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.
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.
Well, Bill could be shocked at first. But then he would likely become very protective. He may take Harry to a safe place, like the Burrow, and gather his family around to support Harry. He would also probably try to find out more about the abuse, like who was behind it and for how long it had been going on. He'd be determined to make sure it never happened again.
Harry might be initially shocked. He could feel a mix of sadness and a sense of responsibility. After all, Sirius was like a father figure to him.
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.
Arthur might be shocked and immediately want to protect Harry. He could start by confronting those who are abusing Harry or find a safe place for Harry to stay.
Viktor might be shocked at first. He could feel a strong sense of protectiveness towards Harry. Maybe he immediately tries to comfort Harry and assure him that he's not alone anymore.
Well, Dumbledore has always had a special connection with Harry. When he finds out about the abuse, he would be devastated. His first instinct would be to comfort Harry. He might also reflect on his own actions and whether he could have prevented the abuse from happening. Dumbledore would then use all of his resources to make sure Harry is safe and happy. He could provide Harry with a mentor or a guide to help him through the difficult times, someone who can teach Harry how to cope with his past and move forward in a positive way.
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.
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.