It could be a jealous rival trainer. This rival has always been in Ash's shadow and decides to plot his downfall. By spreading false information and turning Ash's friends against him, the rival hopes to finally get the recognition he thinks he deserves.
There's also a possibility that it's a misunderstood Pokémon. This Pokémon has some sort of grudge against Ash, perhaps thinking Ash accidentally wronged it in the past. So, it uses its powers to make everyone around Ash believe he's a bad person, thus causing the mass betrayal.
Maybe it's an evil organization that wants to use Ash's popularity against him. They figure that if they can make everyone turn on Ash, they can create chaos in the Pokémon world. They start by brainwashing some of Ash's closest friends to start the chain of betrayal.
He might be initially shocked. Ash has always been surrounded by friends and supporters, so suddenly being betrayed by everyone would be a huge blow. He could be in disbelief for a while.
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.
One possible plotline could be that Ash's friends start believing false rumors about him being a traitor to their group. For example, an enemy might spread lies that Ash is secretly working for a rival organization. So, his friends gradually turn their backs on him, leaving him alone and confused. He then has to prove his innocence while also dealing with the pain of betrayal.
Ash might be heartbroken at first. He'd feel a deep sense of loneliness and confusion.
Another possibility is that there is some kind of dark influence at work. A powerful and malevolent force could be manipulating people's minds to turn them against Ash. This force might see Ash as a threat and use this underhanded method to get rid of him, causing his world to crumble as he is betrayed by all.
One idea could be that Ash is betrayed by his so - called friends in the Pokémon world. They might be jealous of his status as the Chosen One and team up against him. For example, Gary could spread false rumors about Ash, causing other trainers to doubt him.
In this kind of 'ash and riolu betrayed fanfic', it might be that Ash and Riolu are in a Pokémon tournament. They meet a new trainer who seems friendly and offers to team up with them. However, during the tournament, this so - called friend betrays them by using underhanded tactics. This could include things like secretly weakening Riolu before a battle or spreading false rumors about Ash to other trainers. As a result, Ash and Riolu not only lose the tournament but also feel deeply hurt and betrayed.
Perhaps Percy's loved ones are made to believe false rumors about him spread by an unknown enemy. As a result, they turn their backs on him. Percy would need to find a way to expose the real culprit behind the rumors and regain the trust of his friends and family.
The story could end with Ash forgiving everyone. Despite the betrayal, he realizes that his friends were misled or acting out of weakness. So he takes them back and they all learn from the experience, growing stronger together.
In such a fanfiction, Ash might become extremely isolated. He could lose faith in his Pokémon and himself. Maybe he'd leave the Pokémon world in despair, feeling that all his efforts were in vain. Without the support of his friends and Pokémon, his once - strong will to be a Pokémon Master could crumble completely.