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.
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.
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.
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.
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.
Obviously Naruto himself is a main character. He is the one with the pacemaker so he is central to the story. Sasuke might also be involved. He could be initially skeptical of Naruto's new situation with the pacemaker but then come around to help. Sakura too, as a medical ninja, she could play a crucial role in understanding and perhaps even trying to modify the pacemaker.
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.
Well, obviously Gaster is the central character as it's about his pregnancy. But often, other characters from the same fictional world might be involved. For instance, characters who are friends or colleagues of Gaster could be there to support or oppose the situation. It might also include characters who have some sort of special connection to Gaster's unborn child, like a potential mentor or an enemy who wants to harm the baby.
The dragonborn himself is, of course, a main character in these fanfictions. Depending on the story, other DxD characters like Akeno Himejima could also be involved. She has her own powers and personality, and her interactions with the dragonborn can add a lot of depth to the story. Her seductive nature might play off against the dragonborn's more straightforward and perhaps stoic personality.
Naruto is almost always a main character in 'naruto dice fanfiction' as the story is based on the Naruto universe. His friends like Sasuke and Sakura also often appear. Sasuke's cool and brooding nature can add an interesting dynamic when combined with Naruto's more upbeat and impulsive personality in stories related to the dice.