Obviously, Percy would be the main character. But there could also be other characters like his family members who might be trying to figure out why he has such a negative attitude. Maybe there's a best friend from his past who he now hates but who still tries to reach out to him.
There could be a new acquaintance, a stranger who comes into Percy's life and is intrigued by his hatred for everyone. This person could try to break through Percy's walls and understand him better. And of course, there could be antagonists, people who Percy hates for a specific reason, like bullies or people who have wronged him in the past.
Another possible main character could be a mentor figure who sees potential in Percy despite his hatred. This mentor might try to guide Percy towards a more positive view of people. There could also be some side characters who represent the society that Percy hates, like a group of popular kids or a clique that he feels excluded from.
Well, it could be that Percy has had some really bad experiences in his life that made him bitter towards everyone. Maybe he was betrayed by friends in the past, so now he just can't bring himself to trust or like anyone.
Well, Pikachu could still be a main character. Even if Ash hates everyone, Pikachu might be the one trying to bring back the old Ash. Maybe Team Rocket could also be main characters. They are always around Ash, and his new - found hatred could make their interactions even more interesting. They might be confused by his change or even try to take advantage of it.
Dumbledore could also be an important character. He might be the one who tries to figure out what's wrong with Harry and tries to reverse this hatred. Another character could be Snape. He might see this as an opportunity to make Harry's life even more difficult or, on the other hand, might be secretly trying to help him get over this hatred.
It could be Naruto himself. Since he hates everyone, his actions against the people he once cared about could make him the antagonist. He might start attacking the Hidden Leaf Village or other ninja villages, making him the main source of conflict.
Maybe a new, powerful and evil ninja from another land. This ninja could have come to the village and caused a lot of chaos, making Naruto believe that no one in his village was capable of protecting themselves or him. So he blames everyone for their weakness and starts to hate them all, with this new ninja being the root of the problem.
It could be a jealous rival. This rival might have always been envious of Percy's popularity and skills. So, they manipulate Percy's friends into betraying him to get Percy out of the way and take over his position.
There could be his friends like Ron and Hermione. Even though he hates everyone, they would still be important characters as they might try to figure out what's wrong with Harry and how to bring him back from this state of hating everyone.
Perhaps in this fanfiction, Naruto discovers some dark secrets about the village. Secrets that make him feel like he's been used all along. Like the village was only using him as a weapon against their enemies. This realization could make him resent everyone in the village, thus hating everyone.
Family members, if he has any in the fanfiction world, would be devastated. His mother, for instance, would have to deal with the loss of her son. She might become more involved in the affairs of the demigod world, trying to make sure that her son's sacrifice was not in vain. Also, his half - siblings, if any, would be affected. They could be inspired to step up and prove themselves worthy of Percy's sacrifice.
Obviously, Percy would be the main character. His friends like Annabeth, Grover, and the others from his group would also be important characters. They are the ones who forget his birthday, so they play a big part in the story as they deal with the consequences of their forgetfulness.