Obviously, Finn in his dark form is the central character. Jake often appears as well, either trying to help Finn or being conflicted about his now - dark friend. Princess Bubblegum might also be involved, using her scientific knowledge to try and find a cure for Finn.
Well, Finn is the main one, of course. But then there's Marceline. She could be a key character as she has experience with dark powers herself. She might be able to offer some advice to Dark Finn. And Lady Rainicorn could be in it too, as she's connected to Jake and could be part of the group's efforts to deal with Dark Finn.
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 Finn himself. Since it's about him being godlike, he's the central figure. And of course, Jake his best friend is likely to be involved. Jake would probably be there to support Finn, or maybe try to keep his new powers in check if they start to get out of hand.
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.
Princess Bubblegum is also a common character. Given her scientific mind and love for creating things, she could be involved in fanfictions about cherry cream soda. For example, she could be researching the unique properties of the soda or using it in some of her experiments in her candy kingdom.
Varian is, of course, the main focus. However, characters like Eugene could also be involved. He might be in a more antagonistic role towards Varian in this type of fanfiction, trying to protect Rapunzel from what he perceives as Varian's dangerous actions.
The Dark Seducer themselves are obviously a main character. Often, the player character from Oblivion might be involved as they interact with the Dark Seducer. Another could be Mehrunes Dagon, as the Dark Seducer has ties to the Daedric Princes.
The Nine - Tails also plays a major role. In a dark Naruto story, the Nine - Tails could be more of a corrupting influence, whispering dark thoughts into Naruto's mind and fueling his descent into a more menacing figure in the ninja world.
Naruto himself could be a main character. He might be tempted to use dark summons to gain power to protect his village or friends. For instance, in some fanfictions, he discovers a dark summon that could potentially defeat a powerful enemy threatening Konoha, but at a great cost.
In Naruto dark chakra fanfiction, Sasuke is frequently a central figure. Given his complex character arc and his inclination towards power - seeking, it's easy to envision him being affected by or exploring dark chakra. Naruto too can be a main character. His strong will and connection to positive chakra could be put to the test when faced with the allure of dark chakra. Another character often involved is Kabuto. He has a history of being involved in dark and underhanded schemes, so it's not a stretch to see him in stories centered around dark chakra. There are also some original characters created by fanfiction authors that are specifically designed to interact with the concept of dark chakra in unique ways.
Naruto himself is often a main character. He's usually the one who stands up against the dark guild's evil plans. Sasuke might also be involved, as his power and sometimes ambiguous moral standing can add interesting dynamics when dealing with the dark guild.