Who could be the main antagonist in 'ash and greninja betrayed fanfiction'?

It could be a new and powerful trainer who manipulates Anabel into betraying Ash. This trainer might be jealous of Ash's success and see Anabel as a means to bring Ash down.

It could be a new character who is jealous of Ash and Skyla's friendship. This character might spread false rumors about Ash to Skyla, causing her to feel betrayed. So, this new jealous character would be the main antagonist.

The Pokemon Professor in the region could also be the antagonist. Suppose the professor discovers a rare and powerful Pokemon that Ash is also interested in. The professor, driven by greed or a desire for power, might betray Ash by making it seem like Ash is trying to steal the Pokemon, causing all of Ash's friends and the community to turn against him.

It could be a jealous rival trainer. This trainer might have always been envious of Ash's popularity and the loyalty of his harem. So, they plot and scheme to turn the harem against Ash by spreading lies and creating discord among them.

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.

Ash himself could be the main antagonist as he is the one doing the betraying.

It could be a powerful and jealous Pokémon trainer. This trainer might want to get rid of Ash so he can be the best. He manages to brainwash Snivy into betraying Ash and Pikachu. This trainer would be the main antagonist, constantly trying to stop Ash from achieving his goals and causing trouble for him through Snivy's betrayal.

Maybe an ancient evil spirit that has a vendetta against Ash. The spirit could possess the aura guardian and make it turn against Ash. In this scenario, the main antagonist is not really the guardian but the spirit that controls it, and the story would be about Ash fighting this spiritual evil.

It could be a powerful and jealous Galar trainer. This trainer might see Ash as a threat to their own reputation and success in the region. They would then use their influence and charm to turn Ash's friends against him.

A new Team Rocket - like organization could be the main antagonist. They might be after Greninja because of its unique abilities. They could try to capture Greninja to use its power for their own evil purposes, like taking over the Alola region.