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.
Jon Snow is often associated with the ice dragon in many fanfictions. Given his connection to the North and his role in the fight against the White Walkers in the original series, it makes sense for him to be linked to such a powerful and ice - related entity like the ice dragon.
Arya Stark is also a common character paired with the Winter King in fanfiction. Arya's skills as an assassin could be useful to the Winter King's cause. She might be sent on secret missions by him or they could form a bond over protecting the innocent in the North.
Obviously, the King Beyond the Wall himself is a central character. He's often depicted as a strong and fearless leader. Then there are the wildlings who follow him. Characters like Ygritte are also quite common, as she was an important figure among the wildlings in the original series. And sometimes, characters from the Night's Watch get involved too, as they are in direct conflict or potential alliance with the King Beyond the Wall.
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 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.
Well, the Gold King is obviously one of the main characters. There might also be his advisers, like an old and wise man who has been with him since his early days. His enemies are also important characters, perhaps a rival king or a powerful sorcerer who wants to dethrone him.
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.
The dragons themselves can be considered main characters in a way. Each dragon might have its own personality and abilities. And then there are the members of the harem. They could be people from different regions of Westeros, some with noble backgrounds, some commoners, all brought together by their connection to the dragons and the rider. They interact with each other, have rivalries and alliances, which drives the story forward.
In some cases, the Mountain might be paired with other powerful warriors from different regions in fanfiction. For example, a character from Essos who has a unique fighting style. This could lead to some epic battles and exchanges of combat techniques in the fanfiction.
Arya Stark is often paired with tigers in fanfiction. Given her independent and fierce nature, it makes sense that she would be associated with such a powerful animal. Another could be Jon Snow. His connection to the wild and his leadership qualities could be enhanced by having a tiger as an ally in fanfiction.