Obviously Naruto has to be one of the main characters. He'd be the energetic and sometimes clumsy student who is always getting into trouble but has a heart of gold. Sakura would also be a main character, the smart and strong - willed girl who is often exasperated with Naruto's antics. And Sasuke, the cool and aloof student who has a mysterious past and is the object of many students' admiration.
Naruto is definitely one of the most popular characters. His energetic and determined personality makes him a central figure in many fanfictions set in Konoha High School. He's often the one getting into trouble and then finding solutions.
Sakura could be a main character. Since it's Naruto returning to Konoha, Sakura, being his friend and a strong ninja in her own right, might play a big role. Maybe she's the one who first welcomes Naruto back and they team up to face any threats.
The main characters would most likely start with Naruto. His character would be central to the story. Alongside him, we might see the Hokage at the time also playing a role. Additionally, there could be a group of shinobi who are specialized in dealing with demons that Naruto teams up with. And perhaps some of the villains or demons themselves could be considered main characters in a sense, as they drive the conflict in the story.
Another main character could be Kakashi. As Naruto's former teacher, he might be conflicted about Naruto's exile. He could be trying to find a way to bring Naruto back to Konoha or at least help him in some way while also having to deal with the village's decision. And of course, there could be new characters introduced specifically for this fanfic, like a mysterious figure who helps Naruto during his exile.
The main characters are, of course, the Konoha Twelve. This includes Naruto Uzumaki, Sasuke Uchiha, Sakura Haruno, and others. They are the focus of the fanfic and usually the ones driving the story forward with their actions and decisions.
I think Naruto is the central character. His friends, such as Kakashi, who was his teacher, would also be main characters. They would be involved in Naruto's journey of dealing with being labeled as a demon in Konoha. Additionally, any villains or antagonists who oppose Naruto because of his 'demon' status could be main characters. For example, if there are some villagers who are against Naruto and try to drive him out, they could be considered main characters in the context of this fanfic.
The ANBU black ops could also play the role of antagonists. They are the ones who carry out the village's more secretive and sometimes morally gray missions. If they are ordered to hunt Naruto down unjustly, they would be in Naruto's way and could be the reason for his violent reaction towards Konoha.
The hidden enemies within Konoha who oppose Naruto's ideas of peace and unity could be antagonists. They might spread false rumors about Naruto, turning the villagers against him. This growing animosity between Naruto and the village could result in him lashing out at Konoha, making those who spread the rumors antagonists in the fanfic.
The elders of Konoha could be the main antagonists. They are often shown to be power - hungry and willing to do whatever it takes to maintain their control over the village. They might see Naruto as a threat to their authority and decide to betray him. Another possibility is Danzo. He has always had his own agenda and has been shown to be ruthless in achieving his goals. He could easily be the one behind the betrayal of Naruto in a fanfic.
It could be a rogue ninja from another village who has a vendetta against Naruto. This ninja might spread false rumors about Naruto in Konoha, which eventually leads to Naruto leaving. So, this ninja would be the main antagonist as he is the root cause of Naruto's departure.