Well, apart from Luke and Percy, there could be other characters involved too. For example, Annabeth might be there. She could be a witness and later play a crucial role in either getting revenge for Percy or trying to understand Luke's motives. There might also be some minor characters from their world who could be bystanders or somehow related to the events leading up to the beating.
Obviously, Luke and Percy are the main characters. Luke is the one doing the beating, and Percy is the victim in this scenario.
The main characters are surely Luke and Percy. However, we can't rule out the possibility that some of the gods or demigods from their universe could be mentioned or even have an influence on the situation. Maybe Poseidon, Percy's father, could be in the background, and his relationship with Percy might be a factor that Luke takes into account when he decides to beat up Percy.
It could be Percy himself if he's acting unjustly. Even though he's beating up Hercules, if his reasons are not noble, like if he's just being full of hubris, then he would be the antagonist.
Percy could be the instigator if he has a moment of rage or is misinformed. His impulsive nature might lead him to start the fight without fully understanding the situation.
Obviously, Peter and Flash. It's possible that Peter is a hero - type character with a moral code that Flash has somehow violated. Flash could be a sidekick - turned - antagonist or just a regular character who has a conflict with Peter. Their personalities and the nature of their relationship in the original source material would be twisted in this fanfiction to lead to Peter beating up Flash.
In this fanfiction, perhaps there is a misunderstanding between Luke and Percy. Luke might think Percy has betrayed him in some way, like siding with the wrong people in the context of their fictional world, and thus resorts to violence by beating him up.
The main characters are Luke and Percy. Luke might be portrayed as a troubled or jealous character who takes out his frustrations on Percy. Percy, on the other hand, is likely the protagonist who has to deal with Luke's bullying and grow from the experience. There could be other supporting characters in the fanfiction, but Luke and Percy are central to the concept of 'Luke bullies Percy' story.
Chiron, the centaur mentor at Camp Half - Blood, might get involved in the rescue. He has a vast knowledge of the demigod world and could use his wisdom and connections to track down Luke and free Percy. Also, some of the other demigods at the camp who look up to Percy might volunteer to help, like Clarisse. She may not like Percy much, but she wouldn't want a fellow demigod to be in the hands of Luke.
A fan who really likes Sakura and wants to show her strength. They might be tired of seeing Sakura always in a support role and want to create a scenario where she takes the lead and shows her power by beating up Naruto.
Luke, on the other hand, would probably be a more experienced and perhaps a bit jaded character. He could have seen a lot in his adventures and might be a bit overprotective of Percy at times. There could also be other characters from the original source material that interact with them, like their friends or mentors who play important roles in shaping their relationship and their individual growth.
It depends on the situation. If the girl was just acting out of anger without any real provocation, then she could be at fault. But if the boy had been harassing her or others, then he might be the one to blame.
Maybe some of the gods or goddesses play important roles. For example, Apollo, the god of medicine, could make an appearance. He might offer some magical healing or at least some advice on how Percy can recover more quickly. This would add an interesting mythological element to the fanfiction.