There could also be other characters who are either involved in the situation that leads to the betrayal or are affected by the after - math. For example, there might be a mutual friend who tries to mediate between Ash and Lemon after the betrayal. Or there could be an antagonist who somehow influenced Ash to betray Lemon, like by blackmailing him or offering a great temptation.
Obviously, Ash and Lemon are the main characters. Ash is the one who does the betraying, and Lemon is the victim of that betrayal.
Some fanfictions might have Misty betray Ash. Perhaps she gets lured by the promise of power or some valuable reward. She could be tricked into believing that Ash is no longer the friend she thought he was and that betraying him is for the greater good.
The main character is surely Ash. He is the one who is being betrayed. Mew is another important one. It might be that Mew is the cause, directly or indirectly, of Ash's betrayal. Then, Ash's Pokémon partners are likely to be main characters too. They might be confused or forced to side against Ash. And there could be some antagonists in the story, perhaps some evil trainers or organizations who are behind the plot to make Ash look bad and get betrayed.
Gary could be a likely character. He has always been Ash's rival and there could be a plot where his jealousy or some other motive leads him to betray Ash. Another could be one of his so - called friends from the Pokemon League who might be influenced by power or some dark secret. Or, in some really dark fanfictions, it could be a Pokemon that Ash trusted deeply but for some reason turns on him.
One likely character could be a rival trainer. They might see Fem Ash as a threat and betray her to get an edge in competitions. Another could be a so - called friend who is jealous of Fem Ash's success.
Gary Oak could be a likely betrayer. He has always been Ash's rival, and in some fanfiction, he might take things too far and betray Ash in a crucial Pokémon battle, like using underhanded tactics to win. This would surely make Ash cry.
Another possibility is that it could be an old acquaintance who has changed for the worse. They might have been jealous of Ash and Lyra's success or popularity in the Pokémon world. This person could use their knowledge of Ash and Lyra's habits and weaknesses to betray them. It could also be a group of people rather than just an individual, like a team of thieves or poachers who see Ash and Lyra as obstacles to their illegal activities.
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.
Obviously, Ash is one of the main characters. He's the one who experiences the betrayal and then transforms into a badass.
In this fanfiction, it might be that Ash has been friends with Lemon, but then Ash sides with an enemy of Lemon's for some false gain. He might think that the enemy can offer him something better, like power or wealth, and in doing so, he stabs Lemon in the back by giving away Lemon's plans or weaknesses to the enemy.
Definitely his adoptive father, Giovanni. Also, the members of Team Rocket like Jessie and James would probably have a different relationship with Ash. They might see him as a sort of prodigy or future leader. And his original friends like Misty and Brock might cross paths with him, which would lead to some very tense and interesting interactions.