Since it's a fanfiction, it could have Naruto as one of the main characters. And depending on the plot, there might be a new character who is the demon. Maybe Sasuke could also be involved if the story ties into the existing Naruto - Sasuke dynamic in some way.
The main characters could include a sensei - type figure who has some knowledge about the 'monster of the leaf' from the past. This sensei could guide the younger characters, both from the original Naruto series and any new ones, in understanding and dealing with the situation. And of course, the 'monster of the leaf' would be at the center of the story, with its own backstory and motives that gradually unfold as the fanfiction progresses.
Obviously Naruto would be one of the main characters as the story is about getting him back. Sakura could also be a main character. She might be one of the ones leading the effort to bring Naruto back, using her medical skills and her friendship with Naruto as reasons to convince him.
Well, in 'naruto demon of the leaf fanfiction', it could involve a unique plot where the demon has a special connection to the Leaf. Maybe it's a demon that was sealed long ago and now its power is starting to surface, causing chaos in the village. And Naruto, being the main character, might be the one to either stop it or understand it. There could also be elements of mystery, like the origin of the demon and what its true intentions are. It could also be about a demon that is somehow related to the history or secrets of the Leaf Village.
Since it's a fanfiction, it could be Naruto himself as one of the main characters. Maybe there's also a new character created specifically for this story who is the demon of Sabertooth. Sasuke could also be involved, perhaps trying to either stop or understand the new threat.
Naruto is the central figure in most 'demon naruto fanfiction'. In addition to him, there could be an ancient demon lord who serves as the antagonist. There are also times when the other members of Team 7, like Sakura and Kakashi, are involved. Sakura might be worried about Naruto's change, while Kakashi tries to guide him. And sometimes, there are original characters created specifically for the fanfiction, such as a demon hunter who targets Naruto at first but then realizes he's not entirely evil.
Perhaps Sasuke could be one of the main characters. He has always been on a path of power - seeking, and in this fanfiction, he might be involved with the 'demon shredder' concept, either as an ally or an adversary. There could also be a new, mysterious character who wields the 'demon shredder' power and becomes a central figure in the story.
Obviously Naruto would be one of the main characters as it's in his name. And probably the dark demon itself, which would have a major role in driving the story.
Typically, there would be a young and ambitious trainer as one of the main characters. They are on a journey to become the best in the Burning Leaf region.
There could also be new characters introduced specifically for this fanfiction that are related to the demon way. For example, a powerful demon - like being who becomes an ally or an enemy to Naruto and his friends. Or perhaps an ancient demon - hunter character who has knowledge about the demon way that the main characters need to learn from. Also, characters like Kakashi might have a different role in guiding the young ninja in dealing with the new demon - related situations.
Naruto is sure to be a central figure. Then there could be a new character, a half - demon who acts as a bridge between the ninja world and the demon army. Hinata might also be involved, perhaps using her Byakugan to discover secrets about the demon army. And of course, Kakashi could be there to guide Naruto and the others in dealing with this new threat.