One common theme is the shattering of trust. When Percy gets betrayed, it's all about how that trust he had in someone is completely broken. This often leads to him feeling isolated and alone.
Percy would likely feel a great sense of hurt and confusion. He might question his own judgment in trusting Annabeth. He could become more withdrawn and less trusting of others in general.
The boy and the girl were friends turned lovers. The girl was always jealous of the boy's female friends. One day, she made up a false story that the boy had cheated on her with one of his friends. In fact, she was the one who had fallen for someone else and wanted to break up with the boy in this way. This was a cruel betrayal for the boy.
In some 'harry gets betrayed fanfic', it could be Dumbledore who betrays Harry in a way. Maybe Dumbledore has been using Harry all along for a greater plan without fully disclosing it to him. Harry would feel extremely hurt and betrayed as Dumbledore was like a mentor to him. This would lead to a series of events where Harry questions everything he has known and tries to find his own path.
One great 'percy gets betrayed fanfiction' is 'The Betrayal of Percy Jackson' on FanFiction.net. It really delves deep into the emotions of Percy when he's betrayed by someone he trusts. The author does a wonderful job of building up the relationship before the betrayal, which makes it all the more impactful.
One possible reason could be jealousy. Naruto is very powerful and some in Konoha might be envious. For example, some minor clans might feel that Naruto, as an outsider in a sense, gets too much attention and resources. Another reason could be fear. His Nine - Tails power is so immense that some might fear he could lose control and endanger the village, so they betray him to try and eliminate the 'threat' before it's too late.
In such a fanfic, the plot might start with Naruto's friends gradually turning against him due to false rumors spread by a hidden enemy. They could start to exclude him from missions and isolate him within the village. For example, Sasuke might believe the lies and challenge Naruto, thinking he has changed for the worse. This would lead to a series of battles where Naruto has to fight not only his former friends but also his own feelings of betrayal and confusion.
Another reason could be jealousy. Suppose Percy was getting a lot of attention and praise for his recent achievements, and Annabeth felt overshadowed. Her jealousy could fester and turn into a desire to undermine Percy, leading to her betrayal. However, this would be a more complex and less noble reason compared to the others.
One possible reason could be jealousy. Maybe the Weaslys in the fanfic become jealous of Harry's fame or special abilities. Another reason might be some kind of dark magic influence on them, which changes their nature and makes them turn against Harry. It could also be due to a misunderstanding. For example, a false rumor spreads about Harry that makes the Weaslys believe he has done something terrible.