Well, at first, there would be disbelief among many Slytherins. But once they realize it's true, a few of the more empathetic ones might actually feel sorry for Harry. They could start to see him as more than just the 'Boy Who Lived' but as a fellow student who has been through a great deal of pain. They might even try to befriend him secretly, away from the prying eyes of their housemates who might not be so accepting.
The Weasleys are likely to be horrified. They are a warm - hearted family. Mrs. Weasley might immediately take on a motherly role towards Harry, perhaps making him his favorite foods and fussing over him. Mr. Weasley could become very protective, ensuring that Harry is safe from any further harm. The siblings, like Ron, might be angry that Harry had to endure such abuse and vow to always have his back.
The relationship between Slytherin and Gryffindor houses could shift. Slytherins might become more protective of Harry, which would be a huge change from the usual rivalry. This could lead to some Gryffindors being suspicious of Slytherin motives.
They might be shocked and immediately want to protect Harry. For example, they could confront the abuser or find a safe place for Harry.
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.
They would likely be shocked and angry. The Weasleys are a kind - hearted family. They might immediately take Harry under their wing more protectively, like Molly Weasley would probably start fussing over him even more, making sure he has enough to eat and a warm place to sleep.
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.
When Slytherins discover Harry's bullied in fan fiction, it's a complex situation. The more traditional, self - centered Slytherins might see it as an opportunity to gain an upper hand over Harry and his friends. They could try to use the knowledge to their advantage in the Hogwarts hierarchy. However, there are also Slytherins who are more nuanced. These Slytherins might secretly feel bad for Harry. They might start to question the whole system of bullying that exists in the fan fiction world. They might even start to have a change of heart towards Harry, but they would be very cautious about showing it, as they don't want to be seen as going against their house's general attitude.
The others in the seven, like Jason, would probably feel a sense of guilt for not noticing earlier. They would come together as a group to support Percy. Jason might offer his friendship and strength to Percy, telling him he doesn't have to face it alone anymore. They would also likely start to investigate who was responsible for the abuse.
They might be shocked and immediately try to offer protection. For example, they could take Harry to a safe place within the wizarding world where he can't be reached by his abusers.
In many fanfics, Draco's reaction turns into a sense of protectiveness. He feels a need to shield Harry from any further harm, whether it's from the people who abused him in the past or from the judgment of others. For example, in some stories, he'll stand up to those who make snide remarks about Harry's past, using his Slytherin cunning to put them in their place. He also tries to be a source of comfort for Harry, which is a big change from their initially antagonistic relationship.