The main character is Percy Jackson. However, in fanfiction, new characters can be introduced. For example, a mysterious figure who has been trapped in Tartarus for eons and becomes Percy's guide during his escape. Another could be a demigod from a different camp who has a unique power that is essential for Percy's escape. Annabeth is also bound to be a significant character, as she and Percy have a strong bond. She might be on a desperate mission to rescue Percy or be the first one he reaches out to when he finally escapes.
Obviously, Percy Jackson himself is a main character. Annabeth might also be very involved, either helping him escape or being there to welcome him back. Grover could be another main character, perhaps using his nature - related powers to assist in some way.
There could also be some characters unique to the fanfiction. Maybe a new demigod who has some special power related to Tartarus and ends up joining Percy on his quest. Or perhaps an ancient being who was imprisoned in Tartarus long ago and has knowledge that Percy needs to complete his mission.
In some more creative fanfictions, new characters might be introduced. For example, a powerful demigod who was born in Tartarus and has unique abilities. These new characters can add a fresh twist to the story, interacting with Percy and the others in unexpected ways, and often bringing new plotlines related to the mysteries of Tartarus.
He would surely interact with Annabeth. Their relationship is central in the Percy Jackson series, so in a Tartarus fanfic, she'd likely be there to support him or be in a similar situation as him.
Well, perhaps a plot twist is that Percy doesn't escape in the traditional sense. Instead, he finds a way to manipulate the rules of Tartarus so that he can exist in both Tartarus and the mortal world simultaneously. This could lead to some really interesting story developments as he tries to keep this a secret from his friends while also using his new - found ability to fight against the forces of evil. Also, it could be that the escape route he finds is actually a trap set by an enemy, but he manages to turn it around and use it to his advantage in a really unexpected way.
The main antagonist could be Nyx. She is a powerful primordial goddess of the night, and Tartarus is her domain in a way. She might see Percy as an intruder and try to stop him from escaping or achieving his goals there.
Percy himself is often very popular. He's the main character in the original series, so in fanfiction about Tartarus, his journey and how he overcomes the challenges there are always interesting. Annabeth is also a popular character. Fans love to write about her either trying to rescue Percy or being part of his adventure in Tartarus. Nico di Angelo is another one. His powers and his somewhat dark and mysterious nature make him a great addition to Tartarus - related fanfictions.
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.
The Titans or Giants that were banished to Tartarus could be the main antagonists. They might see Percy as a threat or an opportunity to escape and seek revenge on the Olympians.