Obviously, Percy is one of the main characters. Since he's the one sent to Tartarus. There might also be some of his old friends who play important roles, like Annabeth. Maybe she's on a quest to save him, so she becomes a central figure in the fanfiction.
The main character is Percy. But in fanfiction, new characters can be introduced as well. Maybe there's a powerful being in Tartarus that becomes an important character. This being could either be an enemy, constantly trying to thwart Percy's attempts to escape, or a potential ally, with its own motives for helping Percy. And of course, Percy's relationships with his existing friends from the original story, like Piper and Jason if they are included, will also shape the story.
Percy is the main focus as he is sent to Tartarus. However, there could be other characters from the original series that get involved in different ways. For instance, Grover might use his nature - related powers to try and sense Percy's whereabouts or communicate with him in some way. Nico could also be a key character, as he has some knowledge of the underworld and might be able to offer some help in getting Percy out.
Obviously, Percy would be the main character. Since he is the one sent to Tartarus. Annabeth might also play an important role, even if she's not in Tartarus with him at first. She could be on the surface world trying to figure out a way to get to him or send help. Grover could be another likely character. His connection to nature might come in handy in some way, like if there are strange nature - based elements in Tartarus that only he can understand or control.
Obviously, Percy is the central character. But often, Annabeth can also play an important role, even if she's not physically in Tartarus with Percy in some stories. She might be trying to find a way to rescue him from the outside. Another common character is Nico, who has some knowledge of the Underworld and Tartarus and could potentially be a key figure in helping Percy either survive or escape.
Perhaps one of the Titans who were banished to Tartarus. They might see Percy as a threat to their plans of revenge or escape. Kronos, if he still has some form of existence in Tartarus, could be a major antagonist. He would have his own motives and powers, and he could try to manipulate Percy or simply get rid of him.
Annabeth would be a likely character to appear. Since she and Percy are so close, she might try to find a way to rescue him from Tartarus. Another character could be Nico. He has experience with the Underworld and might be able to offer some help or advice. Grover could also show up, using his nature - related powers to try and sense Percy in Tartarus.
Obviously, Percy would be one of the main characters. Since it's about him training in Tartarus. And Annabeth might also be involved. Maybe she's helping him in some way, or she's waiting for him outside Tartarus while he trains.
Annabeth might also be a main character. She could be either with Percy in Tartarus or be a driving force for him from outside, like trying to find a way to rescue him or communicate with him in some way.
Obviously, Percy is one of the main characters. Given the 'dark' aspect, he may have a more brooding and conflicted personality compared to his normal self.
Nico di Angelo could be involved. Since he has some knowledge of the Underworld and Tartarus, he could play a crucial role. He might be the one to guide Annabeth or other rescuers to the right places in Tartarus to find Percy. Also, some of the Titans or primordial deities who have a connection to Tartarus could be important characters, either as obstacles or potential allies for Percy.
It's possible that an unexpected character like a friendly Titan could rescue Percy. There could be a Titan who has had a change of heart and realizes that Percy is important for the balance of the world. This Titan might use its own knowledge of Tartarus and its powers to free Percy from the clutches of that terrifying place.
Obviously, Percy would be the main character. He's the one stuck in Tartarus, so the story would revolve around him. Maybe Annabeth could also be an important character, even if she's not physically in Tartarus with him. Her thoughts and attempts to rescue Percy from afar could be part of the story.