Well, Naruto is obviously one of the main characters. He brings his ninja skills, his determination, and his dream of becoming Hokage. The White Dragon Emperor or a character with its powers would also be a main character. They could be an ally or a rival to Naruto, depending on the plot of the fanfiction.
Another character that could be main in these fanfictions is an original character created specifically for the story. This character could be related to the White Dragon Emperor's backstory. Maybe they are a keeper of the White Dragon Emperor's power or someone who has been searching for the reincarnation of the White Dragon Emperor, which turns out to be Naruto in this case.
One common character paired with the White Dragon Emperor is his rival, the Red Dragon Emperor. Their interactions, whether it's in battle or begrudging cooperation, are often central to the story.
One interesting fanfiction could be where the White Dragon Emperor's powers are combined with Naruto's ninja skills in a unique way. For example, the White Dragon Emperor's strength and elemental control could be used to enhance Naruto's combat abilities, creating new and exciting fight scenes.
It could be Naruto himself. Imagine Naruto obtaining the powers of the Red and White Dragon Emperor. He would be an even more formidable force, using his ninja skills along with the dragon powers to protect the Hidden Leaf Village.
Obviously, Naruto is one of the main characters. He is the one who usually gets involved with the dragon emperor concept. Sasuke can also be a prominent character. In some fanfictions, he might be jealous or intrigued by Naruto's new dragon - related powers. Kakashi could also play an important role, perhaps as a mentor trying to guide Naruto in handling his new abilities.
Most likely Naruto would be the main character as he is the Dragon Emperor. But other Naruto - verse characters might also be involved, like Sasuke, Sakura, Kakashi, etc. They could play different roles, such as Naruto's allies or opponents in this new storyline.
Well, Naruto is most likely one of the main characters since it's a Naruto fanfiction. And probably there will be a character related to the black dragon emperor, whether it's the black dragon emperor itself or someone who has a connection to it. There could also be other Naruto characters like Sasuke or Sakura involved, depending on how the author wants to develop the story.
Often, the main character is an original creation who has the power of a water dragon slayer. For example, in many fanfictions, it could be a young ninja who discovers this power within themselves. They are usually the ones driving the story forward, facing challenges and growing as they learn to control their new abilities.
One main theme is power and its control. The White Dragon Emperor's powers are often very potent, and in the Naruto world, the character has to learn how to control it properly, just like Naruto has to control the Nine - Tails' power.
Sasuke could also be a main character. His relationship with Naruto might change due to Naruto having the power of the Red Dragon Emperor. Maybe he would be jealous at first, but then they could end up working together to face new threats that this power has attracted. Sakura might also be involved, perhaps she tries to help Naruto control the new power or she is worried about the changes in Naruto.
Sasuke could also play a major role. His rivalry with Naruto might take on a new dimension with Naruto having the power of the Black Dragon Emperor. He could either be jealous and try to compete even more fiercely, or he might be intrigued and eventually team up with Naruto in a new way.