Obviously, Ash is a central character. Pikachu is also often there, as it's Ash's main Pokemon. Misty and Brock might be in the fanfiction too, trying to figure out what has happened to Ash and how to stop his evil ways. Sometimes, Gary Oak could be involved, either as an antagonist to Ash's new evil self or as someone trying to help bring him back to the good side.
Another possible plotline could be that Evil Ash is on a quest to conquer a certain region. Serena, with her own Pokemon, decides to oppose him. Along the way, they both discover new things about the Pokemon world and themselves. Serena might have to team up with other trainers to stand a chance against Ash's newfound power.
One interesting concept could be Ash's fall to evil due to some dark influence. Maybe he gets corrupted by a powerful and malevolent Pokémon or an evil organization. This could lead to a complete change in his character, making him use his Pokémon in cruel and unethical ways.
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.
One interesting 'evil Ash' fanfiction might be where he gets corrupted by a powerful and dark Pokemon. He could start using his Pokemon in unethical ways, like forcing them into battles they're not ready for. This would create a whole new dynamic as his friends try to figure out what's wrong and save him.
Well, 'pokemon evil ash fanfic' could be about an alternative version of Ash where he turns evil. Maybe he gets corrupted by some dark power in the Pokemon world. It could involve him using his Pokemon in cruel ways or having different goals that oppose the usual values in the Pokemon series.
In a Pokemon Ash turns evil fanfiction, he might start using his Pokemon for wrong purposes. For example, instead of battling for sport and friendship, he could use them to steal from other trainers or take over Pokemon Centers. His Pikachu, usually so loyal, might be conflicted about following his new, evil ways.
Pikachu is a very likely candidate. It's Ash's most iconic Pokemon and has a deep bond with him. Whenever Ash is in trouble, Pikachu often steps up to protect him.
You can try looking on some fan - fiction websites like FanFiction.net or Archive of Our Own. There are often a lot of Pokemon - related fanfics on those platforms, so there's a chance you'll find 'pokemon evil ash fanfic' there.
Ash could be a Pokémon trainer with a different starting point in this fanfiction. Born in 1987, by 1997 he might have already made a name for himself in local Pokémon battles. He could have a team of Pokémon that he rescued from difficult situations, like a Charmander that was abandoned by its trainer. His story could revolve around giving these Pokémon a second chance and proving that Pokémon don't need to be 'perfect' to be strong.