Sometimes there are also human characters. For example, a young wizard or warrior who befriends the dragon and kyuubi. This human character might act as a mediator between the two powerful creatures, helping them understand each other better. In a story, this human could be on a quest to learn magic from the dragon and kyuubi, and in the process, they all form a deep bond.
The dragon and the kyuubi are obviously the main characters. The dragon is often depicted as strong, with the ability to fly and breathe fire or other elements. The kyuubi, on the other hand, is known for its fox - like appearance and powerful magic, especially illusion - based magic.
Another possible main character could be an ancient spirit that has a connection to both the dragon and the kyuubi. This spirit might be the one who initially brought them together or set in motion the events that led to their encounter. It could have its own motives, such as restoring balance to the magical world that the dragon and kyuubi inhabit.
Often, you'll find dragon - riders as main characters. They are the ones who have a special bond with the dragons. There are also powerful mages who might interact with the dragons, either as friends or foes.
The girl herself is of course a main character. She's usually brave and curious. The dragon is also a key character, often with unique abilities like breathing fire or having the power of invisibility. There might also be a mentor figure, perhaps an older dragon rider who teaches the girl how to ride and care for her dragon.
Obviously, the Dragon King himself is a main character. He is usually depicted as a powerful and complex figure, with his own goals and desires for the kingdoms. Daenerys Targaryen is also often present, either as an ally or a rival to the Dragon King, given her own connection to dragons in the original series.
Sometimes, characters from the Forbidden Forest might play important roles. Centaurs, for instance, with their deep knowledge of magic and nature. They could either be allies or foes to the dragon master. And then there are the dragon handlers at Hogwarts, like Hagrid. He might be involved in helping the main character on their journey to becoming a true dragon master, teaching them the basics about dragon care and behavior.
Naruto is usually a main character as he has a connection with the Kyuubi. Since he already has the Kyuubi sealed within him, it's logical that he would be the one to potentially unlock or develop the Kyuubi doujutsu. Sasuke might also be involved. His rivalry with Naruto could lead to interesting interactions in relation to the doujutsu, either trying to obtain it for himself or learning to counter it.
Well, Naruto is the top one. As the Jinchuriki of the Kyuubi, he's at the center of it all. But don't forget about Jiraiya. He has a huge influence on Naruto and his past self could be a key factor in any time travel plot. And then there's Hinata. Her relationship with Naruto could be explored in new ways if time travel is involved. For example, she could help Naruto in the past in ways she didn't before, adding a new dimension to their relationship.
Sasuke often appears. Since he is Naruto's rival and friend, his relationship with Naruto might be affected by the Kyuubi's influence on Naruto. Kakashi could also be there, as Naruto's teacher, he might notice the changes in Naruto due to the Kyuubi's new role in his life.
Sometimes, other gods and goddesses make an appearance. Athena might show up to collaborate with Hephaestus on a project. Their interactions can add a lot of depth to the story. For example, they might have different ideas on how to create a new weapon, and their debates and compromises can be really interesting to read about. And of course, there could be a villainous god who tries to steal Hephaestus' secrets related to his snaps.
Vegeta is often a main character in dark saiyan fanfiction. Given his prideful and sometimes volatile nature in the original series, it's easy to imagine him being swayed towards the dark side. Writers might explore what would happen if he fully embraced his darker impulses.
There are also cases where historical figures themselves are main characters in a different context. For example, a famous historical hero who is taken out of their own time and dropped into a future or an alternate past. They have to adapt to the new situation while also dealing with the fact that they are out of their own time.