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.
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.
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.
The main characters are Kaguya and Miyuki. Kaguya is a complex character with her own pride and feelings. Miyuki is also an important character, and their relationship forms the core of the story.
Kaguya is most likely a central character. Additionally, there could be characters that play important roles in her life. For example, there might be a love interest, a mentor figure, or an antagonist. These characters would interact with Kaguya and help drive the story forward through their relationships and actions.
Kaguya herself is a great character to focus on. Her complex personality, with her pride and growing feelings for Miyuki, offers a lot of room for exploration. Miyuki is also a good choice. His intelligence and his own internal struggles while dealing with Kaguya make for interesting stories. And Chika, with her bubbly and sometimes mischievous nature, can add a lot of humor and unexpected twists to a fanfic.
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.
The main characters in the 'Princess Kaguya' original story are Princess Kaguya herself, the bamboo cutter who discovers her, and his wife who helps raise her. There are also the various suitors who come to court Princess Kaguya.
It's also possible that some non - human characters could be part of the harem. For example, some of the tailed beasts in human form (if that's possible in the context of the fanfiction). They would bring their own set of powers and a very different kind of relationship with Naruto compared to the human characters in the harem.