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.
Some common themes could be self - discovery. Kurt might be finding his voice for the first time, both literally and figuratively. It could also involve themes of acceptance, as he steps into a new world of performing or expressing himself through song.
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.
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, 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.
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.
Sonic himself, of course. And his best friend Tails is very likely to be there. Tails is always by Sonic's side and would surely be there when Sonic is sick.
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.
Marinette and Adrien are also very likely to be in it. Since they are the civilian identities of Ladybug and Chat Noir respectively, they could be at the beach with their friends, like Alya and Nino. Maybe Alya could be trying to get some juicy photos for the Ladyblog while at the beach.
Zuko is very likely to appear. As her brother, he would play a crucial role in her life after the war. He could be the one trying to help her redeem herself or dealing with her if she still poses a threat. Mai and Ty Lee could also be there. They were once her friends/companions, and their relationships would be interesting to explore post - war. Maybe they would be hesitant to trust her again at first. Aang would be another possible character. As the Avatar, he might be involved in Azula's fate, whether it's in terms of forgiveness or making sure she doesn't cause more harm.
Iruka sensei is almost certain to be in it. He has always been important in Naruto's life. Also, Sakura might be there. She could be one of the first to welcome Naruto or perhaps she might initially be a bit cold because of her past feelings for Sasuke. Other characters like Kakashi sensei could also make an appearance, guiding Naruto in his new life in Konoha.