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.
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.
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.
In certain fanfiction, Naruto is indeed the Ten - Tails. This could be a really cool concept as it combines Naruto's already powerful nature with the ultimate power of the Ten - Tails. It might lead to stories where Naruto has to struggle with the dark side of such power, or he could use it to bring about a new era of peace in the ninja world. Maybe he would have to face off against old enemies who are now even more afraid of his new - found strength.
One great 'tails grows up fanfiction' story is 'Tails' New Adventures'. It shows Tails as he matures and faces new challenges in a really engaging way. Another one could be 'Growing Up Tails' which focuses on his growth not just in skills but also in his relationships with other characters.
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.