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.
Definitely Mikasa is the main focus. Historia could play a role in these fanfictions. Maybe Historia's own experiences and growth could contrast with Mikasa's snapped state. She could offer a different perspective or be a source of support. Reiner might also be involved. His own complex backstory and actions in the original story could be related to Mikasa's snapping, either as a cause or as someone who has to deal with the consequences of her snapped state.
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.
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.
Obviously, Butters is the main character. His friends like Stan, Kyle, and Kenny often show up in these fanfictions too. They might either be helping Butters with his snapping - related adventures or be the ones who are in trouble and need Butters to use his snapping powers to rescue them.
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.
Sometimes, other gods and goddesses make an appearance. Athena might show up to collaborate with Hephaestus on a project. Their interactions can add a lot of depth to the story. For example, they might have different ideas on how to create a new weapon, and their debates and compromises can be really interesting to read about. And of course, there could be a villainous god who tries to steal Hephaestus' secrets related to his snaps.
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 many Beast Boy Snaps fanfictions, the villain characters from the original Teen Titans series can also be main characters. For instance, Slade. His menacing presence and his constant attempts to undermine the Titans can create a lot of tension and conflict in the fanfiction. He might target Beast Boy specifically because of his unique powers, or try to use Beast Boy's 'snaps' to his advantage.
Alina is also frequently involved. Her connection to the Darkling in the original work makes her a natural addition in many fanfictions. Their complex relationship can be further explored or even rewritten in different ways in fanfiction.
Naruto himself is often a central character in 'naruto destruction fanfiction'. Since he is the main protagonist of the original series, fanfiction writers like to put him in situations where he has to deal with destruction. Sasuke is also frequently involved as his rival, and their relationship can be explored in the context of destruction, like if they have to work together to stop a greater threat of destruction.