At first, Ash would be in shock. But then, he would likely turn his focus on finding out the truth. He's not one to give up easily. He'd start investigating what led to his betrayal. He might even go on a journey within the RWBY world to find evidence to clear his name and expose those who really are at fault. He'd still hold out hope that his former friends would see the truth and come back to his side.
One possible plot could be that Ash, being from a different world, is initially welcomed into the RWBY universe. But then, he might be betrayed when his Pokémon powers are seen as a threat. Maybe the RWBY characters fear that his abilities could disrupt the balance of their world. For example, if his Pikachu's electricity causes unexpected power surges in a world that has its own unique energy sources like Dust.
His rival, Gary, could be a likely candidate. Gary has always been competitive with Ash, and in this fanfiction, his jealousy could reach a boiling point, causing him to betray Ash for his own gain, like trying to steal his Pokémon or get credit for his achievements.
Well, without actually reading the specific fanfiction, it could be about Ash Ketchum, who is in a 'godlike' state, being betrayed by someone. Maybe his friends turn against him due to jealousy of his newfound power. Or perhaps an enemy manages to deceive those close to him, making them think Ash has done something wrong when he hasn't.
One common theme could be Ash being betrayed by his friends in the Pokémon world. For example, they might leave him for a more powerful trainer or because of a misunderstanding. Another theme could be betrayal by the Pokémon League, like being wrongly disqualified from a major tournament. Also, some fanfictions may explore Ash being betrayed by a new character who has ulterior motives and tries to undermine his journey.
Ash could get angry. He has a strong sense of justice, and he might feel that May has violated their friendship. He could challenge her, not in a violent way, but more in a way to get an explanation and to try to make her see the error of her ways. He would likely be determined to show her that what she did was wrong.
Ash would likely be shocked at first. He's so used to seeing the good in people, especially fellow trainers. But then he'd probably get angry and determined to set things right. He has a strong sense of justice, so he wouldn't let the betrayal go unpunished.
Ash would be extremely devastated. Pikachu is not just his Pokémon but also his closest friend. He would likely first try to reason with Pikachu, not believing that Pikachu could truly betray him. He might be in denial for a while.
Perhaps in this fanfiction, Ash is betrayed by a mentor figure. This person has been guiding him all along but has a hidden agenda. The rated m aspect could mean there are some intense battle scenes and moral dilemmas. Ash will have to reevaluate his beliefs and skills to overcome this betrayal and the challenges that come with it.
Ash would probably be initially shocked. He is used to seeing the good in people and would not expect such a betrayal from Cynthia. He might stand there in disbelief for a moment, unable to process what has just happened.
Ash might be heartbroken at first. He'd feel a deep sense of loneliness and confusion.