One interesting character could be a long - lost relative of Naruto who also has a connection to the Nine - Tails. This relative could have been in hiding all this time and now comes forward to help Naruto understand and master the power. Another could be a powerful sage from a far - off land who has knowledge about the Nine - Tails that no one in the Hidden Leaf Village has. And perhaps a rogue ninja who was once experimented on with tailed - beast chakra and now has a vendetta against Naruto for having the Nine - Tails' power.
Well, we could add a character who is a spirit of the Nine - Tails' past. This spirit could have memories of how the Nine - Tails was created and could guide Naruto in using its power more effectively. Also, a young prodigy from another village who is jealous of Naruto's power and tries to steal it for themselves. And a mysterious figure who has the ability to nullify the Nine - Tails' power temporarily, which would create a great challenge for Naruto in his adventures.
In a 'naruto with nine tails power fanfiction', an interesting character could be an ancient warrior whose soul was trapped within the Nine - Tails' chakra. This warrior could have unique fighting techniques that Naruto could learn. Another character could be a girl who has a special ability to calm the Nine - Tails' rage, which would be very useful for Naruto. And there could be a group of outcasts who worship the Nine - Tails as a god and they either try to help or hinder Naruto depending on their own motives.
A character who is a descendant of an ancient clan that once had a connection to the Nine - Tails could be added. This character could have some knowledge about the Nine - Tails that was lost over time and could help Naruto better understand his power.
You could create a character who is a descendant of an ancient clan that has a special connection to the Nine - Tails Chakra. This character could be a rival to Naruto at first, thinking that Naruto doesn't deserve the power of the Nine - Tails. Another option is a mysterious healer who can manipulate the Nine - Tails Chakra in a unique way to heal even the most severe wounds.
One idea could be Naruto using the Nine - Tails' power to travel to different dimensions and encounter versions of his friends with different personalities. Another could be him training with the Nine - Tails to create new and unique jutsu that no one has ever seen before. Maybe he uses the power to go back in time and change certain events in the ninja world's history for the better.
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.
One interesting type of original character is a wandering ninja who has been on a quest to find the true nature of the Nine - Tailed Fox. This character might cross paths with Naruto and share their knowledge or experiences. Another is a young girl who can communicate with the Nine - Tailed Fox on a spiritual level, which causes Naruto to be both intrigued and a bit jealous. And then there are the members of a secret organization that wants to harness the power of the Nine - Tailed Fox for their own dark purposes, and they create original characters within this organization who have unique abilities and motives.
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.
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.
A great plot in 'naruto of the nine tails fanfiction' could be that Naruto has to face an enemy who has a similar power to the Nine - Tails. He then has to rely not only on his own skills but also on his bond with the Nine - Tails to overcome this new threat. The Nine - Tails might offer him advice or even share some of its hidden memories to help Naruto in the fight. This could lead to Naruto growing stronger both as a ninja and in his understanding of the Nine - Tails.
There could be a fanfiction where the Nine - Tails takes on a more human - like form temporarily. In this form, it accompanies Naruto on a journey outside of the Hidden Leaf Village. They face various challenges together, and along the way, the Nine - Tails starts to understand human emotions better, and Naruto learns more about the ancient history of the tailed beasts from the Nine - Tails itself.
Hinata is another character that can be included. Her love for Naruto can be intertwined with the story of the Nine Tails. For example, she could be worried about the toll the Nine Tails' power is taking on Naruto and tries to find ways to support him both emotionally and in battle.