It could be one of his close friends. In many fanfics, friends often play a crucial role in uncovering such secrets. They might notice Percy's unusual behavior, like being overly jumpy or withdrawn, and start to suspect something was wrong.
Perhaps a mentor figure. This person has more experience and wisdom. They might pick up on signs of abuse through Percy's performance in certain tasks or his attitude towards authority. If Percy has been abused, he may have a negative reaction to figures of authority, and the mentor could notice this and start to dig deeper.
Perhaps there's a power struggle in the story world. Percy might be seen as a threat to certain groups or individuals who then resort to abuse as a means to gain control over him. Another reason could be related to his family background. If his family has some sort of enemies, they might take out their anger on Percy.
Nico might be initially shocked. He could feel a deep sense of anger towards whoever was responsible for Percy's abuse. Then he would probably become very protective of Percy, vowing to keep him safe from any further harm.
She might be shocked and angry. Thalia is a protective character. She could immediately vow to find those who abused Percy and make them pay. For example, she might start by gathering information about the abuse, asking Percy for details in a gentle way to not further traumatize him.
Well, Annabeth is a strong character. When she finds out Percy was abused in the fanfiction, she likely starts by trying to comfort him. She may use her intelligence to figure out how to help him deal with the trauma. Maybe she'll also plot some sort of revenge against the abuser in her own clever way, all while being a great source of emotional support for Percy.
She might be shocked at first. Then she could become extremely protective of Percy. She would likely confront those who are responsible for the abuse and make sure Percy is safe from further harm.
The abuser could face immediate death. Gods like Hades might be quick to drag the abuser down to the Underworld for punishment.
Well, in such a fanfic, Percy's abuse might make him more guarded. He could become distrustful of others at first. For example, he may be hesitant to form new relationships or open up to people, fearing more harm. This could also lead to him being overly self - reliant, not wanting to depend on anyone in case they turn out to be like his abuser.
Well, in many such fanfics, the gods might react with anger. They could seek revenge on the abuser. For example, Zeus might send a bolt of lightning to strike the place where the abuse took place as a sign of his wrath.
They might be shocked at first. They could feel a sense of guilt for not noticing earlier. Maybe they immediately try to comfort Percy and offer their support, like giving him a safe place to stay and assuring him that he's not alone anymore.
The gods might be outraged. They could see Percy as one of their own, especially since he has done so much for them. They might immediately seek to punish the abuser. For example, Zeus could send a bolt of lightning towards the person who abused Percy.