Another character could be Mr. Satan. He might misunderstand Goku's situation and try to take advantage of it by claiming to be the one who can save Goku from his vampire state, when in reality he has no idea what he's doing. This would add some comical elements to the otherwise dark and serious concept of Goku as a vampire.
Bulma could also be a significant character. She might use her scientific knowledge to try and find a cure for Goku's vampirism. Or she could be in danger from Goku's vampire instincts, which would add a lot of drama to the story.
Obviously, Goku is the main character. But sometimes, characters like Vegeta might also be involved. Vegeta could be either an ally trying to help Goku deal with his vampire side or a rival who sees Goku's new form as a threat.
Future Mai could also be a key character in Goku Black forced fanfiction. Given her relationship with Trunks and her own skills, she could be part of the group that is fighting against Goku Black, or she might even have some unique interactions with him that drive the plot forward.
The main characters also include Ash's mother in some fanfictions. She might be worried about Ash's newfound powers and his connection to Goku. In the Dragon Ball universe, Kami could be involved. He could sense Ash's unique existence as a Pokémon - Saiyan hybrid and either try to help him or be wary of his potential. And of course, there are always the various villains from both universes that could be a threat to Ash in these fanfictions.
Princess Bubblegum could also be a main character in such fanfictions. She might be the one trying to find a scientific solution to Finn's vampirism. Her intelligence and resources in the Candy Kingdom could play a big role in the story. Another character could be Marceline. As a half - vampire herself, she could either be a mentor to Finn or a source of conflict as she has her own views on vampirism.
Obviously, Edward is a key character. He is not only Bella's love interest but also someone who has a lot of knowledge about the vampire life. He can help Bella understand her new powers and limitations. Alice is also popular in these fanfictions. With her ability to see the future, she can foresee any potential problems or challenges that Bella might face as a vampire. And then there's Rosalie. Her initial dislike for Bella might change or intensify after Bella turns, adding more drama to the story.
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.