Well, the brother is the central figure as the whole premise is about him replacing Percy. But we can't forget about Percy's mother. She would surely have a strong reaction to her son being replaced. Her relationship with the brother would be complex, filled with emotions like confusion, suspicion, or maybe even acceptance over time. And there could be other characters like Grover, who was Percy's friend. Grover's role would be interesting as he might be the one to first notice the differences between Percy and his brother and try to figure out what's really going on.
Obviously, Percy's brother is one of the main characters. He is the one who is replacing Percy. And the characters who were associated with Percy like his friends, family members, and maybe some of his foes are also likely to be main characters as they would be affected by this replacement and would interact with the brother.
One possible plotline could be that the brother has different abilities or a distinct personality. Maybe he approaches the same challenges Percy faced in a completely new way. For example, if Percy was known for his courage in battles, the brother might use his intelligence to outwit the enemies instead.
Some of the gods might also be important characters. Poseidon, Percy's father, could have an interesting reaction to Percy's new relationship. Hades, as the god of the underworld where Styx is located, might also be involved, either as an ally or an obstacle depending on the direction of the fanfiction.
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.
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.
Well, Karin is for sure a main character as she is taking over Sakura's position. Sakura, although being replaced, still has an important role as the benchmark for Karin's replacement. Then, other characters in their shared universe who interact with either of them could be main characters too. For instance, Naruto could be involved, seeing as he is friends with Sakura and may have some sort of reaction to Karin replacing her. Also, characters from their respective teams or groups might be main characters as they would be affected by this replacement.
It's likely that Chiron, the centaur, is also a main character. He has a wealth of knowledge and experience and can guide Percy and the other demigods in using their powers effectively to protect the camp. He might also be involved in training them and preparing for different types of threats that Percy's powers need to counter.
Obviously, Percy would be one of the main characters. His friends like Annabeth and Grover would also be important. They would be the ones to support Percy through his injury and help him recover. Maybe there would be some new characters introduced as healers or other allies who help Percy during this tough time.
Annabeth could be one of the main characters interacting with him. She might tease him about taking his shirt off, or she could be concerned if he's taking it off because of an injury. Their relationship is complex, and this situation could add a new layer to it.
A group of rogue demigods could also work well as antagonists. They might be jealous of Percy and his brother's abilities or have their own twisted ideas about how the demigod world should be run. They could create all sorts of trouble for the brothers, from stealing their quests to setting traps for them.
Grover could be a popular character. Since he's Percy's best friend, he might be involved in the fanfiction, perhaps helping Percy protect his mom or going on adventures with them. Another could be Annabeth. Her intelligence and skills could be useful in the stories, like when they are facing a difficult situation and she comes up with a clever plan.