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.
In this 'naruto acts the fool fanfic', perhaps Naruto acts the fool to make his friends lower their guards and have some fun with them. It could also be a way for him to hide his true strength and new jutsu from potential spies or threats in the village. For example, he might make silly mistakes during training on purpose, but in reality, he's secretly perfecting a powerful new technique.
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.
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.
I'm not sure as I haven't read the novel. But typically, a novel will have characters who initiate the acts of service and those who are the recipients. Maybe there's a protagonist who is on a journey of self - growth through service.
I'm not sure specifically as I haven't read the novel. But typically, there could be a protagonist who is in a desperate situation, like a young person trying to make it in a tough city. There might also be an antagonist who either causes or exacerbates the desperate situations.
Given the title 'a fool and a girl novel', the two main characters are probably the fool and the girl. However, there could be other supporting characters that play important roles in the story. Maybe there are friends or family members of the girl or people who interact with the fool in various ways that add depth to the plot.
Obviously, Naruto is the main character. He is the one who becomes this 'god - like' figure in the fanfic. But there are also other important characters like Sasuke. Sasuke might play a role in either supporting Naruto's new power or being in conflict with him because of it.
The main character is Gimpel, of course. He is the one around whom the whole story revolves. He is a simple man who is often deceived by others.
I'm not sure specifically without reading the novel. But usually, there's a central character who drives the story. It could be a young person trying to make their way in the world.
The main characters in a 'fool me once story' really depend on the nature of the story. It might be a young person starting out in the world, full of hope and trust, like a fresh graduate entering the job market. They could be fooled by a more experienced and unethical colleague. Or it could be a lonely old person being deceived by a seemingly friendly neighbor. There are so many possibilities depending on the context of the story.