There could be a detective character. Since Percy is in the business of protection, there may be times when he needs to work with the police or a private detective to solve a case related to the threats against his client. They might have different approaches, but they need to cooperate.
Another character could be Percy's mentor or an older, more experienced bodyguard. This person could offer Percy advice and guidance when he faces difficult situations in protecting his client. Maybe this mentor has a different philosophy about bodyguarding, which Percy has to either accept or reject as he grows in his role.
There could be new characters as well, like a devil mentor figure who teaches Percy about his devil heritage. This character could be ancient and have a lot of knowledge about the devil realm. Or there could be an angelic character who is initially an enemy but later realizes Percy is not fully evil and they end up forming an unlikely alliance.
His friends like Annabeth would surely be main characters. She is smart and resourceful, so she would be leading the search for Percy. Grover could also be a major character. He has a strong sense of loyalty and his connection to nature could be useful in tracking Percy down.
He could interact with his old friends at Goode, like Rachel. She has a special connection to the demigod world, and their relationship could be further explored. Also, there might be new teachers or students that Percy meets. Maybe a new student who is actually a demigod in hiding too, and they start to figure out each other's secrets.
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.
There could be a new character, a jailmate of Percy. This character could be either a friend who helps Percy survive in jail or an antagonist who makes his life more difficult. For example, a tough guy who wants to bully Percy at first but later has a change of heart.
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.
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.
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.