Well, Cedric might first be in disbelief when he finds out. Then, he would likely gather evidence about Harry's abuse. He might also seek help from his friends or some authority figures within the wizarding world to ensure Harry's safety and get justice for him.
In this kind of fanfiction, Cedric might feel shocked and angry. He could decide to protect Harry from further harm. Maybe he'll try to find the abuser and put a stop to it.
Cedric would probably be very angry.
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.
Cedric might first talk to Harry privately to get all the details. Then he could go to Dumbledore for help.
Cedric would be deeply troubled. In the wizarding world, they have their own code of honor and respect. Finding out about Harry's abused fanfiction would go against everything Cedric stands for. He might start to question the morality of the Muggle world that allows such things to be written. He would also feel a sense of sadness for Harry, knowing that someone out there has created a story that portrays him in such a negative and harmful light. Cedric might even consider taking some sort of action, like talking to Harry about it or trying to get the fanfiction removed if he could, though that would be a complex matter given the nature of fanfiction and the different worlds they live in.
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.
Well, if we consider the 'umbridge finds out harry was abused fanfiction', it really depends on the writer's take. One possibility is that Umbridge, being the power - hungry character she is, might try to cover it up to maintain the status quo at Hogwarts. On the other hand, she could also see it as a way to discredit Dumbledore, thinking he failed to protect Harry. There's also a chance that the discovery could lead to a change in Umbridge's relationship with Harry, from being a tormentor to an unexpected ally, though this would be a huge shift in her personality.
One possibility is that Narcissa would try to use her influence within the pure - blood society. She could spread the word among the more reasonable pure - blood families about Harry's situation. This might lead to some sort of support network being formed for Harry, even if it has to be done in secret to avoid Voldemort's wrath. She might also try to subtly nudge Draco into being nicer to Harry, which would be a big change in their relationship.
In such a fanfic, Snape might initially be shocked. Given his complex relationship with Harry in the original series, he could struggle between his old grudges and his sense of duty. He might take it upon himself to protect Harry secretly at first, perhaps by using his position at Hogwarts to keep an eye on Harry's abusers. For example, if it's the Dursleys abusing Harry, Snape could find ways to make sure they don't go too far without revealing his true intentions.
Cedric could also try to find out more about the origin of the fanfiction. He might use his connections in the wizarding world or even try to understand Muggle ways of tracking down information. Once he has more information, he could decide whether to confront the writer, if possible, or simply try to protect Harry from the negative impact of such a story. For example, he could make sure Harry doesn't accidentally come across it and get hurt by it.
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.