Sometimes, characters like Kakashi might also play a role. As Naruto's mentor in a way, Kakashi could notice the strange relationship between Kaguya and Naruto and try to give Naruto some advice, which in turn affects how Kaguya and Naruto's relationship develops.
Sasuke can also be involved in some cases. Since he and Naruto are often together in many situations, Sasuke's presence might add some complexity to the relationship between Kaguya and Naruto. For example, Sasuke might be suspicious of Kaguya's intentions towards Naruto at first.
In addition to Naruto and Kaguya, Hinata can be a major character in some fanfictions. Her relationship with Naruto can add an interesting dynamic, especially if the story involves Naruto's growth and how Hinata supports him in the face of the challenges related to Kaguya. Also, characters from the Ootsutsuki clan other than Kaguya, such as Hagoromo and Hamura, might be included to provide more backstory and context to the whole Kaguya situation.
Naruto's chakra and Kaguya's chakra are both very powerful but in different ways. Naruto's chakra is more about positive energy and his willpower, while Kaguya's is ancient and more raw. If they were to interact, Naruto's positive chakra might start to influence Kaguya's, perhaps making her more empathetic. For example, when they fight side by side, Naruto's chakra could boost Kaguya's attacks in a way that gives them a more moral or just - focused energy.
Sasuke would most likely be involved. Since he has a strong bond with Naruto, he would sense something is wrong and try to free Naruto from Kaguya's control. Sakura might also be part of it as she cares about Naruto and would be determined to save him. Kakashi could play a role too, using his wisdom and experience to figure out a way to break Kaguya's hold on Naruto.
In some fanfictions, Kaguya might be reincarnated in a different form and meet Naruto during his early days of training. Their encounter could be filled with mystery as Naruto senses a strange power in her, and this could lead to a love story that also involves Naruto learning about ancient powers from her.
Obviously Naruto would be one of the main characters since it's about him being the heir of Kaguya. Sasuke could also be a main character as Naruto's rival and friend. Sakura might be involved too, as she is part of Naruto's team. And there could be other characters like Kakashi, who would play a role in guiding Naruto as he discovers his new identity. There might also be some new characters introduced that are related to Kaguya's past, who either help or hinder Naruto on his journey.
Kakashi can be a popular character too. His wisdom and combat skills can be useful in fanfic scenarios involving Kaguya. He could be a mentor figure to Naruto, guiding him in dealing with Kaguya - related threats or helping to decipher ancient ninja knowledge related to Kaguya.
At first, it might be one of suspicion. Naruto would be wary of Kaguya's power and her past as a sort of villain. But as they face challenges together, they could start to respect each other. Naruto might see the good in Kaguya that was buried under her past actions, and Kaguya could come to admire Naruto's tenacity and his strong sense of justice.
The two main characters would be Naruto and Kaguya for sure. However, depending on the plot, other important characters from the Naruto universe could make an appearance. It could be the tailed beasts, as Naruto has a strong connection with them. If the situation involves something related to the chakra of the ninja world, the tailed beasts might be involved in some way. Or perhaps characters from the Otsutsuki clan could be relevant if Kaguya's actions are related to her clan's history or powers.
Obviously, the reincarnation of Kaguya would be a main character. Naruto or Sasuke could also be main characters as they might be involved in the discovery of the reincarnation and have to decide whether to trust her or not.
Obviously Naruto and Kaguya are the most important. Naruto as the one to free Kaguya, and Kaguya as the one being freed.