One possible scenario is that Greninja is tricked into believing that Ash is endangering its kind. So it decides to turn on Ash. Haruka, on the other hand, having known Ash for a long time, might try to convince Greninja of Ash's true nature. However, Greninja's loyalty is so shaken that it doesn't listen easily. In the end, Ash has to prove his worth and friendship to Greninja all over again.
In this fanfiction of betrayal, Greninja could be lured by a false promise of greater power. It then betrays Ash, who is heartbroken. Haruka, being Ash's friend, tries to help him. She might use her own Pokémon to fight Greninja if necessary, but she also hopes to resolve the situation peacefully. Ash, meanwhile, refuses to give up on Greninja and starts a journey to understand what went wrong and how to win back his friend.
Well, they could be betrayed by a new acquaintance in a strange town they visit during their travels. This new person could pretend to be friendly, but then steal their Pokemon or important items.
When writing about Ash, Haruka and Greninja's betrayal in fanfiction, you can start with a normal day in their journey. Then, something unexpected happens that leads to Greninja's change of heart. You need to build up the tension and drama. Use dialogue effectively to show the conflict between the characters. For instance, the conversations between Ash and Greninja when the betrayal occurs. And Haruka can be the mediator or the one who takes action to fix things. Also, think about the setting and how it can affect the story. A spooky forest or a mysterious cave might add to the atmosphere of betrayal.
They might have an epic battle against a common enemy together. Greninja could show off some new moves it has learned during their separation, and Ash could use his experience from other adventures to lead them to victory.
In this fanfiction, Greninja might come back with new powers. Maybe it has been on a special training journey and returns to Ash to help him in a big battle.
Well, perhaps Greninja will encounter some new and strange Pokémon in Alola that it has never seen before. Ash could use this opportunity to train Greninja further, and they might participate in some Alola - specific Pokémon contests or battles together. Greninja's presence could also stir up some friendly rivalries among the local Pokémon and trainers, leading to exciting adventures and growth for both Ash and Greninja.
In this fanfiction, Ash might introduce Greninja to his family. His mother could be surprised but also happy to see such a unique Pokémon. Maybe Greninja would show off some of its cool moves in the backyard.
Well, one possible plot could be that in the Kalos region, Ash makes a new powerful rival. Haruka, being in Kalos for some reason, gets influenced by this rival's charm and power. In Alola, Ash discovers that Haruka has been secretly helping this rival against him, thus feeling betrayed. Maybe the rival promised Haruka something she couldn't resist, like a rare Pokémon or a chance to become a top - tier trainer in a different way.
In fanfiction, a story of Ash, Haruka (May in the English version), and Dawn could involve betrayal and hurt in many ways. For example, perhaps Ash makes a promise to both Haruka and Dawn about their Pokémon journey together, like he'll always be there to support them in battles and training. But then, he might break that promise by suddenly leaving them for a more 'glamorous' opportunity in another region without any prior notice. This would make them feel betrayed and hurt, as they relied on him not just as a friend but also as a partner in their Pokémon adventures.
The antagonist could be a powerful and devious Team Rocket member who has infiltrated the Kalos and Alola regions. This member could manipulate Haruka, perhaps by blackmailing her with something from her past in Kalos. In Alola, the Team Rocket member uses Haruka to get to Ash, making her betray Ash without really wanting to.
In fanfiction, Ash and Greninja might have all kinds of adventures in Alola. They could encounter new Pokémon species that are unique to Alola. Greninja could show off its skills in the new environment, impressing the local trainers. Maybe they'll participate in the Alola Pokémon League together and face strong opponents.