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.
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 might be heartbroken at first. He'd feel a deep sense of loneliness and confusion.
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.
Pikachu would probably be very curious. It might zap the lightsaber a bit to see what would happen, and then be in awe of the bright blade. It could also get a bit protective of Ash when he wields the lightsaber, as if it senses the added danger.
Ash would feel a deep sense of betrayal that could shatter his usually upbeat attitude. He might go through a period of self - doubt. However, his love for his Pokémon and his sense of justice would pull him out of it. Mewtwo, having already experienced betrayal from humans in the past, would be filled with a cold fury. It would use its psychic abilities to gather information about the betrayers. Ash and Mewtwo would work together, Ash learning from Mewtwo's insights and Mewtwo being inspired by Ash's tenacity in the face of betrayal.
I think Ash would be angry as well as hurt. He's a passionate trainer and his friends mean a great deal to him. He might challenge May in a Pokémon battle, not to defeat her but to make her see how wrong her betrayal is. And during the battle, he would keep asking her to come back to their old friendship.
Ash would be extremely hurt. He's a very loyal person, and having someone he cares about like Lillie betray him would be a huge blow. He might initially go into denial, not believing that Lillie could do such a thing.
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.