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.
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, 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.
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.
Well, I've heard about 'Leo's Snapping Point'. In this fanfiction, it really builds up the tension leading to Leo's snap in a very engaging way. There's also a story where Leo snaps because of a betrayal from a close friend. It's a heart - wrenching read. And then there's a more action - packed one where Leo's snap triggers a series of events that could save or destroy the world they live in.
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.
Obviously, the main characters are the Flash Pain and Leo Valdez.
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.
Well, obviously Darth Vader is the central character. But often, other characters like Luke Skywalker can be involved. Maybe Luke tries to reach out to Vader during his 'snap' moment, hoping to turn him back to the light side.
I don't think the gods read Leo Valdez fanfiction. Gods in that universe are often busy with more important things. They have to manage their powers, deal with other gods, and look after the affairs of the demigods. Fanfiction is a form of human creativity, and it's not something that would likely be on the gods' radar. It's more of an entertainment for the fans of the Leo Valdez stories.