It might also be a case of sibling rivalry taken to an extreme. In the fanfic, he could be trying to prove himself as superior to you in some way. For instance, if you are both competing for a role in a big event in the story, he may bully you to get an edge.
One reason could be jealousy. Maybe you have something he wants, like a special power or a closer relationship with another character in the fanfic.
Sometimes, in a fanfic, a character's backstory or personal issues can lead to bullying behavior. Perhaps your brother in the fanfic had a difficult past that made him bitter and he takes it out on you.
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.
Maybe the brother is seen as more obedient and studious. In a ninja village, those traits are often highly valued. Naruto, on the other hand, is very boisterous and doesn't always follow the rules. So, in the fanfic, the family or those around them might give more attention to the brother. Also, it could be that there's some sort of prophecy or family legacy associated with the brother that overshadows Naruto, causing the neglect.
First, you can try to communicate with your brother within the context of the fanfic. Maybe he has some misunderstandings that can be cleared up through honest conversation.
One reason could be for financial gain. Maybe someone thought they could get money from sharing a sob story. Another is for revenge. They might want to get back at someone or an organization.
One possible reason could be that she's different. In many fictional worlds, being unique can make a character a target. Maybe her appearance or her abilities set her apart in a way that makes others jealous or feel threatened.
One possible reason could be the extreme pressure he faces in the fanfic. Maybe he has to deal with much more powerful enemies alone, which overwhelms his mental state. Also, if he experiences a lot of betrayals in the fanfic, like friends turning against him, it could lead to his mental instability.
One reason could be his quirk. His electric quirk might make him a target for villains to manipulate. They could use his power for their own evil plans.
In some fanfics, Naruto may be sad because of unrequited love. He has strong feelings for someone, but that person doesn't return his feelings. Naruto is a very emotional character, and love is an area where he can be easily hurt. So this could be a source of his sadness in the fanfic.
It could also be about narrative tension. Fanfic authors want to keep the story interesting. By having the Avengers be 'dicks' to Tony, they create a source of conflict. This conflict can drive the story forward, whether it's leading to Tony leaving the team for a while, or him trying to fix the relationships. It gives the author more room to explore different emotions and plot developments, like Tony's journey to regain the trust and respect of his fellow Avengers.
One possible plotline is that the main character is a new student at school. They are not a big fan of One Direction, and the school is full of die - hard fans. These fans start to bully the character by excluding them from social activities, like lunch tables or parties. As the story progresses, the character tries to find ways to fit in without having to pretend to like One Direction.