Obviously, Ash would be the main character as he is the one stuck in the costume. His Pokémon like Pikachu, Bulbasaur, and Charmander could also be important characters. They could be the ones trying to help Ash get out of the costume, using their different abilities.
Perhaps Ash is at a Pokémon festival and gets stuck in a traditional costume. He then has to go on a mini - adventure within the festival grounds to find someone who can help him out of it. Along the way, he meets new Pokémon and trainers, and they all work together to free him. This could also be a chance for Ash to learn more about the local Pokémon culture.
Perhaps in this fanfiction, the main antagonists are people from the past who are threatened by Ash and Serena's presence. They see them as a danger to their way of life or their power. For example, a local tyrant in the past might not like Ash's ideas about fairness and equality when it comes to Pokémon training. Serena's modern - style cooking might also be seen as a threat to local food traditions, so those who uphold these traditions could oppose them and become antagonists.
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.
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.
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.
They might encounter advanced Pokémon species that don't exist in their time. For example, new evolutions or completely new types of Pokémon. This could lead to new adventures and challenges as they try to understand and befriend these Pokémon.
In this fanfiction situation, they could end up using their knowledge of the future to help people in the past. For example, Ash could use his Pokémon training skills to solve some problems that people in the past were having with wild Pokémon. Serena could perhaps use her cooking skills to introduce new recipes. And together, they would be constantly looking for clues or a magical item that could send them back to their time.
Obviously, Ash would be a main character even in his baby form. Pikachu is bound to be a key character as well, being Ash's closest Pokémon partner. His other Pokémon like Bulbasaur, Charmander, and Squirtle could also play important roles in protecting and taking care of baby Ash.