Well, in addition to Naruto and the succubus, there could be other characters from the Naruto universe. For instance, his friends like Sasuke or Sakura might make an appearance. They could either help Naruto in his fight against the succubus or be used by the succubus in some way to try and manipulate Naruto. Also, Kakashi Sensei could potentially show up to offer some sage advice to Naruto on how to deal with this new and strange threat.
It's possible that new characters could be introduced in this fanfiction as well. Maybe there's a hermit who knows a lot about succubi and gives Naruto some crucial information on how to break their power. Or perhaps there's another ninja from a different village who has had a similar encounter with a succubus and teams up with Naruto to defeat her.
In this 'naruto breaks succubus fanfiction', perhaps Naruto has to face the succubus in a battle of wills. He might draw on his experiences with difficult opponents in the original Naruto series. His strong sense of friendship and loyalty could give him the strength to break the succubus's hold. For example, if the succubus tries to use his friends against him, Naruto's unwavering belief in his comrades could be his key to victory.
Obviously, Harry would be the main character as the title indicates his transformation into a succubus. His friends like Hermione and Ron are likely to be important secondary characters. They would play a role in either helping Harry come to terms with his new nature or being initially suspicious of the changes in him.
There might also be other characters involved, like Sakura. Since she was Tsunade's apprentice, she could be part of the story. Maybe she's caught in the middle when Tsunade has to take some action regarding Naruto. Also, Kakashi could be in the fanfic. He has a close relationship with Naruto and might be affected by whatever Tsunade does to Naruto or be involved in the events that lead to Tsunade's actions.
Obviously, Harry Potter and Severus Snape are the main characters. Harry is the young wizard who is constantly under Snape's scrutiny at Hogwarts. Snape, on the other hand, is the complex and often - misunderstood professor.
Hermione Granger might also be a main character. Given her intelligence and knowledge of magic, she could be helping Harry in his quest to break magic. She might be researching ancient spells or looking for loopholes in the magical laws. Ron Weasley could be there too, providing support and his own brand of humor and loyalty as Harry undertakes this dangerous mission.
Obviously, the succubus would be one of the main characters. She could be joined by other demons, like her friends or enemies in the demon world. And then there would probably be some human characters. For example, a priest who tries to exorcise her or a young man who is attracted to her despite knowing what she is. These characters would drive the plot forward through their interactions.
The main character is surely the succubus. However, there might be other important characters such as a human she has a special connection with. This human could be either a target of her seduction or someone who she forms an unexpected bond with. There may also be an antagonist, perhaps a powerful demon who wants to control her or a hunter who is after her.
Obviously, Bella is the main character as she is the succubus. There might also be a love interest, perhaps a human or another supernatural being. For example, in some fanfics, there could be a vampire character who becomes intrigued by Bella's succubus nature.
It could be Orochimaru. He might use Naruto's actions of breaking something related to Tsunade (like a seal she was relying on) to his advantage. Orochimaru could manipulate the situation to make Tsunade look weak or to cause chaos in the village.
It could be a powerful demon who wants to control the Succubus Queen and use her powers for evil. Since Naruto is with the Queen, he becomes a target too.