Gary could be the main antagonist. He has always been Ash's rival and seeing Ash turn evil, he might feel responsible to stop him. Gary could gather his own team of powerful Pokémon and use his wits to oppose Ash at every turn.
Another possibility is that an unknown, powerful Pokémon trainer emerges. This trainer has a pure heart and a strong sense of justice. He or she decides to take on the challenge of stopping evil Ash, seeing it as their duty to protect the Pokémon world from his tyranny.
It could be a powerful and evil Pokemon trainer who wants to capture Ash in his new Pokemon form. This trainer might be after Ash because he believes that a human - turned - Pokemon has some special power or value.
It could be a trainer who hates Pikachu and wants to capture all of them. This would put Ash, as a Pikachu fan, in direct conflict with this trainer.
It could be a trainer who has a dislike for Charmander. This trainer might see Ash's new - found love for Charmander as a threat and try to undermine Ash's relationship with his Charmander. For example, this antagonist could try to steal Charmander or set up difficult battles where Charmander is at a disadvantage.
The Guys in White could be the main antagonists. If Danny becomes evil, they might see this as their chance to finally capture and experiment on him. They would be constantly chasing him, which would make Danny see them as the main obstacle in his new evil plans, and thus they would be the antagonists in the story.
It could be a rogue Pokémon trainer who wants to capture Ash - now as Pikachu - for his own collection. This trainer might have a cruel streak and not care about the well - being of Pokémon.
It could be Team Rocket. Since they always cause trouble for Ash in the original series, in this fanfic, they might see Ash's transformation into a Pokémon as an opportunity to capture him and use him for their evil plans.
Based on the title, Evil Ash is surely one of the main characters. But without reading it, it's hard to say who else might be involved. It could be characters from the original source material that Evil Ash has a relationship with, like his friends or enemies.
One possible plotline could be that Ash is betrayed by his friends. This heartbreak and sense of abandonment turns him evil. He then starts using his Pokémon in cruel ways to seek revenge on those who wronged him.
It could be a jealous rival trainer who mocks Ash for wearing a dress and tries to use it as a way to undermine his confidence and stop him from competing in Pokemon battles.
Well, clearly Ash and Serena are the main characters. Ash, in this case, is the 'evil' version, which is a departure from his usual good - natured self. Serena is likely to be the one who tries to bring back the old Ash or at least stop his evil deeds.