If it's a story where the black dragon is the center, there might be a wizard or a sage who has knowledge about the dragon. Maybe there are also villagers who live in fear of the dragon's presence. The black dragon could be the main character, and its actions and nature drive the plot. For example, if it's stealing livestock from the village, the villagers' attempts to stop it and the dragon's reactions to their efforts would be part of the story.
As I don't know the exact 'Black Dragon Story', it's hard to say the main characters. But generally, aside from the black dragon itself, there could be a hero who tries to either defeat or befriend the dragon.
Since I don't know the specific story, it could have a brave knight as one of the main characters who is determined to face the black diamond dragon. There might also be a wise old wizard who gives advice on how to deal with the dragon.
Since I don't know the exact 'black and green dragon story', the main characters would likely be the black dragon and the green dragon.
Most likely, the black knight is one of the main characters in 'Black Knight Dragon Story'. He would be the one taking on challenges and showing heroism. And of course, the dragon is also a main character. It could have unique abilities like flying and breathing fire, and it might be a key factor in the knight's adventures.
Vegeta could also be a main character. He's Goku's rival and often teams up with him to face powerful enemies. Maybe in this 'Black Story', Vegeta has some new development in his power or character arc related to the 'Black' aspect of the story.
Obviously, the Black Dragon Emperor himself is a main character. He is likely to be at the center of the story, with his actions and decisions driving the plot forward.
Obviously Naruto would be one of the main characters. And since it's about the black dragon, the dragon itself would likely be a main character too. There might also be other Naruto - related characters like Sasuke or Sakura who could be involved in the story in some way.
Since I haven't read it, it's hard to say for sure. But most likely there would be a character who is having tea with the black dragon. There could also be other characters in the background that are related to either the tea - serving or the world where the black dragon exists.
Perhaps there is a conflict over a particular object or being. Maybe there is a magical crystal that has the power to enhance a dragon's abilities, and both the black and blue dragons want it for themselves. This would cause them to engage in battles and schemes against each other in order to obtain the crystal.
The main challenges are the powerful enemies. The black dragon's minions are tough to defeat.
I'm sorry for not being able to give definite answers as I'm not acquainted with this novel. However, in such a story, the black dragon boss is the central figure. There might be a group of loyal minions, which could be considered important characters in a way as they support the black dragon's rule. There could also be a prophetic character, like an oracle or a seer, who foretells events related to the black dragon's future. And there may be a character from a different realm or dimension that somehow crosses paths with the black dragon and affects the story's development.