It's possible that some new characters are introduced in this fanfiction. They could be powerful underworld figures that Naruto has to deal with. These characters could have their own motives and backstories, and they would interact with Naruto in various ways, either trying to ally with him or overthrow him as the king of the underworld.
There could also be original characters created specifically for the fanfiction. These characters could be hybrids of the Naruto and Selene universes, perhaps having a mix of ninja abilities and Underworld powers. They could add new dimensions to the story and help drive the plot forward, whether as friends or foes to Naruto and Selene.
Sasuke could also be a main character in this fanfiction. He might be interested in the power of the lycans, seeing it as a way to gain more strength for his own goals. His brooding nature and his search for power could lead him into the lycan underworld, where he has to make choices about whether to ally with the lycans or use them for his own purposes.
In the 'underworld queen and king novel', the main characters are the queen and the king. The queen might be a strong - willed character with a mysterious past. She could have special powers or abilities that help her in ruling. The king, on the other hand, might be more of a brute force type, but also very intelligent in matters of war and strategy. Besides them, there could be characters like the seers in the underworld who can predict the future and play a role in guiding the queen and king's decisions.
Sasuke is often a main character in such fanfictions. He could be Naruto's right - hand man, using his skills and intelligence to support Naruto's rule. Their relationship might be more complex, with Sasuke having to come to terms with Naruto's new position of power.
Most likely Naruto would be one of the main characters. Since it's in the title, he's probably the central figure in the story.
Obviously, Naruto is one of the main characters as it's about him being the king of foxes. The nine - tailed fox, Kurama, is also likely to be a central figure as it's closely related to Naruto's new title.
Naruto is surely the central character in this fanfiction. Besides, characters like Kakashi could be involved. He might be Naruto's mentor in controlling and understanding his new 'king of fire' powers. And villains from the Naruto universe might also appear. They could be envious of Naruto's power and try to take it away or use it for their own evil purposes. So, a variety of characters from Naruto's world could be main characters in this fanfiction depending on the plot.
The main characters could include a group of thieves, each with their own special skills. For example, one might be an expert in lock - picking, another in distraction techniques. And then there could be the ninja who are after them, such as Kakashi. Kakashi's intelligence and combat skills would make him a formidable opponent in the cat - and - mouse game with the thieves.
Obviously Naruto would be one of the main characters as he is the 'King of Darkness' in this fanfiction. Sasuke could also be a main character as his relationship with Naruto is so central in the original series and would likely be explored in a new way in this dark version. Sakura might also be a main character as she may try to bring Naruto back from the dark side.
Obviously Naruto would be one of the main characters. There might also be some makai - specific characters like powerful demons or other supernatural beings that play important roles, either as allies or antagonists to Naruto.