Even one of his Pokémon could betray him in this fanfiction. Maybe a Pokémon that he caught later in his journey was actually planted by the enemy. It could act normal for a while but then suddenly turn on Ash at a crucial moment, like during a big battle, causing him to lose not only the battle but also the trust of his other Pokémon and friends.
His rival could be the one to betray him. Rivals are often jealous and competitive. If Ash's rival saw an opportunity to gain an advantage by betraying him, they might do it. For instance, they could lie about Ash to the Pokémon League, getting him disqualified from important battles.
It could be Gary. Gary has always been a rival to Ash. Maybe in this fanfiction, Gary gets jealous of Ash's progress and decides to betray him by making false accusations to the Pokemon League about Ash using illegal training methods.
Maybe Team Rocket. They've been after Ash for a long time. If Ash comes back as a champion, they might see it as a threat and betray him in some way. They could pretend to be his friends at first and then stab him in the back to get what they want, like his powerful Pokémon or his championship title.
A fellow trainer in Alola could betray Ash. This trainer might be envious of Ash's success as a champion. He could secretly work with Team Skull or some other antagonists to undermine Ash. For instance, he might give false information to Ash about a battle location, leading Ash into a trap.
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.
Some fanfictions might have Misty betray Ash. Perhaps she gets lured by the promise of power or some valuable reward. She could be tricked into believing that Ash is no longer the friend she thought he was and that betraying him is for the greater good.
Perhaps in the fanfiction, Ash was betrayed by a powerful mentor figure. This mentor, who was supposed to guide him, saw Ash as a threat to their own power or plans. So they leaked Ash's secrets to his enemies, which led to his downfall. It could also involve a love interest. Say, a girl Ash trusted and liked turned out to be working for the villains all along and used his feelings for her to betray him.
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.
It could be a rival trainer. Rival trainers are often very competitive in the Pokémon world. Maybe this rival is jealous of Ash and May's progress and decides to betray them to slow them down. They might spread false rumors about Ash and May, causing other trainers to be distrustful of them.
Well, Ash could lose his trust in people. If he's betrayed, he might become more guarded and less likely to form new friendships. This could change his entire approach to his Pokémon journey. He might start to rely more on his Pokémon for emotional support rather than other humans.
One idea could be that Ash is betrayed by his so - called friends in the Pokémon world. They might be jealous of his status as the Chosen One and team up against him. For example, Gary could spread false rumors about Ash, causing other trainers to doubt him.