Well, James is a loyal friend. So, when he discovers Sirius is abused, he would most probably drop everything else he is doing to focus on Sirius. He'd be angry, not just at the abusers but also at himself for not realizing it sooner. He'd make sure Sirius knew that he wasn't alone and that he had someone who would always be there for him. James might also encourage Sirius to learn self - defense magic more intensively so that he could protect himself in case the abusers tried to harm him again.
Sirius would likely be extremely angry. He already has a protective streak towards Harry. He might immediately confront those he suspects of the abuse, whether it's the Dursleys or others at Hogwarts. He would probably also try to remove Harry from the situation as quickly as possible.
She might be horrified and immediately take steps to protect Sirius. For example, she could confront the abuser or report it to the proper authorities within the wizarding world.
Remus is likely to be shocked and angry. He might immediately want to protect Sirius. He could confront those who are abusing Sirius or try to find a way to get Sirius out of that situation.
He might be filled with rage. James Potter was a loving father figure in his own right. So, when he finds out about Harry's abuse, he could immediately seek to confront the abuser. He would want to protect Harry at all costs and might use his magical skills to ensure Harry's safety.
Sirius might be completely devastated. He could go into a state of shock at first, not believing the news. Then, he might be filled with guilt for not being there for his brother, and anger towards the situation that led to Regulus' death.
Well, in a number of these fanfictions, Draco has a very emotional reaction. He may be angry at himself for not realizing it sooner. His reaction can also vary depending on his relationship with Harry prior to the discovery. If they were enemies before, his discovery might lead to a complete shift in his attitude. He could start to see Harry in a new light and do whatever it takes to make sure Harry is no longer in harm's way. He might even change his own behavior and values in the process of helping Harry.
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.
In the fanfiction, if Ron finds out Harry is abused, it's a complex situation. Ron's reaction would be multi - faceted. Emotionally, he'd be a mess. He loves Harry like a brother. He might feel guilty for not noticing earlier. Ron would also start to view Harry in a new light, with more compassion and understanding. He would use his Weasley family connections to find resources or help for Harry. Maybe he'd ask his mother for advice or even get his older brothers involved if the abuse was from a powerful source within the wizarding world.
He might be shocked at first. Then he could feel a strong sense of protectiveness towards Harry. Maybe he immediately tries to find ways to keep Harry safe from further harm, like staying closer to him at all times.