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.
In such a fanfiction, the plot could involve Ash being betrayed by his long - time friends. Maybe they envy his success in the Pokemon world. For example, Gary could spread false rumors about Ash in the new region, causing the local trainers to distrust Ash. This would make Ash's journey in the new region much more difficult as he has to not only deal with new Pokemon and challenges but also regain his reputation.
Ash would probably be really hurt at first. He's always been so trusting of his friends. He might go off on his own for a while to clear his head, like he did when he had a falling - out with some of his friends in the past.
Perhaps in this new region, Ash discovers a powerful new Pokemon that others want to control. His friends, fearing the power of this Pokemon, betray him to protect the region. They might think that Ash is being too reckless with this new discovery and that it could lead to destruction. It could also be that there's a prophecy in the new region that Ash is somehow involved in, and his friends misinterpret it and believe he's the cause of potential harm.
One possible analysis could be that in such a fanfiction, the idea of Ash being betrayed might be a way to create drama. It could involve his friends or companions in the harem turning against him for various reasons like jealousy or misinformation.
Professor Kukui is another common character. He serves as a guide and mentor to Ash in the Alola region. He might provide Ash with new Pokémon knowledge, especially about the unique Alola Pokémon species. And of course, there are the various Pokémon trainers that Ash meets and battles in the Alola region. These trainers can range from novice trainers just starting out to more experienced ones who pose a real challenge to Ash.
Well, in many such fanfictions, it could involve Ash being betrayed by Clair perhaps in a Pokémon battle strategy. Maybe Clair teams up with an unexpected rival against Ash, or she might break a promise regarding Pokémon training or a joint adventure.
In such a fanfiction, perhaps Ash could be betrayed by his friends in Alola. Maybe they misunderstand his actions or are influenced by some false information. For example, they might think Ash is secretly working for an enemy, which leads to the betrayal.