Rey is the main focus, of course. Snoke could also play a role in some fanfictions. Even though he was killed in the official movies, in fanfiction he could be resurrected or exist in some form to manipulate Rey into the dark side. Luke Skywalker's spirit might also be involved. He could be trying to guide Rey from beyond, but she ignores his advice and still falls. And then there's Maz Kanata, who might have some knowledge about Rey's dark side tendencies and try to warn others.
Obviously, Rey is the central character as the story is about her fall to the dark side. Kylo Ren might also be a key figure. He could be either a catalyst for her fall, perhaps by tempting her with the power of the dark side, or he could be someone who tries to bring her back later on. Finn and Poe are also likely to be involved. They would probably be the ones trying to save Rey from the clutches of the dark side.
Palpatine is often involved. Since he is a master manipulator in the Star Wars universe, he could be the one who subtly guides Windu towards the dark side. He might offer false promises or twist Windu's sense of justice to make him question the Jedi way.
Obviously Rey herself is the central character. Kylo Ren often plays a role too. In some fanfictions, he might be the one who either tries to stop her from fully turning to the dark side because he has some sort of connection with her, or he might be the one who encourages her to embrace the dark side to be with him.
The Emperor or some other powerful Sith lords would likely be involved. They would see Luke as a great prize to turn to the dark side and would use all their cunning and power to manipulate him. Yoda's spirit could also be present, trying to guide Luke from beyond the grave but perhaps being unable to prevent his fall. Han Solo might also play a role, either trying to rescue Luke or being a victim of Luke's new dark side - influenced actions.
It could be the remaining Jedi. Since Rey is turning, the Jedi who still uphold the light side would see her as a threat. They would try to stop her from causing more harm with her newfound dark side powers.
There can be original characters created specifically for the fanfiction. For instance, a detective who is obsessed with bringing down the Joker but slowly starts to lose his own sanity in the process. This character can provide a new perspective on the Joker's reign of terror.
In some rey falls to the dark side fanfiction, her turn could be due to a sense of betrayal. If she discovers some dark secrets about the Jedi Order or the Resistance, she might feel that the dark side is the only way to get true justice. For instance, if she finds out that the Jedi had some hidden, unethical practices in the past. Then she starts to embrace the dark side teachings and uses her powers in a more aggressive and self - serving way.
Varian is, of course, the main focus. However, characters like Eugene could also be involved. He might be in a more antagonistic role towards Varian in this type of fanfiction, trying to protect Rapunzel from what he perceives as Varian's dangerous actions.
The Dark Seducer themselves are obviously a main character. Often, the player character from Oblivion might be involved as they interact with the Dark Seducer. Another could be Mehrunes Dagon, as the Dark Seducer has ties to the Daedric Princes.
The Nine - Tails also plays a major role. In a dark Naruto story, the Nine - Tails could be more of a corrupting influence, whispering dark thoughts into Naruto's mind and fueling his descent into a more menacing figure in the ninja world.
Naruto himself could be a main character. He might be tempted to use dark summons to gain power to protect his village or friends. For instance, in some fanfictions, he discovers a dark summon that could potentially defeat a powerful enemy threatening Konoha, but at a great cost.