One reason could be a power struggle. If there was a rare and powerful Pokémon at stake, Ash might be tempted to betray Korrina to get it for himself. In the Pokémon world, power and rare Pokémon are highly coveted.
It could be due to a difference in goals. Suppose Ash has a goal that he believes is for the greater good of the Pokémon world, but Korrina's actions are hindering it. In that case, he might feel the need to betray her to achieve his objective. This could involve things like protecting a certain Pokémon habitat or preventing a powerful enemy from rising, and Korrina's methods are not in line with what Ash thinks is right.
One idea could be that Korrina and Ash team up for a special Pokemon tournament. They train together, with Ash sharing his experiences from his previous travels and Korrina teaching Ash some new fighting - type Pokemon moves. Their Pokemon also form unique bonds during the training process.
It could be due to a misunderstanding. Suppose Ron overhears a conversation between Harry and Hermione that he misinterprets completely. He might think they are plotting against him or don't really consider him a true friend. So in a fit of anger and hurt, he decides to betray them.
One possible reason could be jealousy. Maybe Ash has achieved more in his Pokemon journey than his friends, and they can't handle it. Another reason might be some misunderstanding. For example, an enemy could trick Ash's friends into thinking Ash has done something bad. Or perhaps there's some sort of power struggle. If there's a powerful item or a rare Pokemon involved, and Ash is the main contender, his friends might be tempted to betray him for their own gain.
I'm not sure specifically if there is an existing fanfiction with that exact title, but it's a very possible concept in the Pokémon fanfiction community. Writers might explore Ash's darker side in relation to Korrina, perhaps due to a conflict over different ideologies within the Pokémon world, like how to train Pokémon or what the true purpose of battling is.
One possible reason could be jealousy. Maybe she sees Percy getting more attention or praise than her in the fanfic world, and that envy turns into betrayal. Another reason might be a misunderstanding. For example, if there's some misinformation spread about Percy that makes Annabeth believe he's done something really bad.
Another reason might be related to Pikachu's own growth. As Pikachu becomes more powerful and self - aware, it might feel that it needs to go on its own journey without Ash. It could be seeking new challenges and experiences that it believes Ash can't provide, like exploring regions that Ash has no intention of going to or training with Pokémon of a different caliber.
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.
One common theme is the journey of self - discovery for both characters within the relationship. Another is the power of friendship turning into love. Also, their shared love for Pokémon often plays a role in bringing them closer.
Maybe he loves her deeply. In the Pokémon world, love can be a strong motivator. He might see a threat coming towards Serena and know that sacrificing himself is the only way to save her.
In some fanfictions, Ginny might be written to betray Harry because of a prophecy. The prophecy could mislead her into thinking that Harry was going to bring doom, and so she decides to act against him to prevent that. It's a different way of looking at their relationship compared to the original story, where their bond was based on love and trust.