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.