Hermione would probably be very angry and start researching ways to help Harry legally in the wizarding world. Ron, on the other hand, would be ready to fight anyone who had hurt Harry.
Well, usually Remus would be shocked and heartbroken. He'd try to create a safe space for Harry at Hogwarts. For example, he might let Harry stay in his office when he feels threatened. Remus would also likely try to get Harry some kind of emotional support, like introducing him to some of his more trustworthy friends. He'd be determined to break the cycle of abuse and help Harry grow into a confident and healthy wizard.
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.
There could be a plot development where Remus and Sirius go on a journey to find a new place to start over, away from the abuser. Another development could be that Remus uses his intelligence to outwit the abuser legally. The fanfiction could also show how Remus helps Sirius regain his confidence and self - esteem. As Sirius heals, they might start to explore their relationship in a deeper way, whether it's a stronger friendship or something more romantic. There could also be a sub - plot where other characters get involved in different ways, some trying to help and some causing more trouble.
In the fanfic 'Malfoy's Revelation', Malfoy is paired with Harry for a project. As they spend time together, Malfoy notices some strange behaviors from Harry, like flinching at sudden movements. After some snooping around, he discovers that Harry has been abused. This new knowledge makes Malfoy more empathetic towards Harry and he tries to help him in his own way, which includes standing up for Harry against some of their mutual enemies at Hogwarts.
Well, first of all, the students would probably be shocked. His friends, like Ron and Hermione, would be extremely angry at the Dursleys. They might try to comfort Harry more often. And the school itself could start providing special resources for Harry, such as counseling or extra protection spells. Also, there could be a big change in the way the wizarding community views Harry. Instead of just seeing him as the 'Boy Who Lived', they would see him as a victim who needs support.
The other students could have a range of reactions. Some might be sympathetic and try to befriend Harry more. Others might be afraid, especially if the abuser is still around. But overall, it would create a stir in the wizarding community, and there would be a general outpouring of concern for Harry's well - being.
The abuser might face the wrath of the DA. They could be ostracized within the wizarding community. The DA members could use their influence to make sure the abuser is held accountable for their actions.