Perhaps a young wizard or sorceress. They would have the magical abilities to detect the demon's magic and might be in a unique position to combat it. Their inexperience could also add an interesting dynamic as they learn more about the demon and their own powers in the process.
A street - smart thief could also be the main character. They might stumble upon the demon's activities while on one of their heists. Their skills in stealth and getting in and out of difficult situations could be useful in dealing with a demon that lurks in the shadows. They may start off only interested in their own survival but gradually become more involved in saving the whole city or world from the demon.
It could be a rogue ninja who has risen to power within the League of Shadows. This ninja might have a personal vendetta against Naruto or the Hidden Leaf Village and uses the resources of the League of Shadows to carry out his or her revenge.
It could be a rogue ninja who has a personal vendetta against Naruto. Maybe someone from his past that he thought was defeated but has now returned with new powers and a thirst for revenge.
It could be Orochimaru. He's always been interested in new and powerful jutsus. He might see Naruto's shadow - controlling ability as something he wants to steal or use for his own evil purposes.
Well, it could be about a hidden demon that is secretly influencing the events in a particular fictional world. Maybe it's causing chaos among the characters without them fully realizing it at first.
Maybe a new enemy emerges who wants to use the power of the League of Shadows for their own evil purposes. They would target Naruto as the heir, trying to either kill him or turn him to their side. This antagonist could have some connection to Naruto's past, like an old enemy of his family or something.
Perhaps it's a human who has somehow discovered the existence of vampires and werewolves and has decided to take matters into his own hands, using his knowledge of the town's layout and the creatures' weaknesses to become a killer in the shadows. He might be motivated by a desire to protect the town from what he sees as a threat, but his methods are extreme.
Obviously Naruto would be one of the main characters. There might also be characters like Sasuke who could be involved in the shadow - related plot, as he has his own dark side and secrets.
Well, it could be a new vampire that the author has created. This vampire might be a loner, living in the shadows away from the main vampire clans in Twilight. Another could be a human character who stumbles upon this hidden vampire world.
Well, Naruto is surely one of the main characters. He is at the center of the story, exploring the shadows.
A new character that could be interesting is a scientist who was working on a project to reverse the effects of the symbiote. This character could have a complex backstory, like they were once a student of Dr. Connors and turned to this research after seeing the chaos the symbiote brought.