Grover could be another key character. He has always been Percy's friend and would surely not sit idly by while Percy is in Tartarus. He could use his nature - related powers to try and find a way into Tartarus. Additionally, some of the monsters in Tartarus might become key characters in their own right, for example, if Percy befriends one of them or if they have some important information that Percy needs to get out of Tartarus.
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.
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.
Annabeth could be a likely candidate. She's always been by Percy's side, and she'd stop at nothing to save him. Her intelligence and combat skills would be useful in the dangerous environment of Tartarus.
The plot could revolve around Percy's journey of self - discovery in Tartarus. Sent there unjustly by the gods, he could start to question his past actions and his relationship with the divine. As he battles the horrors of Tartarus, he may develop new powers or skills. He could also be on a mission to expose the true nature of the gods' actions, which led to his banishment to Tartarus. Along the way, he would face challenges that test his physical and mental strength to the limit.
Percy could initially be heartbroken. He's a very loyal person, and to be betrayed by those he considered family would be devastating. In Tartarus, he would have to rely on his instincts and his powers. He might also experience a lot of self - doubt, wondering if he really was as bad as they made him out to be. As time passes, he would start to plan his revenge against those who wronged him while also looking for an escape route.
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.
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.
In some fanfics, Nico di Angelo could be the one to help Percy. Since Nico has a connection to the Underworld and has some dark powers, he might be able to enter Tartarus more easily than others. He could use his powers to fend off the monsters and guide Percy to safety.
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.
It could be another powerful god or goddess who wants to use Percy's betrayal of Tartarus for their own gain. Maybe a god of chaos who sees this as an opportunity to create more disorder in the world. They might manipulate Percy into the betrayal and then turn on him later.