There might be a monstrous creature that Percy befriends instead of fighting. This creature could have unique abilities that help Percy survive, like being able to sense danger from afar.
I'm not sure specifically since I don't know the exact fanfiction, but it could be his friends who might be either involved in his hacking activities or completely unaware of them. Maybe there's a mentor - like figure who guides him in the world of hacking.
In this fanfiction, Percy might also have to deal with his own sanity. Growing up in Tartarus, the constant threat and the dark, desolate place could be a huge mental strain. He could struggle with nightmares and hallucinations, and part of the plot could be about him overcoming these mental challenges while trying to find a way back to the world above.
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.
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.
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.
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 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.
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.