There's also a chance that an old enemy from Ash's past resurfaces. This enemy, who has a long - standing grudge against Ash, could find a way to use Ash's Pokemon to get revenge. They might have developed new techniques or powers over time that allow them to influence the Pokemon's loyalty.
Ash would probably be devastated. He would feel like his world has been turned upside down. At first, he might not believe it and think it's all a big mistake. As the reality sinks in, he could start to isolate himself from others, even from his Pokemon for a while. However, his strong will would eventually kick in. He would use this as motivation to become stronger, not just in his Pokemon battles but also in his ability to face challenges alone, without relying on his so - called friends.
Ash would probably be really hurt at first. He's a very trusting person, so it would be a big shock. He might start to question his own judgment in trusting her.
Gary Oak is a likely character. He has always been a rival to Ash and in fanfiction, he might betray Ash out of jealousy or a desire to be the ultimate Pokémon trainer. Another could be Misty. Maybe she gets influenced by some external force or her own insecurities and turns against Ash. Brock could also be a candidate in some stories. If he falls in love with a girl who has a vendetta against Ash, he might be forced to choose between his loyalty to Ash and his new love interest.
His friends like Misty and Brock would probably visit him.
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.
Some of the gym leaders could also be involved. Maybe they feel threatened by Ash's new - found power as a champion. They might think that Ash will change the way the Pokemon battling world works and in order to stop that, they betray him. They could spread lies about Ash misusing his Pokemon or something similar.
Gary could be a likely character. He has always been Ash's rival and there could be a plot where his jealousy or some other motive leads him to betray Ash. Another could be one of his so - called friends from the Pokemon League who might be influenced by power or some dark secret. Or, in some really dark fanfictions, it could be a Pokemon that Ash trusted deeply but for some reason turns on him.
It could be his rivals. For instance, Tracey or Paul might see Ash's potential in the new region as a threat and decide to betray him. They could try to sabotage his battles by giving false tips to his opponents or stealing his battle strategies.
He might meet an old Pokémon expert who gives him tips on hatching the egg.
It could be a new character, someone who has a long - standing grudge against Ash's group. This character might manipulate Ash into betraying his friends by preying on his insecurities or desires.