Eventually, he could become very determined. Ash has a strong spirit. Instead of being defeated by this betrayal, he might see it as a challenge. He could set out to clear his name and expose the truth behind the false accusations that led to his being betrayed.
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.
Ash might be heartbroken at first. He'd feel a deep sense of loneliness and confusion.
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.
Charizard could be a bit more indifferent. It's a proud and strong Pokémon. It might just snort when it sees Ash in a dress, as if to say 'This is a strange human thing, but it doesn't affect our relationship'. However, if other Pokémon made fun of Ash because of the dress, Charizard might get protective.
A plotline might involve Ash being betrayed by a new acquaintance he meets on his journey. This person could pretend to be his friend, gain his trust, and then use that trust to steal his Pokémon or important items. Or perhaps Ash is betrayed by a Pokémon he just caught. The Pokémon could have been planted by an enemy trainer to spy on Ash and then turn on him at a crucial moment.
One reason could be power. Maybe he was offered a great deal of power by an outside force in exchange for betraying the harem. Another could be jealousy. If there was someone in the harem who was getting more attention or was more successful than him in some way, he might have been driven by envy to betray them.
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.
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 possible plotline could be that Ash and Cynthia are betrayed by a powerful villain who they once trusted. This villain might have been secretly working against them all along, perhaps stealing their Pokémon or sabotaging their quests. For example, the villain could be a rival trainer who was jealous of their success.
One possible plot could be that Ash discovers Anabel has betrayed him by siding with a rival trainer in a crucial tournament. This could lead to a rift in their friendship and Ash having to find a way to overcome this setback and prove himself without her support.
One possible plot could be that Ash is falsely accused of a serious crime within the Pokémon world, and Greninja, due to some misinformation or outside influence, turns against him. Maybe a powerful villain manipulates Greninja's mind, making it believe that Ash is a threat to the Pokémon world. This could lead to intense battles between them, with Ash being heartbroken and trying to prove his innocence while also trying to save Greninja from the villain's control.