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.
The main characters would surely include Latios itself. Since Ash is so into Latios, trainers who have also interacted with Latios in the past could make an appearance. For example, a trainer who once saved a Latios and has valuable knowledge about it. And of course, Ash's other Pokémon friends like Bulbasaur or Charmander might also be involved, as they are part of Ash's Pokémon family and would be affected by his new interest.
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.
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.
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.
In this fanfiction, Ash could have a life - changing encounter with a Latios. He might be in awe of its majestic appearance and gentle nature. Ash could then decide to focus his Pokémon training around understanding and emulating Latios. This could lead him to explore new areas where Latios are known to inhabit, meet other trainers who have also been influenced by Latios, and develop new training techniques inspired by Latios' powers.
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.
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.
Maybe a mysterious organization that wants to experiment on Latios. They believe that Latios holds the key to some sort of genetic or power - enhancing technology. They send out their agents, who are skilled in Pokémon capture and combat, to try and get Latios. They don't care about Ash at first, but when he stands in their way, they see him as an obstacle to be removed.