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.
Typically, in a 'king of the underworld novel', the main character is the king of the underworld, who is often portrayed as a powerful and intimidating figure. There may be other important characters like his rivals, who are constantly trying to dethrone him. And then there are the characters who are in his inner circle, like his most trusted advisors or his family members if they are involved in the underworld business. These characters play important roles in shaping the story, either by supporting the king or causing trouble for him.
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.
Obviously Naruto himself would be a main character. He could be joined by Sasuke, who might have his own reasons for being interested in the underworld. For the 'lemon' aspect, Sakura could be involved as a love interest for Naruto. Also, characters from the underworld like a powerful underworld lord or a female warrior who catches Naruto's eye could be main characters.
Well, Naruto is likely to be one of the main characters since it's in the title. He would probably be involved in the interactions with the Arrancar King. Sasuke might also be there, as he is a major part of the Naruto story. And of course, the Arrancar King himself would be a central figure, driving the plot forward with his actions and motives.
Naruto is surely a main character. Additionally, there might be some original characters created specifically for this fanfiction. For example, there could be a powerful spirit or a guide in the world of the dead who helps Naruto understand his new powers. And characters like Kakashi could also be involved, perhaps providing advice or assistance to Naruto as he deals with the implications of being the king of the dead.
Since it's a Naruto - related fanfiction, Naruto is bound to be a main character. Regarding the Shadowkhan, there could be a powerful Shadowkhan leader or a group of Shadowkhan who play important roles. Maybe there's a Shadowkhan character who becomes Naruto's ally or rival in his journey to be the king. And of course, Naruto's sensei, Kakashi, might also make an appearance to guide Naruto in this new and strange situation.
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.