Obviously, Naruto himself is the central character as it's about him 'snapping'. Sasuke is also often involved. He might be a factor in what causes Naruto to snap, or he could be on the receiving end if Naruto's snapping is directed at his actions or decisions.
In Gohan snaps fanfiction, Gohan is the main focus. However, antagonists can play a big role too. Characters like Cell or Frieza could be in the story, if it's set in an alternate universe or a what - if scenario. For example, if Frieza comes back and attacks his family again, that could be the reason for Gohan to snap. And then there are his friends who will support him, like Krillin and Tien.
Obviously, Leo Valdez is the central character. But often, his friends from the series like Percy Jackson, Annabeth Chase, and Piper McLean are also involved. They play important roles in either causing the situation that makes Leo snap or in helping him deal with the aftermath of his snapping.
Obviously, Ruby herself is the central character. But often, her team members like Weiss, Blake, and Yang are also involved. They play important roles in either causing the situation where Ruby snaps or in helping her get through it.
In some fanfictions, antagonists can also be important in relation to Naruto's mind snapping. For instance, Orochimaru might be interested in Naruto's new state. He could see it as an opportunity to either use Naruto for his own evil plans or study him to gain some new knowledge. So, the villains can have a significant role in the story of Naruto's mind snap.
Summer can also be a main character in Morty Snaps fanfiction. She could be jealous of Morty's power, or she could be involved in the adventures that Morty's snaps create. Maybe she helps Morty figure out how to control his power better or uses his snaps for her own gain in some humorous way.
Often, the main characters are Naruto and his friends. Naruto, as the protagonist, is naturally drawn to the mystery of the egg. His friends like Shikamaru, who is intelligent and can strategize about how to deal with the situation regarding the egg, and Choji, who might provide some physical protection if needed. Hinata could also be involved, perhaps having a special connection to the egg because of her Byakugan. And then there are the villains, like the members of the Sound Five, who might want to get their hands on the egg for their own evil purposes.
Naruto himself is almost always involved as he is the main character of the Naruto series. His friends like Sasuke and Sakura may also be part of the story. Sometimes, even villains like Orochimaru or Madara could be involved if the plot of the fanfiction is related to a big threat where their knowledge or power is needed.
In 'naruto angry fanfiction', characters like Hinata can also be involved. If she is in danger or if Naruto feels that he couldn't protect her well enough, it can stir his anger. And villains like Orochimaru or Madara can be present as they are often the source of problems that make Naruto angry, whether it's through attacking the village or harming his friends.
Obi - Wan Kenobi is frequently in Anakin Snaps fanfiction. As Anakin's master, he has a big role to play. Their relationship is often explored, and Obi - Wan's reactions to Anakin's snapping are crucial. Sometimes he tries to bring Anakin back from the brink, while other times he might be the cause, albeit unintentionally, of Anakin's breakdown.
Well, Mikasa herself is obviously a main character in these fanfictions. Since it's about her snapping, she is at the center of the story most of the time. Eren is also often involved. Given their complex relationship in the original series, his actions or words could be the trigger for Mikasa to snap. Another character that might be involved is Armin. He could be the one who tries to calm Mikasa down or understand what made her snap.