There could also be some side characters. For example, Kakashi might notice Naruto's strange behavior and try to figure out what's going on. And maybe some of the female characters in the village would befriend Naruto in his 'girl' form and introduce him to different aspects of being a girl, like shopping for kunoichi gear or sharing girl - talk.
In addition, villains might also be part of the story. They could be deceived by Naruto's girly act and underestimate him, only to be surprised when he reveals his true strength. This would add an interesting twist to the fanfiction as Naruto uses his 'girl' guise as a strategic advantage against his enemies.
Sasuke could be an important character. He might be initially confused and a bit annoyed by Naruto's cat - like behavior, but as the story progresses, he could start to find it endearing or even useful. Another character could be Kakashi. He would probably be the one trying to figure out what caused Naruto to act this way and how to reverse it, if needed.
Peter Pettigrew is a very likely character to be involved in more acts of betrayal in fanfictions. Since he already betrayed James and Lily Potter in the original story, it's easy to imagine him continuing his traitorous ways. Another could be Draco Malfoy. In some fanfictions, he might be made to do more extreme acts of betrayal rather than just being a bully. He could be forced to betray his schoolmates in a more serious way for the sake of his family's safety.
Well, Naruto is the main one. But also, Kakashi could be involved. He might be a bit amused at first but then help Naruto figure out how to use his new body to his advantage in combat. And then there's Hinata. She could have an interesting reaction, perhaps being more shy around Naruto as a girl, or she could be more empathetic as she also has her own insecurities.
Well, Naruto is surely one of the main characters since the title has his name. Other than that, it could include his friends like Sasuke and Sakura. They might be involved in the situations where Naruto acts the fool and react to his behavior in different ways.
Well, one plot could be Naruto dressing up as a girl to go undercover on a mission. He might initially struggle with acting girly but then gradually get into the role, like learning how to walk gracefully and use a more feminine tone of voice.
There could also be other characters from Hogwarts, like Neville or Ginny. Neville might be more accepting of Harry's change, as he has his own experiences with being different. Ginny could be in a unique position as she has a special relationship with Harry. She might be one of the first to notice the change in Harry and could have her own thoughts and feelings about it, either positive or negative.
Naruto himself is often a central character in 'naruto destruction fanfiction'. Since he is the main protagonist of the original series, fanfiction writers like to put him in situations where he has to deal with destruction. Sasuke is also frequently involved as his rival, and their relationship can be explored in the context of destruction, like if they have to work together to stop a greater threat of destruction.
Well, Naruto is obviously a main character in most of these. But there could also be Sasuke. He might be either an ally or an adversary in relation to the dagger. For example, if the dagger has a power that can either be used for good or evil, Sasuke could be tempted by its darker potential.
Some fanfictions might also include characters like Kakashi in important roles. As Naruto's former teacher, he could be a guiding figure or someone who has to adapt to Naruto's new level of power. He might also be involved in trying to keep Naruto's power in check if it starts to have negative effects on the ninja world.
It could be Naruto himself. He might be trying to create new weapons to better fight his enemies. Another possible main character could be Sasuke. He has a great understanding of weaponry and chakra, so he would be interested in making unique weapons. And then there's also Kakashi. His knowledge and experience would make him a great asset in the weapon - making process in the fanfiction.