The main character is Naruto, of course. But other characters like Jiraiya would be relevant too. Jiraiya has had a great influence on Naruto. If Naruto snaps, it would be a big blow to Jiraiya's teachings. And then there are the antagonists. People like Orochimaru or Madara might take advantage of Naruto's snapped state. They could try to manipulate him for their own evil purposes, which would add more complexity and conflict to the fanfic.
In a 'naruto finally snaps fanfic', Naruto is the main focus, but characters like Kakashi would be involved. Kakashi has been Naruto's teacher and mentor, so he would be affected by Naruto's change. Also, characters from other villages such as Gaara could be part of the story. Gaara has his own connection with Naruto, and Naruto's snapping would have an impact on their relationship and the balance between villages.
Obviously Tsuna would be the main character. His friends like Gokudera and Yamamoto are likely to be in it too. They might be the ones who are shocked by Tsuna's snapping and try to help him get back to normal. Reborn could also be there as he has been guiding Tsuna all along and would surely play a role in this situation, either trying to understand what made Tsuna snap or trying to fix it.
Voldemort would still be a major character. After all, he is Harry's main antagonist. If Harry snaps, it would likely be in relation to his ongoing battle with Voldemort. And the Death Eaters, as Voldemort's followers, would also be involved. They would be on the receiving end of Harry's new, more aggressive stance, and their reactions would shape the story further.
There could also be some new characters created specifically for the fanfic. Maybe a foreign diplomat who comes to Konoha and Naruto is the first one to welcome him and befriend him. This would open up new storylines about international relations in the Naruto world and how Naruto's friendship can have a positive impact on a larger scale.
Naruto is the obvious one. His connection to the Uzumaki clan makes him the driving force behind the rebuilding. Hinata would also be likely involved, as Naruto's wife, she would support him and might contribute to the new Uzushiogakure in areas like family support and community building.
Obviously Naruto himself is the central character. Sasuke would also likely be involved. Given their history and Sasuke's own complex relationship with Konoha, he might side with Naruto or at least be a key figure in the events. Kakashi too, as he has been Naruto's teacher and is an important figure in Konoha. He could be caught in the middle, trying to understand Naruto's motives and also trying to uphold Konoha's stability.
Definitely Hinata. Her gentle yet strong - willed nature could be an interesting contrast when dealing with the often - mischievous yokai from Nurarihyon. Gaara too, as his own experiences with being an outcast and having a powerful force within him could have parallels in the Nurarihyon world where some yokai are feared or misunderstood. And Rock Lee, his determination and unique fighting style could bring a new dynamic to the interactions in the fanfic.
Obviously, Naruto is the central character. But other characters like Sasuke could also be involved. Sasuke might be torn between his loyalty to Konoha and his friendship with Naruto. He could either try to bring Naruto back or join him in his exile.
Sasuke is a very likely character. He already has a strong connection to hate in the original series due to his clan's massacre. In a fanfic about the power of hate, his journey could be further explored, like how he might be tempted by even darker forms of hate - driven power. Another one is Obito. His fall into darkness was also driven by hate, and in a fanfic, his character could be used to show the extreme lengths that the power of hate can take someone.
Tsunade for sure. As the Hokage and also a medical expert, she would be furious about Naruto escaping the hospital. She might send some ninjas to bring him back.
Naruto himself is the most obvious one. Since it's a Naruto - related fanfic, he would likely be at the center of any crossovers with god - like entities. His strong will and desire to protect his village make him a prime candidate for interacting with powerful gods.