Well, often in such a 'ash gets betrayed fanfiction', the story might start with a seemingly normal day for Ash. Then, out of the blue, someone he trusts deeply, like Pikachu or one of his human friends, does something that he perceives as a betrayal. This could be something as simple as not believing in him during a crucial battle or siding with an enemy for a reason that's not clear to Ash at first. It can create a lot of emotional turmoil for Ash and make for a very engaging read.
One possible reason for Ash getting betrayed in the fanfiction could be misinformation. A villain might spread false rumors about Ash, making his friends believe he has done something wrong. His friends, without verifying the truth, turn against him. It's a sad situation where lack of communication and trust plays a big role.
There was a boy who was head over heels for a girl. They had plans for the future. But then the girl met someone at her work who she thought was more exciting. She began to lie to the boy about where she was going. Eventually, the boy discovered the truth through a mutual friend. He felt betrayed not just because she left him for someone else, but because of all the lies she told him along the way. It was a very difficult time for him to accept this harsh reality.
In the '4 kunoichi gets betrayed fanfic', it could be that the four kunoichi are working for a noble cause. However, one of their own, due to jealousy of the attention the others get, decides to betray them. She spreads false rumors about them, making the whole ninja community turn against them. The four kunoichi have to prove their innocence and also figure out a way to deal with the traitor within their group.
One common theme is trust. When Sonic is betrayed, it shatters the trust he had in his friends or allies. This often leads to a journey of rebuilding that trust or finding new trustworthy companions.
It could be their master. Maybe the master has been corrupted by dark forces. He sees the potential in the kunoichi and fears they will become more powerful than him. So, he betrays them to stop their progress.