct personalities based on the nature of their parent Pokemon. For instance, the child of Articuno could be calm and collected, while the child of Zapdos might be more impulsive. And there could also be a trainer or two who befriend the children and help them on their journey.
Often, characters who are hybrids of different bloodlines tend to be popular. For instance, a character who is part Uzumaki and part Senju could be really interesting as they would potentially have access to both the Uzumaki's life - force and sealing abilities, as well as the Senju's natural affinity for all types of ninjutsu.