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.
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.
Annabeth Chase could also be a main character. Her wisdom and knowledge of architecture and mythology would help in understanding the Horsemen's motives and weaknesses. She could be the one to come up with plans to stop them, like finding ancient artifacts that could be used against the Horsemen.
Obviously, the baby gods themselves could be main characters. Each with their own unique personalities and powers. For example, a baby god of the sea might be curious and mischievous like Poseidon. Another could be a demigod who discovers the baby gods and takes on the role of their protector. This demigod would have to deal with the challenges of protecting these powerful but inexperienced beings. And there could also be an older god who has an interest in the baby gods, either for good or bad reasons, and becomes a central figure in the story.
Definitely the children of Percy and Annabeth. Their kid would inherit some of their parents' skills and personality traits. Maybe they'd be a great strategist like Annabeth and have Percy's water - related powers. Another main character could be the son or daughter of Nico di Angelo. They'd likely have some spooky underworld - related powers and a brooding personality.