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.
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, 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.
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.
In this fanfiction, Harry Potter is the central figure. However, other characters could play important roles too. Dumbledore might be involved as he has great knowledge of magic and could either oppose or guide Harry in his attempts to break magic. And Voldemort could also be a significant presence, perhaps his actions or the threat he poses are what drive Harry to consider breaking magic in the first place.
In this fanfiction, Harry Potter is the main focus. But it's also possible that some original characters that were not as prominent in the books could become important. Characters like Luna Lovegood, with her unique perspective on the wizarding world, could be crucial. She might understand Harry's need to break away and offer him support or advice. Additionally, there could be original characters created by the fanfiction author. These could be wizards or witches from other parts of the world that Harry meets during his breakaway journey and who help shape his new experiences.
Another possibility is that there is no real 'antagonist' in the traditional sense. Instead, it could be the circumstances that act as the 'antagonist'. For example, the differences in their worlds, their duties, and the fact that they come from such different backgrounds could be what causes Erza to break Naruto's heart.