The main characters would surely include Percy. Then, there could be Tyson, Percy's half - brother. Tyson could be really worried about Percy and use his strength to protect him while he's sick. Also, Chiron in his disguised form as a teacher at the school could play a major role. He has a lot of knowledge about demigod illnesses and could guide Percy's friends on how to help him.
Obviously Percy would be one of the main characters as he is the one who is sick. His best friends like Annabeth and Grover would likely also be main characters as they would be taking care of him or trying to find a cure for his illness.
Well, maybe Percy could be poisoned by a monster in disguise at school. He starts feeling really sick, like having a high fever and being super weak. His friends, like Annabeth, suspect something is wrong and start investigating. They might find out that there's a new threat in the school that's targeting demigods.
Obviously, Percy would be the main character as the story is about him getting sick. But his friends like Annabeth and Grover would also be important characters. They would be the ones trying to help Percy get better, so they play a big part in the fanfiction.
A new character could be a transfer student from another demigod camp. This student has a different set of powers and a unique backstory related to a less - known Greek god. They could initially clash with Percy but then become great friends as they face challenges together.
For sure Percy. A merman who is an outcast among his own kind because he has a unique ability that others are jealous of. And Nico di Angelo could be another main character. Nico's connection to the underworld might play an interesting role when dealing with the merman's world which is so different from the underworld but still has some similarities in terms of being a hidden and often misunderstood realm.
Perhaps an old enemy from Greek mythology who has found a way to infiltrate the school in a modern form. This could be a minor god or a powerful monster. They want to get revenge on Percy and Nico for past defeats. They might be using the school's resources, like the science labs or the library, to create chaos or find a way to regain their full power.
Percy Jackson himself would likely be one of the main characters. Also, Annabeth Chase as she is always by Percy's side. Grover would probably be there too since he's Percy's best friend.
Silena Beauregard is also underrated. She could be the focus in a fanfiction set in the school. Since she was a daughter of Aphrodite, she could use her charm and beauty skills in a different context at school. For example, she could use her powers to boost the confidence of shy students or mediate between feuding groups by using her charm to calm everyone down.
Obviously, Artemis. She is in charge of the Hunt, and her relationship with Percy would be central to the story. Then there could be Zoe Nightshade. She was a very important member of the Hunt in the original Percy Jackson lore. If Percy is adopted by the Hunt, she would have a big influence on him, either as a mentor or as someone who initially opposes his presence. And the wolf is also crucial. It could be a mysterious figure, maybe a messenger of the Hunt or a guardian of some sort, and it would be involved in Percy's journey every step of the way.
Maybe the child of one of Percy's friends from Camp Half - Blood could also be a main character. For instance, the son or daughter of Grover. They could have unique abilities related to nature and prophecy, and bring a different perspective to the story compared to Percy's kids.