Obviously Naruto himself would be a main character. He could be the one to face the challenges posed by the puppet sannin. Sasuke could also be involved as he has his own powerful skills that could be used against the puppet - related threats.
The main characters are the Sannin - Orochimaru, Jiraiya, and Tsunade. These three are central to the fanfiction as it explores their characters in a darker context.
Well, Naruto himself would be a central character of course. Sasuke could also be involved, as he often has a role in Naruto - related stories. Sakura might also play a part, perhaps she's the one who first discovers some clue about the fourth sannin.
Perhaps an old enemy of the Sannin resurfaces. This antagonist could see Naruto's attempts to follow in the Sannin's footsteps as a threat. They might target Naruto to get revenge on the Sannin or to prevent a new powerful force from emerging in the ninja world. For example, if there were a group that was once defeated by the combined efforts of the Sannin, they could try to stop Naruto before he becomes as strong as the Sannin were together.
One possible main antagonist could be a rogue ninja who was once an apprentice of Orochimaru and is jealous of Naruto's connection to the Sannin. This ninja might want to steal Naruto's new - found powers related to the Sannin. Another could be a hidden organization that sees Naruto as a threat because of his potential with the Sannin's abilities. They fear he will disrupt their plans for power in the ninja world.
Sasuke could be one of the main characters. Given his power and brooding nature, he could be drawn into the world of human puppets and end up in a harem situation. Maybe a puppet master sees his potential and tries to recruit him, and along the way, a group of female characters start to form a harem around him. Another possibility is Naruto himself. His kind heart and strong will could attract a harem while he's exploring the new concept of human puppets. He might try to use the puppets for good, and in the process, gather a group of followers who become part of his harem.
One idea could be that the puppet sannin create a super - powerful puppet that requires the combined skills of Naruto and his friends to defeat. Maybe this puppet has the ability to absorb chakra and uses it to grow stronger.
Often, the main characters include the Puppet King himself. He is usually a complex character with a deep backstory. Then there could be a female ninja who is initially an enemy of the Puppet King but later realizes his true intentions and joins forces with him. Another important character could be an old master who has knowledge about the ancient puppet arts and guides the Puppet King in his journey.
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.
The main characters are, of course, the members of the Phoenix Sannin. They are usually well - known characters from the Naruto series who are given a new spin in these fanfictions.
It's hard to say for sure without reading it, but likely there will be three main characters who are the new sannin. They could be students of the original sannin or completely new characters with no direct connection but similar in their skills and importance.