In a Sakura Cardcaptors fanfic where Chelsea finds out, she might react in different ways. She could be shocked and initially not believe what she has discovered. For example, if she finds out about Sakura's secret cardcaptor identity, she may question how such a thing could be possible in the seemingly ordinary world they live in.
He could be furious. In many fanfictions, Snape is a strict character. Discovering something about a cupboard might lead him to suspect some sort of rule - breaking or hidden secrets. He might start an investigation right away to find out what's really going on.
In the Supernatural fanfiction, if Mary finds out about Jess, she might be quite shocked. She could initially be confused as Jess is Dean's girlfriend and Mary has her own complex backstory. She may try to understand the relationship and how Jess fits into Dean's life, given her own experiences with love and loss in the Supernatural universe.
In some fanfictions, Naruto is completely devastated. He might go into a state of shock at first, unable to believe the news. Then he could be filled with intense grief and anger, blaming himself for not being there to protect Jiraiya.
Well, if Clint discovers Coulson is alive in a fanfic, it could start with Clint being in disbelief. He might question how it's possible at first. Then, as he realizes it's true, he could feel a mix of emotions. There could be a sense of relief, because Coulson was like a friend and mentor to him. They might start to share stories of what they've been through since they last saw each other. And this new - found reunion could be the start of a whole new adventure for them, perhaps even leading to a new team - up against a common enemy that they thought they couldn't face without each other.
Well, Dudley might be jealous at first. He's always been used to being the 'big shot' in his own little world. If he finds out Harry is famous in fanfiction, he could start to feel overshadowed all over again. Maybe he'll try to downplay it, saying it's just some silly made - up stories.
Charlie might be shocked and filled with anger. He could immediately seek to comfort Harry and ensure his safety. Maybe he'll vow to find the perpetrator within the context of the fanfiction's story.