Obviously, Percy is the main character. He is the focus of the story as it is about him in solitude.
There might also be a mysterious figure that haunts Percy's solitude. This figure could be an antagonist, creating challenges for Percy in his solitary state. Maybe it's an old enemy who has found a way to reach Percy even in his seclusion, or it could be a manifestation of Percy's inner demons, making his time alone even more difficult.
Superman is surely one of the main characters as the Fortress of Solitude is his. Lois Lane might also be involved as she is often close to Superman in the Superman - related stories. Maybe Lex Luthor as well, since he is always interested in Superman's secrets and the Fortress of Solitude could hold many of those.
As I don't know this novel well, I can't tell you the main characters specifically. But usually in a solitude - related novel, there could be a single protagonist who is the focus of the story during their time alone.
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 many 'Percy is a biker' fanfictions, Percy is the central figure. However, new characters could be introduced. For example, there could be a rival biker who turns out to be a demigod from another camp. This would create a lot of tension and interesting plotlines as Percy has to deal with both his biker rivalries and his demigod duties.
Percy is the main focus. There could be other demigods that are either intrigued or threatened by Percy's difference. Some might want to be his allies, while others might see him as a threat to the status quo at Camp Half - Blood. And his family, like his mother, would also play an important role in dealing with the changes in Percy.
In most 'Percy is wise' fanfictions, Percy is central. His character development towards being wise is the focus. Other characters often include Grover, his satyr friend. Grover might rely on Percy's wisdom more than ever. Then there are the minor characters like the various nymphs and demigods who interact with Percy and are affected by his wise decisions.
There could be other gods as well. For instance, Zeus might play a role, perhaps being suspicious of Percy's newfound godly powers or seeing him as a threat to his own authority. Hermes could also be there, acting as a messenger or a guide to Percy in the world of the gods.
Percy is the central figure in 'percy is immortal fanfiction'. Other characters like Grover might play important roles too. Grover could be worried about Percy's immortality and try to help him come to terms with it. And of course, there could be new characters introduced who are also immortal and interact with Percy in different ways.
Percy is usually a central figure, and his backstory and powers play a big role in the fanfiction. Peter may be a new character introduced into Percy's world or vice versa. Their interactions are what drive the story forward. Whether they are friends or start off as rivals, their relationship is at the heart of the fanfiction, and their individual traits are what make the story unique.
Obviously, Percy is the main character as he is the captain. But depending on the type of fanfiction, there could be other important characters. For example, if he's a sports captain, his teammates would be significant. They could have different personalities and skills that interact with Percy's leadership. If he's a ship captain, his crewmates, like the navigator or the first mate, would play important roles in the story.