Obviously, Hermione herself is the central character as it's about her brain drain. Harry and Ron are usually also important characters as they are Hermione's closest friends. They are often the ones who first notice the changes in Hermione and try to figure out what's wrong.
Obviously, Hermione and Ron Weasley are the main characters. Since it's about the marriage law related to the Weasleys. Harry Potter might also be involved, as he is their best friend and could be dragged into the situations that arise from the law. The other Weasley family members, like Molly and Arthur, could play important roles too. Molly might be concerned about how the law affects her family, and Arthur could be involved in any political or magical - government - related aspects of the law.
Obviously, Harry and Hermione are the main characters. They are the focus of the love potion's effect. Whether it's Hermione being affected by the potion and having new - found feelings for Harry, or vice versa, their relationship is at the center of the story.
One good 'hermione brain drain fanfiction' could be 'The Altered Intellect of Hermione'. It explores how Hermione's intelligence is drained in a unique magical situation and how she copes with it. Another might be 'Hermione's Lost Brilliance' which delves into the emotional and social impacts on her and those around her when her brain power starts to fade.
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.
Harry Potter often shows up. Since he's their best friend, he might be caught in the middle of their divorce, trying to mediate or just being there for both of them. His presence can add a lot of emotional depth to the story as he struggles with loyalty to both Hermione and Ron.
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.