His family member, if there is one in the story who is perceptive and caring. They might have seen some small signs over time but not fully understood what was going on. Once they find out about the abuse, their natural instinct to protect their family member would kick in, and they would be one of the first to react, showing concern and a determination to make things right for Percy.
In this kind of fanfiction, there could be a lot of shock and disbelief among the characters at first. His friends might feel guilty for not noticing the signs earlier. They would probably become extremely protective of Percy.
Poseidon might be extremely angry. He could immediately seek revenge on the abuser. After all, Percy is his son and he has the power to protect him.
She would probably be furious. Artemis has a strong sense of justice and protecting the vulnerable. So, she would likely immediately start to plan how to deal with those responsible for Percy's abuse.
She might be shocked and feel a great deal of sympathy for Percy. She could immediately try to comfort him and assure him that he's safe now.
Another reaction could be that Percy is worried about Sally getting into trouble because of him. He might try to convince her to stay out of it, fearing that the abuser or those associated with the abuser could harm Sally. But at the same time, deep down, he wants her support and protection.
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.
Some characters might be shocked. For example, Harry Potter could feel a sense of pity and might try to help Draco in some way. Hermione might start to analyze the situation and look for ways to protect Draco from further abuse. Ron, on the other hand, might be a bit more hesitant at first but would eventually come around to support Draco.