There could be some original characters created specifically for the fanfiction. For example, a fellow inmate in hell who has a unique backstory and becomes an important part of Katherine's journey in hell. This character could help her or make her situation even more complicated.
The devil or some sort of high - ranking hell authority figure could be present. This character would be important as they would likely be the ones in charge of Katherine's fate in hell. There might also be other characters from 'The Vampire Diaries' universe who have died and ended up in hell, and they could interact with Katherine, either as friends or foes.
Obviously, Katherine herself. Also, there could be appearances from demons or other hellish creatures that are part of the hell setting. Given her history, Stefan or Damon might make appearances in her memories or even in some sort of spectral form if the fanfiction has elements of the afterlife being connected to the living world.
Perhaps it could explore her past actions in more detail while she's in hell, like how her wrongdoings on earth are now being punished or how she reflects on her relationships, especially with Stefan and Damon. It could also show her facing her inner demons, both literally and figuratively, as she tries to find a way out of the hellish realm.
There could also be some minor characters from other families that become important in this fanfiction. For instance, a distant relative of the Lockwood family who shows up and causes a stir in the family affairs. They could bring new secrets and power dynamics to the story, changing the way the existing families interact with each other.
Obviously, Katherine would be one of the main characters as the story is centered around her time in hell and what happens after. Damon and Stefan would also be important as they have a long history with Katherine. Their reactions to her return would be crucial to the plot.
Obviously, Kurt himself. Then, there could be his friends from the Glee club like Rachel, who might be there to support or compete with him. Finn could also be present, perhaps as an encouraging figure or someone who is a bit skeptical at first.
Sasuke would be a likely character to appear. Since he and Naruto have a deep connection, Sasuke might cross paths with Naruto during his wanderings, either as an ally or an antagonist depending on the plot.
Obviously Naruto himself is very likely to appear. He is the main character after all. Sasuke would also be a common character as his relationship with Naruto is so central to the story. Then there are the important figures like the Hokages, especially the ones like Minato and Hashirama who had a big impact on Naruto's life. And of course, characters like Kakashi who played a major role in Naruto's growth.
Obviously Percy himself. And likely his friends like Annabeth. She would probably play a role in either helping him adjust or trying to find a way to reverse the situation if Percy doesn't want to be a god.
Obviously, Paul himself is the central character. His brother Reggie might also appear as he could be concerned about Paul's condition and offer support. Ash could be in it too. He might be someone who initially doesn't get along with Paul but then shows compassion because of his illness. And of course, Paul's Pokemon would be there, like his Torterra or Electivire.
Hermione would likely be one of the characters. She's Harry's best friend and would probably try to find him and help him out. Ron might also be there, as he's always been by Harry's side. Dumbledore could be another character. He might be trying to keep an eye on Harry from afar, or perhaps he has a plan for Harry's new situation. Hagrid would also be a likely character. He has a soft spot for Harry and would want to make sure he's okay. And of course, the Dursleys themselves might make appearances, either to continue their mean - spirited ways or to have a change of heart.
Sakura's teammates from her original team could be likely characters. They would be worried about her and try to figure out a way to get her out. There could also be some local Kirigakure ninja who are curious about this outsider in their prison and interact with Sakura in different ways, either as friends or foes.