Perhaps Leaf betrays Ash because she becomes jealous of his friendship with Pikachu. She might feel like Pikachu gets all of Ash's attention and decides to side with a group that wants to separate them. This could lead to Ash having to fight not only to save Pikachu but also to win Leaf back.
It could be a new and powerful trainer who has a vendetta against Ash. This trainer might see Ash as a threat to their own rise to the top in the Pokemon world. They could manipulate Leaf into betraying Ash to further their own goals.
Perhaps Ashley betrays Ash by revealing his battle strategies to other trainers. In the Pokemon world, strategies are crucial, and this could lead to Ash losing battles. As a result, Ash might feel extremely disappointed and have to come up with new strategies while also dealing with the betrayal.
One possible scenario could be that Ash betrays May for a powerful Pokemon. Maybe he gets offered a super rare and strong Pokemon but has to cut ties with May to get it. Another could be that May betrays Ash because she is influenced by a rival trainer who convinces her that Ash is holding her back from reaching her true potential as a Pokemon trainer.
Perhaps a new and powerful trainer arrives in the story. This trainer manages to deceive Ash's harem members, making them think that Ash is a hindrance to their growth as trainers. As a result, they side with the new trainer and betray Ash, leaving him feeling confused and alone.
In addition to Ash's friends, there could be some villains in the fanfiction. They might be the ones who initiate the betrayal. For example, some Team Skull members could play a role. They might manipulate the situation to make Ash's friends believe that he is not trustworthy. Also, some mysterious new trainers in Alola could be involved, either as instigators or as characters who try to help Ash later.
Betrayal could also stem from a misunderstanding. Maybe Ash or Cynthia accidentally offend someone without realizing it. For example, they might unknowingly step on a trainer's dreams or beliefs. And that trainer, feeling deeply hurt, decides to betray them. Or it could be that a Pokemon they both have a connection with has a different loyalty that they were not aware of, which leads to the Pokemon causing their betrayal.
One possible storyline could be Serena feeling heartbroken and then deciding to focus on her own Pokemon journey independently. She might strive to become a better Pokemon trainer on her own terms, proving herself not just for Ash but for her own self - worth.
One possible storyline could be that Ash is a knight in training and Serena is a noble lady. Ash has to prove his worth in a jousting tournament to win Serena's hand in marriage. Another could be that they are both on a quest to find a magical Pokemon that is said to have the power to save the medieval kingdom from an evil sorcerer. And a third could be that Serena is a healer and Ash is a warrior, and they team up to fight off a horde of dark - type Pokemon that are attacking the villages.
It could be that Mew has some sort of special power or knowledge that certain groups in the Pokémon world are after. They see Ash as an obstacle to getting what they want from Mew. So, they scheme to make everyone think Ash is untrustworthy. His friends, not knowing the truth, end up betraying him. Ash then has to fight against all odds to protect Mew and regain the trust of his friends.
One possible plotline could be that her so - called friends in the Pokemon journey turn against her for some reason, like jealousy over her Pokemon's strength. Maybe they try to steal her Pokemon or spread false rumors about her to isolate her.