Misty could also play a major role. She might be both confused and intrigued by Ash's transformation. There could be some comical moments as well as some deeper emotional exchanges between them as they navigate this new situation.
Gary could be another important character. His rivalry with Ash might take on a new dimension. He could initially be dismissive of Ash as a girl, but as Ash continues to succeed in her Pokémon training, Gary might start to respect her in a whole new way.
Pikachu would definitely still be there. It's Ash's most loyal companion, and it would be interesting to see how their relationship might change now that Ash is a girl. Maybe Pikachu would be more protective or show a different kind of affection.
Latios would obviously be a key character. Also, Pikachu, Ash's loyal Pokémon partner, would be there. Maybe Misty or Brock could also be part of the story, with them either supporting or being skeptical of Ash's newfound admiration for Latios.
A young girl named Mia might be a character too. She is also a Glacion fan and is very competitive. She challenges Ash to various Glacion - related challenges, like a Glacion knowledge quiz or a race to find a hidden Glacion item in the forest.
Brock could also be a main character. Since he has knowledge about Pokémon care and breeding, he could be Ash's colleague in the fanfiction. They could work together on projects related to Pokémon health and nutrition.
May could be one of the main characters. She could join Ash on his journey and share her knowledge about Hoenn's Pokemon with him. Another could be Brock, still tagging along to offer his expertise in Pokemon care.
Well, Pikachu could still be a main character. Even if Ash hates everyone, Pikachu might be the one trying to bring back the old Ash. Maybe Team Rocket could also be main characters. They are always around Ash, and his new - found hatred could make their interactions even more interesting. They might be confused by his change or even try to take advantage of it.
A love interest who breaks the girl's heart can also be a main character. His actions, like cheating on her or suddenly leaving her without an explanation, can be the catalyst for her transformation into an evil character. Then the story could revolve around her trying to get back at him in devious ways.
His best friend from before the war could be a main character. This friend might have been waiting for Ash's return and now has to deal with the changed Ash.
Mallow is another great choice. With her love for cooking and Pokémon, she could use her skills to create special Pokémon food for Ash's team. This could lead to some fun and heartwarming moments between them, and also bring the whole harem together as they enjoy the food.
It could be that there is a young, impressionable character who looked up to Ash Aura. After the betrayal, this character is confused and starts to question his own beliefs. There might also be a powerful mage in the story who has the ability to see the truth behind Ash Aura's actions and is trying to prevent the chaos that the betrayal has caused.
Obviously, Ash (as a girl) would be the main character. Pikachu would still be by her side, being her loyal and cute partner. Her friends like Brock and Misty would also be important characters. Brock might be even more protective of her now, and Misty could become a sort of role model for her in some ways.