As a result of the bullying, Percy might develop stronger empathy for others. He knows what it's like to be picked on, so he'll be more sensitive to the feelings of those who are also in difficult situations. His social skills could improve too. He may learn how to better read people and situations to avoid getting into trouble. And in the fanfiction, this could lead to him making more genuine friends who appreciate him for who he is.
Percy could use his combat skills. If the bullies physically attack him, he can defend himself without seriously harming them. He's trained at Camp Half - Blood after all. Another way is to use his wits. For example, if they mock his dyslexia, he can come up with smart comebacks about how his different way of thinking gives him an advantage.
One possible scenario could be that Percy is bullied because of his dyslexia. The bullies might mock him for his reading difficulties, making him feel even more isolated. Another situation could be that his connection to the Greek gods is somehow revealed, and the bullies think he's crazy, so they pick on him. Or perhaps they're jealous of his skills in combat training that he got from Camp Half - Blood and target him out of envy.
One way could be for Percy to find a group of true friends who can support him. They can stand up to the bullies together. For example, if there are some nice classmates, Percy can join them during breaks and lunches, and the bullies might be less likely to target him when he's in a group.
Naruto sometimes reacts with humor. He'll make jokes about the bullies' actions, which can either make the bullies angrier or make some of the bystanders see how ridiculous the bullies are. For example, if a bully tries to trip him, he might do a silly tumble and then laugh it off, saying something like 'Wow, that was a new kind of training exercise!'. And he also might use these situations to prove his own strength and determination. He'll train even harder to show the bullies that he won't be broken by their actions.
It could be because Percy is different. If he comes from a different background or has a unique personality trait, the bullies might target him. In the fanfiction, if Percy is from a different race or has an unusual way of speaking, the bullies might pick on him for not being like them.
Percy could use his wits. For example, he could outsmart Luke by using his knowledge of the fanfiction world's rules or magic. He could also find allies. Maybe there are other characters who dislike Luke's bullying ways and would be willing to help Percy.
Start with their first encounter. Maybe it's on the first day of high school when they accidentally bump into each other and immediately get on each other's nerves. Percy could spill his coffee on Annabeth's books, and she gets really mad. This sets the tone for their ongoing enmity.
It could develop slowly. At first, they might just be acquaintances who exchange polite greetings in the hallways. Then, they could be grouped together for a school project. Through working on it, they'd start to see each other's strengths and weaknesses, and a friendship would begin to form.
In fanfiction, Issei might also develop morally. He could be faced with tough decisions where the line between right and wrong is blurred. This forces him to think about his actions more carefully and develop a stronger sense of justice, rather than just acting on impulse like he sometimes does in the original story.
Issei's character often develops by becoming more powerful. His Boosted Gear evolves, and he learns to control it better. For example, he might gain new abilities or forms in fanfiction.