Viserys is the main one, no doubt. However, in some stories, there are characters like a powerful sorcerer or sorceress who either helps or hinders Viserys. There could be a young page who is always by his side and through whose eyes we see a lot of the events. And his councillors, who are responsible for the day - to - day running of the kingdom. Each of these characters interacts with Viserys in different ways and shapes the story of his reign.
One possible plotline could be Viserys facing challenges from within his own court. Maybe there are lords who secretly oppose his rule and plot against him. Another could be his relationship with other kingdoms and how he manages diplomatic relations. For example, he might have to deal with a neighboring kingdom that is threatening war. Also, his personal growth as a king, learning to make tough decisions for the good of the realm.
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.
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, Naruto is obviously a main character in most of these. But there could also be Sasuke. He might be either an ally or an adversary in relation to the dagger. For example, if the dagger has a power that can either be used for good or evil, Sasuke could be tempted by its darker potential.
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.
Obviously, the Reaper is a main character. Ruby Rose is also often involved as she is one of the central characters in RWBY and her connection to the concept of death (which the Reaper represents in a way) makes for interesting interactions. Weiss Schnee can also be seen in some fanfictions, as her family's past and her own story can cross paths with the Reaper's storyline in creative ways.
Some fanfictions might also include characters like Kakashi in important roles. As Naruto's former teacher, he could be a guiding figure or someone who has to adapt to Naruto's new level of power. He might also be involved in trying to keep Naruto's power in check if it starts to have negative effects on the ninja world.
Obviously, Sakura herself is a main character. She is the focus with her intelligence being a central aspect of the story.
The Volturi are frequently in 'evil bella fanfiction'. Since Bella's actions, if evil, would likely draw their attention. They would be the ones trying to maintain order in the vampire world and would see Bella as a threat. Alice might also be involved as she can see the future and would be trying to predict Bella's next moves.