Obviously Naruto himself, as he is the one turning into the Nine - Tailed Fox. Sasuke would also likely be involved. He might be the one to try and reach Naruto when he is in his fox - form, either to fight him or to bring him back. Sakura too, she could use her medical ninjutsu to try and help Naruto if he gets injured during the transformation.
The main characters are most likely Naruto, since he has the Nine - Tailed Fox. Sakura could also play an important role as she is one of Naruto's closest friends and a strong kunoichi. Then there's Jiraiya. He has a deep connection with Naruto and his training might be crucial in helping Naruto handle the Nine - Tailed Fox's fury. Additionally, characters like Tsunade might be involved in the political aspects related to the situation with the Nine - Tailed Fox.
Obviously, Naruto himself is the main character in this situation. His friends like Sasuke and Sakura are very likely to be involved. They would be the ones taking care of him, trying to figure out how to turn him back to normal. Kakashi Sensei would also probably be there, guiding them and trying to keep Naruto safe.
The villains in the Naruto universe could also play a role. Maybe the one who turned Naruto into a fox was an enemy seeking revenge or trying to gain some advantage. Orochimaru, for example, could be behind it, hoping to use Naruto's new form for his own evil experiments. And then Naruto, in his fox form, would have to fight against Orochimaru's minions to protect himself and his friends.
When it comes to Sasuke, the Nine - Tailed Fox Faunus could be seen as a threat at first. But as the story progresses, they might find common ground in their pursuit of power and their desire to protect their own kind. The faunus could show Sasuke a different side of power, one that is more in tune with nature.
His relationship with the villagers could become more complex. Some might see him as a greater threat, while others who have come to know Naruto as a hero might be more accepting. His transformation could also lead to new alliances as some might want to use his power for their own purposes, which Naruto would have to navigate carefully to protect his relationships.
Obviously Naruto himself is the main character as he is the one who turns into a baby fox. His friends like Sasuke and Sakura would also be involved. Sasuke might be initially confused but then determined to help Naruto. Sakura would probably be more nurturing and try to take care of the baby fox Naruto. Kakashi Sensei could also play an important role, using his wisdom to figure out what happened to Naruto and how to reverse it.
One interesting plot could be Naruto fully transforming into the Nine - Tailed Fox during a battle against a powerful enemy. His friends are initially shocked but then try to find a way to bring him back to his senses.
One possible plot twist could be that Naruto loses control over his transformation into the Nine - Tailed Fox in a completely unexpected situation. Maybe during a friendly spar with his friends, and this causes them to fear him in a new way. It would add a whole new layer of drama to their relationships.
Obviously Naruto himself is the central character. Sasuke would also likely be involved. Given their history and Sasuke's own complex relationship with Konoha, he might side with Naruto or at least be a key figure in the events. Kakashi too, as he has been Naruto's teacher and is an important figure in Konoha. He could be caught in the middle, trying to understand Naruto's motives and also trying to uphold Konoha's stability.
It could be Orochimaru. He is always interested in powerful forces and might see Naruto's release of the Nine - Tailed Fox as an opportunity to gain that power for himself.