Kakashi is likely to be a main character too. As Naruto's former teacher, he could be guiding Naruto in the ANBU or having some sort of conflict with the ANBU higher - ups over Naruto's training and assignments.
Sasuke could be one of the main characters. Maybe he and Naruto have a new kind of relationship now that Naruto has retired from the ANBU. They could be working together to protect the village in a different way.
Maybe there are new characters created specifically for this fanfiction. They could be local inhabitants of the wet area, such as a water nymph - like character who has knowledge about the secrets of the place and either helps or hinders Naruto's journey.
Naruto's adaptation to the Anbu in this fanfiction could be a complex process. At first, he'd probably struggle with the strict discipline and the need for secrecy. But his previous experiences in tough situations would come in handy. He'd gradually understand the importance of stealth and how to use his skills in a more covert way. He might also learn from the more experienced Anbu members, and his own determination would drive him to become a valuable part of the Anbu.
Obviously, Naruto is one of the main characters. He brings his own set of skills, values, and personality to the ANBU setting. Another main character could be Kakashi. Since he has experience in ANBU and also has a connection with Naruto, he could play an important role in guiding Naruto or being involved in the same missions.
Sasuke is often a main character in such fanfictions. He could be Naruto's right - hand man, using his skills and intelligence to support Naruto's rule. Their relationship might be more complex, with Sasuke having to come to terms with Naruto's new position of power.
In addition to the ANBU, sometimes other Naruto characters play important roles. Naruto himself might cross paths with the ANBU in some fanfictions. His positive and determined attitude could influence the ANBU members. Or perhaps villains from the Naruto world might target the ANBU, making them the main antagonists in the story. So, it really depends on the specific fanfiction.
Obviously, the foxes would be the main characters. There could be an old, wise fox that acts as Naruto's mentor among them. It could have a name like Kyo, and it would be the one to teach Naruto the most important lessons.
Well, Sasuke is likely to still be a main character. His relationship with Naruto would be even more complex if Naruto is the son of Apollo. He might be jealous of Naruto's new powers or intrigued by them. Another could be Kakashi. He would play a role in guiding Naruto in understanding and controlling his powers inherited from Apollo.
Sakura might also play an important role. She could be one of the first to notice the changes in Naruto due to his divine connection. Maybe she tries to help him understand and control his newfound abilities, while also dealing with her own feelings for him in this new context.
The demihumans themselves would be important characters. There could be a demihuman leader who Naruto teams up with.