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 plotline could be Naruto's struggle to control his new tails. He might face challenges in his daily life, like accidentally using too much power during training. Another could be how his friends react to his new form. Some might be scared at first, while others try to help him adapt.
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.
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.
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.
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.
A major theme in 'tails grows up fanfiction' is facing challenges. As Tails gets older, he has to deal with more difficult situations. This could be things like fighting stronger enemies or dealing with complex moral dilemmas. Also, the theme of growth in power and abilities is quite common. We see Tails learn new skills and become more powerful as he grows up.
Power is a common theme. Since the ten - tails is an extremely powerful entity, many fanfics explore how characters deal with that power, whether it's Naruto trying to control it or villains seeking to harness it.
One great 'Naruto Nine Tails Fanfic' is 'The Nine - Tailed Fox's Legacy'. It delves deep into the relationship between Naruto and the Nine - Tails, exploring how they grow together and face new challenges. Another interesting one is 'Nine Tails' Power Unleashed' which focuses on Naruto's journey to fully master the power of the Nine - Tails in unique and unexpected ways.
One great Naruto Three Tails fanfic is 'The Tale of the Three - Tailed Beast's Bond'. It focuses on a young ninja who discovers a unique connection with the Three - Tails. The story is filled with exciting battles and character development as the protagonist tries to understand and control the power of the Three - Tails.