Ash would be heartbroken. He has a deep bond with Charizard, and seeing it turn against him would be like losing a part of himself. He would probably first try to reason with Charizard, thinking that there must be some mistake. When that doesn't work, he might become a bit angry, but mostly he'd be sad and determined to find out what's wrong and fix it.
The other Pokémon in Ash's team would be confused and protective. Pikachu, for example, would be very angry at Charizard for turning on Ash. It would try to stand up to Charizard and protect Ash. The other Pokémon might also try to comfort Ash in their own ways, like snuggling up to him or trying to cheer him up while he's in his state of despair.
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.
One possible plot could be that Ash is forced to give up Charizard in a high - stakes Pokémon battle. Maybe a powerful trainer challenges Ash and demands Charizard as the prize if he loses. Ash, being a proud trainer, doesn't expect to lose but due to some unforeseen circumstances, like a new and unknown Pokémon move used by the opponent, he loses the battle. Charizard, being sent to the new trainer, initially resists but is somehow forced to obey. This makes it seem like Charizard has betrayed Ash as it has no choice but to follow the new trainer's commands.
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 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.
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.
Ash would likely be extremely heartbroken. He has always had a deep connection with Pokemon and believing that Arceus, a powerful and supposedly benevolent being, has betrayed him would be a huge blow. He might become withdrawn at first, not knowing who to trust anymore.