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.
Then there are other Grisha characters. They might play different roles depending on the plot of the fanfiction. For example, some might be loyal allies of the Darkling, while others might oppose him. Their powers and personalities add depth and variety to the stories.
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.
Ruby is often involved. Since she's the main protagonist in RWBY, it makes sense for her to interact with Nightcrawler in fanfictions. Their different powers and personalities can create interesting dynamics.
Naruto himself is often a main character in naruto helmet fanfiction. Since he is the protagonist of the original series, it makes sense that he would be involved with a special helmet in fan - created stories. He could discover the helmet's powers and use them to become even stronger.