Obviously, Naruto is the main character as the story focuses on his growth of the nine tails. Kakashi could also be an important character. He might be the one guiding Naruto in controlling his new power, given his experience and knowledge. Sakura could play a role too, perhaps as a healer in case Naruto gets injured during his power - growth process.
The main characters would surely include Naruto. Another could be Tsunade. She, as the Hokage, has to manage the situation regarding Naruto's growth of the nine tails. She might be worried about the village's safety as well as Naruto's well - being. Shikamaru, with his intelligence, could also be involved, maybe coming up with strategies to help Naruto control his power and deal with any potential threats that come with the growth of the nine tails.
In addition to Naruto, the nine - tails itself can be considered a character in a sense. It has its own will and desires. Jiraiya might also be a significant character. He could have left some knowledge or training methods for Naruto to deal with the growing nine tails before his death. And then there's Sasuke. His actions and reactions to Naruto's growth of the nine tails can be a major part of the story.
One possible plotline could be Naruto's struggle to control the new power of the nine tails as it grows. He might face challenges from both within, like dealing with the increased aggression and power surges, and from without, such as enemies who are more afraid of his growing strength and target him more aggressively.
Sometimes, other characters from the Naruto universe might also play important roles in these fanfictions. For example, Kakashi could be involved as Naruto's mentor, giving advice on how to better communicate with the Nine - Tails. Or Sakura might be there to support Naruto in his journey of befriending the Nine - Tails, even if she doesn't directly interact with the tailed beast.
Obviously, Naruto himself would be the main character. Since it's about him growing tails, he's at the center of it all. The Nine - Tails would also play a crucial role. It's the source of his tail growth. And his friends like Sasuke and Sakura are likely to be important too. They'll probably be involved in Naruto's journey and how he deals with his new situation.
One possible reason could be some kind of strange chakra infusion. Maybe he comes into contact with a powerful and mysterious chakra source that mutates his body in a way that causes tails to grow. Another reason might be related to some ancient curse or prophecy. It could also be the result of a new jutsu he attempts that goes wrong and has unforeseen physical consequences.
Well, the Nine - Tails itself can be considered a popular 'character' in this type of fanfiction. It has its own personality and power, and many stories revolve around Naruto's interaction with it. Another character that might be popular is Gaara. Since he also has a tailed beast within him, there could be interesting interactions between him and Naruto regarding their respective tailed beasts, especially when it comes to the Nine - Tails.
Maybe through new jutsu or abilities. The fanfiction could give Naruto some unique power that allows him to communicate or negotiate with the Nine - Tails on a different level. He could use this power to show the Nine - Tails a different perspective, like the potential for harmony between them instead of constant struggle.
Yes, in some fanfictions, Naruto is often depicted as having a very special connection to the Nine - Tails that goes beyond the normal relationship in the original story. For example, he might be able to fully control the Nine - Tails' power in ways not shown in the canon, or they could have a mental bond that allows for unique forms of communication and cooperation.
Obviously Naruto and the Nine Tails themselves are extremely popular. Naruto is the main character of the original series and his relationship with the Nine Tails is central to the fanfiction. But other characters like Kakashi can also be popular in these fanfictions. Since he is Naruto's teacher, he might be involved in training Naruto when the Nine Tails' influence is changing Naruto's abilities. Also, Sasuke can be a popular character. His rivalry with Naruto can take on new dimensions in Naruto x Nine Tails fanfiction, especially if the Nine Tails' power gives Naruto an edge that Sasuke wants to equal or surpass.
At first, Naruto might be confused and a bit scared of the tails growing. But as he realizes he can control them, he becomes more confident. He starts using the new power associated with the tails in battles, which makes him more respected among his peers. His understanding of his own body and chakra also deepens.