Well, obviously Luke himself is a key character. He is the one who is falling to the dark side, so his actions, thoughts, and decisions are central to the story.
There might be a dark side mentor. This character would play a crucial role in guiding Luke towards the dark side. They would use their knowledge of the dark side's power and manipulation techniques to influence Luke.
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.
One main factor could be the influence of a powerful dark side figure. Such a figure might manipulate Luke's emotions, like his fear or anger. Maybe they prey on his insecurities about his abilities or his place in the galaxy.
Luke's friends from the light side could also be important. Characters like Leia, Han Solo, or Chewbacca. Their reactions to Luke's turn and their attempts to bring him back would be significant aspects of the story. There could also be new characters created specifically for this novel, perhaps some Jedi who were thought to be dead but are actually in hiding and oppose Luke's new path.
In some fanfictions, Luke might fall to the dark side after being betrayed by his closest friends. For example, if he discovers that they have been hiding important information from him that he thinks could have saved lives. This sense of betrayal could push him towards the dark side as he feels abandoned by the light. Also, a powerful dark side user could use mind control techniques on Luke, slowly turning him over time.
In some fanfictions, 'dark side Luke' might be paired with Palpatine. Palpatine, as a master manipulator, could have a role in Luke's fall to the dark side, and their partnership would be full of political intrigue and dark Force - related activities. Palpatine could use Luke as a tool to further his own evil plans, while Luke might be seeking more power from Palpatine.
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.
Since I don't know which 'the dark side novel' you're referring to, I can't tell for sure. But usually in a novel about the dark side, there might be a protagonist who is either fighting against the dark side or being lured into it. There could also be an antagonist who represents the dark side fully.
One of the main themes could be the struggle between good and evil. Luke Skywalker is typically a symbol of the light side, so exploring his journey towards the dark side would likely involve his internal battle with his own moral compass. Another theme might be the allure of power that the dark side offers and how it can corrupt even the most noble of characters like Luke.
Palpatine could still be the main antagonist even if Luke turns to the dark side. He might try to use Luke as a more powerful weapon against any remaining Jedi or the Rebel Alliance. Palpatine would see Luke's newfound dark side power as a means to further his own control over the galaxy.
It could be the remaining Jedi. Since Luke is turning to the dark side, the Jedi who still uphold the light side values would be against him. They would see him as a threat to the balance they are trying to maintain and would try to stop him from fully embracing the dark side.