It could be the Nine - Tailed Fox. Since it already has a connection to Naruto, it might take him under its wing in a more fatherly or family - like way. Another possibility is some of the lesser - known demons from the Naruto lore that haven't been explored much.
One of the main demons could be a shadow - based demon. These types of demons are often associated with mystery and stealth in many fictional universes. They might be attracted to Naruto's own hidden power within him. Also, there could be a fire - demon, considering Naruto has an affinity for the Rasengan, which has a sort of fiery or explosive nature. The fire - demon could see this as a sign that Naruto is meant to be part of their family.
The plot could involve Naruto being adopted by demons after some sort of tragedy. The demons could see his strong will and decide to make him one of their own. He would then grow up with a different set of values compared to regular ninjas. He might struggle with his identity, not fully belonging to the demon world or the ninja world. He could use his new demonic abilities to protect both the demons and his ninja friends from a greater threat that neither side could face alone.
It could be an ancient demon that was sealed away long ago in the Land of Demons but has now broken free. This demon could be extremely powerful and have a vendetta against the ninja world.
It could be a group of lesser - known demons. For example, there might be a demon named Zargoth who has the ability to manipulate time and space a little bit. He could use this power to save Harry from dangerous situations. Another could be Lythra, a female demon with the power of illusion. She can create false images to deceive Harry's enemies.
It's possible that Kakashi could be a main character as well. His experience and wisdom would be valuable in the face of the threat of the 100 demons. He could be mentoring the younger characters on how to deal with the demons, and his Sharingan might give him some unique insights into the nature of these demonic entities.
The main antagonist could be a demon who is threatened by Naruto's singing ability. This demon might be using people's inner demons to gain power, and Naruto's singing that calms those demons is ruining its plans.
One of the most interesting demons could be a shape - shifting demon. This demon could change its form to deceive enemies or help Naruto in stealth missions. For example, it could transform into an exact replica of an enemy ninja to gather information. Another interesting demon might be a demon with the power of time manipulation. Naruto could use this demon's power to go back in time and prevent certain disasters from happening in the ninja world, like the attack on the Hidden Leaf Village by Pain.
One possibility is that Harry Potter, in this fanfiction, might be exposed to a whole new set of dark magic and powers that demons possess. The demons could see his potential and decide to raise him in a way that is completely different from the wizarding world he knew. Maybe they would teach him to use his magic more aggressively or in ways that are considered unethical in the normal wizarding community.
In the 'dancing with demons naruto fanfiction', Naruto is almost surely a main character. Given the nature of the title, he could be the one who is either lured into the world of demons or is trying to protect others from them. Sakura might also be involved as she is one of Naruto's closest friends. Her medical skills could be useful in a story full of dangerous demons. And of course, Kakashi could be there as well. His experience and wisdom would be valuable assets in dealing with the demons and guiding Naruto and the others through this strange 'dance' with the demons.
Naruto is likely to be one of the main characters, as the title implies his connection to the 'god of demons' concept. His friends like Sasuke and Sakura could also be important characters, as they might be involved in Naruto's journey with his new identity.
He might meet a powerful demon king who initially sees Naruto as an intruder but later realizes his good intentions. There could also be a young demon who is curious about Naruto and helps him navigate the land.