There could be villains in the story too. Maybe the ones who thought they had killed Percy are there to gloat over his death. When Percy crashes the funeral, they become the main antagonists again. Also, Percy's family members, such as his mother, would be at the funeral. Her reaction to seeing Percy alive would be a very emotional and important part of the fanfiction.
Other characters could include Percy's comrades from his previous adventures. They would all be gathered at the funeral, reminiscing about Percy. When he crashes the funeral, they might be confused at first, but then quickly realize that they need to help Percy deal with whatever situation led to the false report of his death. There could also be some minor characters from the world he inhabits, like local shopkeepers or other demigods who knew Percy, all having different reactions to his unexpected return.
His friends could be involved, like Annabeth. She might be mourning at the funeral and then be extremely surprised and happy when Percy shows up. Grover could also be there, and he'd probably be the first to give Percy a big, emotional hug.
Well, perhaps Percy shows up at his own funeral in the fanfiction because he wasn't really dead. He might be confused as to why everyone thought he was gone and start revealing secrets that change everyone's perception of his 'death'.
There could be other non - human characters like a wise old wizard who knows about Percy's true nature and tries to guide him. Maybe a shape - shifter who at first seems like an enemy but later becomes an ally.
There could be a wise old monster who acts as Percy's mentor. Maybe a female monster who becomes Percy's love interest. And of course, there would likely be some villainous monsters who oppose Percy's new existence and try to harm him.
Obviously Percy himself is the main character. There could also be his friends like Annabeth. She might be supporting him in his training, or even training with him. Grover could also be involved, perhaps using his nature - related powers to help Percy in some unique training exercises.
There could be a rival within the family. Someone who wants Percy out of the way, perhaps for inheritance reasons or because they're jealous of Percy's abilities or popularity. This rival could be the one who instigates the events that lead to Percy's disownment. And then there might be a mentor - like figure outside of the family who takes Percy in after he's disowned and helps him to grow and face his new situation.
Jason is likely to be another main character. Since he has his own set of skills and knowledge, he might have an interesting relationship with Percy in this story. Maybe he's a bit jealous at first of Percy's control over the ship, but then they end up working together really well. And of course, Piper could be there too. She might use her charm - speak abilities to help the crew communicate with any strange creatures they encounter during Percy's time controlling the Argo 2.
Grover can also be quite prominent. His loyalty to Percy remains a key factor. In some fanfictions, his connection to nature might be enhanced to better support Percy in his overpowered adventures. For example, he could be able to communicate with more powerful nature spirits to assist Percy in battles or quests.
Another possible main character could be a rival. A person who is always competing with Percy and who might use the discovery of her secret as a weapon against her. This would add a lot of conflict to the story.
Annabeth could be a main character. She might be trying to stop Percy from wanting to die. She could use her intelligence and her deep understanding of Percy to figure out what's wrong and try to talk him out of it. For example, she could remind him of all the good things they've been through together and all the people who need him.
There could be a strict but kind music teacher. This teacher sees Percy's potential and pushes him to practice harder and improve his skills. Another character could be Percy's best friend who is always there to support him during his musical journey, even when things get tough.