Magnus Chase could also be a main character. Given his connection to the Norse world and Percy's new situation in it, they could cross paths. Magnus could offer Percy insights into the Norse world, and together they could face challenges that neither could handle alone. This interaction between the two different 'worlds' of Percy's Greek heritage and Magnus's Norse knowledge would make for an exciting story.
Obviously, Percy Jackson himself is a main character. Since it's about him being the son of Triton, his powers and his journey are central. Annabeth Chase also often appears as she is an important part of Percy's life in the original series. Grover, his satyr friend, is another character that frequently shows up. He provides comic relief and is a loyal companion to Percy. And sometimes, other demigods from Camp Half - Blood are included, like Jason or Piper, as they can interact with Percy in interesting ways.
Grover Underwood can be a main character in some winter fanfictions. His connection to nature could be explored in a winter context, perhaps he has to communicate with frozen plants or hibernating animals to help Percy and the others. Nico di Angelo is another one. His control over the dead might come into play if there are undead creatures awakened by the cold winter magic.
Percy Jackson is obviously a main character. He's usually at the center of most fanfictions. Annabeth Chase is also very common, as their relationship is a big part of the original series. Grover Underwood often makes an appearance too, since he's Percy's best friend. Nico di Angelo is another popular character in these fanfictions, especially when it comes to exploring the darker side of the demigod world.
Annabeth Chase also frequently appears. Since she's Percy's love interest and a key figure in his life, she often has a role in fanfictions set in Elysium. She might be helping Percy with his quests or dealing with her own challenges in this afterlife place.
Obviously, Percy Jackson himself is a main character. He's the one living underwater in these stories. Annabeth Chase might also be a key character as she's Percy's close friend and often involved in his adventures. Grover, as Percy's satyr friend, could also be featured, perhaps trying to visit Percy underwater in some way.
Obviously, Percy Jackson himself is a main character. Since he is the God of Tartarus in this fanfiction concept. Annabeth Chase might also be a key character as she is Percy's long - time friend and love interest. Her reactions to Percy's new status and how she supports or challenges him can be a big part of the story.
The hellhound is also a key character. Depending on how the fanfiction is written, it could be a villain at first and then turn into an ally. It might have unique abilities and a backstory that makes it an interesting addition to the story.
Well, of course, Percy Jackson himself is the central character. He's the one being raised by the gods, so everything revolves around him.
There are also times when a character who is an outcast within the Apollo cabin is the main character in fanfiction. Maybe they don't seem to fit in because their powers are different from the others, or they have a different view on how the cabin should be run. Their journey to find their place within the cabin and among the other demigods can make for an interesting story.
Obviously, Percy Jackson is the central character. But often, Annabeth Chase also plays an important role. She might come to visit Percy on Olympus and get involved in his adventures there. Also, the gods themselves, like Poseidon (Percy's father), Zeus, and Athena, are frequently main characters as they interact with Percy in the Olympian setting.