Well, in such a fanfiction, it would likely shatter the trust between them. Harry would probably feel betrayed and hurt deeply. He might become withdrawn and less likely to confide in Hermione. Their friendship could be on the verge of collapse as Harry struggles to come to terms with what she has done.
One possible reason could be that she was in love with someone else and couldn't resist the temptation. It could also be that she was under a great deal of stress, and her judgment was clouded. Maybe she thought Harry was taking her for granted and wanted to get back at him in a very wrong way. Or there could be some dark secret in her past that made her act this way.
In many fanfictions, it could be Ginny. Since they have a relationship in the original series, it would be a shock if she were to cheat on Harry. It would create a lot of drama and give the writer a chance to explore Harry's vulnerability.
The story could also explore the relationship between Harry and the person who cheated on him. Maybe there were signs all along that Harry ignored. For example, his partner being overly secretive or distant at times. By delving into these aspects, the fanfiction can become more complex and engaging.
In fanfiction where Spiderman gets cheated on, it often explores the emotional turmoil he goes through. It can be a way to add more depth to his character. Writers might show his vulnerability and how he copes with the betrayal. Maybe he throws himself more into his superhero duties to distract from the pain.
Well, it could also be Harry who is wrongly thought to be cheating on his partner. In the fanfiction, perhaps some false rumors are spread by his enemies. They create a situation where his friends start to doubt him, and his relationship is put under great strain.
There's also a chance that Neville Longbottom could be involved in a cheating situation. He might have grown into a very different person in the fanfiction, and due to some complex emotional and social factors at Hogwarts, he cheats on his significant other. This would be quite a shock as Neville is usually seen as a very loyal character.
One possible outcome in this fanfiction could be Percy feeling a deep sense of betrayal. He might initially be in denial, not wanting to believe that he has been cheated on. This could lead to a downward spiral in his self - esteem as he questions what he did wrong in the relationship.
There could be many interpretations in such a fanfiction. Maybe it shows Naruto's vulnerability in relationships. It might be a way to explore his character deeper by putting him in a difficult emotional situation.
Often, Hermione reacts with a mix of shock and anger. She's a strong - willed character, so she doesn't just take it lying down. In some fanfictions, she immediately confronts the cheater and gives them a piece of her mind.