A young mermaid named Ariel could be a main character. She could be Percy's friend and help him explore the underwater world. She could teach him how to swim faster and find the best hiding places from the bigger sea creatures.
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.
One possible plot could be Percy as a baby merman being discovered by a kind fisherman. The fisherman, instead of harming him, decides to protect Percy and hides him in a secret cove. As Percy grows, he starts to show unique powers like being able to communicate with sea creatures even at a young age.
Grover could also be a main character. He might be at the scene when Percy gets drunk and try to use his nature - related powers to sober Percy up. Grover could also provide some comical relief with his reactions to Percy's drunken state, like his horrified expressions or his attempts to herd Percy like he would a wild animal.
Well, there could be a wise old wizard who discovers Percy. The wizard takes it upon himself to protect Percy from the forces that might be after him because of his mysterious origins. He becomes like a mentor to Percy, teaching him about the magical world and how to control his powers.
Nico di Angelo could be a central character. Given his connection to the underworld and his own complex relationship with the gods, he might be the one who has the most knowledge about what really happened to Percy and how to reverse it. He could be on a quest to bring Percy back from the dead, facing many challenges along the way.
There could be a new, made - up character. This character could be a seer who had foreseen Percy causing the earthquake but was unable to prevent it. Now, this seer has to work with Percy to understand why it happened and how to prevent future disasters. Or it could be a young demigod who looks up to Percy and is now caught in the middle of the chaos Percy has caused and has to learn to be brave and helpful during this crisis.
Annabeth could be one of the main characters. She might be torn between staying with the gods and following Percy. Their relationship could be a central part of the story as she tries to convince him to stay or decides to go with him.
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.
There could be a strict drill sergeant as one of the main characters. This person would be constantly pushing Percy to his limits and teaching him discipline. Another character might be a fellow student who becomes Percy's best friend. They could support each other through the tough times at military school.
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.