The main characters are the white dragon and the red witch. The white dragon is the character driving the plot forward as it has a score to settle with the red witch. The red witch, on the other hand, is the one who has presumably done something to incite the dragon's wrath. Depending on the fanfiction, there could be other important characters as well. For example, there might be a wise old wizard who tries to guide the white dragon in its quest for revenge, or a loyal sidekick for the red witch who helps her defend against the dragon's attacks.
Obviously, the white dragon and the red witch are the main characters.
I'm not sure specifically as there could be many different interpretations in fanfiction. It might involve a story where the white dragon has some sort of vendetta or conflict with the red witch. Maybe the white dragon was wronged by the red witch in the past and is now seeking revenge.
The red dragon itself is likely to be a main character. There might also be a young wizard or sorceress who befriends the dragon. Maybe an old sage who guides the dragon on its journey could be another main character.
Often, there are dragon riders. These characters form a special bond with the red dragon and go on adventures together. They might be brave warriors or young apprentices learning the ways of dragon riding.
I'm not sure specifically as I haven't read the novel. But typically in a novel like this, there might be a brave protagonist. This could be a young adventurer who is drawn into the events related to the Red Witch.
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.
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.
One of the main characters is Will Graham, the investigator. And there's Francis Dolarhyde, the serial killer.
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.
Given that we have no information about 'chasing the white dragon fanfic' other than its title, we can only make general assumptions about the main characters. It could be that the main character is an outcast who discovers the legend of the white dragon and decides to pursue it in order to prove themselves or find their place in the world. There could also be an antagonist who tries to stop the main character from getting to the white dragon, either out of jealousy or because they have their own plans for the dragon. Or perhaps there are multiple main characters, each with their own motives for chasing the white dragon, and their interactions and conflicts form the core of the story.
Since I haven't read it, I can't say for sure. But I would expect the white dragon to be one of the main characters.