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.
Tony might be really surprised. He could have a range of reactions from being amused to being a bit uncomfortable, depending on what the fanfiction is about.
She might be initially shocked. If the fanfiction contains some untrue or inappropriate things about Clarke, Abby could be protective and try to find a way to stop it from spreading.
Abby might react quite strongly. Given her relationship with Clarke, she would likely be very interested in what is being written about her in the fanfiction. She may try to find the author to ask about their intentions. If the fanfiction has Clarke in some sort of dangerous or inappropriate situation, Abby would probably be quite worried and might even try to get it removed. However, if it's a positive and creative take on Clarke, she could see it as a compliment to her daughter - like figure and might encourage others to write more positive fanfiction about her.
In certain fanfiction scenarios, Brienne's discovery about Jon could lead to an interesting dynamic. Brienne is a character who values integrity above all. When she realizes who Jon is and what he stands for, she may feel a kinship with him. They both have faced hardships and prejudice in their own ways. She might view Jon as a glimmer of hope for a better Westeros. She could become his champion, not just in battle but also in the political arena, using her influence to gather support for Jon's leadership. This could also lead to some great character interactions and development for both of them as they navigate the complex world of Westeros politics and warfare together.
He might be shocked. Fanfiction can often be quite different from the original source material, and the nogitsune concept is already pretty out - there. So the shock value could be high.
Well, often in fanfiction, when Harry discovers his inheritance, it can completely change his social standing. He could be revealed as part of an ancient and noble magical family. This might lead to him getting more respect from the wizarding community and also open up new opportunities for him in terms of education and alliances. For example, he could be given access to rare magical libraries or training from the most skilled wizards in the family. He might also find out about family secrets that have been hidden for generations, which could help him in his fight against Voldemort or other threats.