Gary could play an interesting role. He might be jealous of Ash's new shapeshifting ability and try to outdo him in some way. Professor Oak would also be involved as he could be the one Ash turns to for scientific explanations about what's happening to him. And then there are all the Gym Leaders. They might react differently to Ash's shapeshifting. Some might be suspicious, while others might see it as an advantage in battles and offer to train him further.
Obviously, Ash's main friends like Misty and Brock would be involved. Misty could be the one who first notices Ash's strange new ability and helps him keep it a secret at first. Brock, with his knowledge of Pokemon, could try to figure out how to reverse the shapeshifting if it becomes a problem. Also, Team Rocket would be great characters in this. They could constantly be trying to capture Ash in his different shapes, thinking they've discovered a new and valuable Pokemon.
Team Rocket, of course. Jessie, James, and Meowth would see Ash as Ashley and might come up with new schemes to catch Pikachu. They could be completely thrown off by Ash's new appearance and behavior. Professor Oak could also play a role. He might be the one to figure out how to turn Ash back to his original form or provide some scientific explanations for the transformation. Another potential character could be Dawn. She could meet Ashley and be inspired by her unique approach to Pokemon training, which was originally Ash's but has a different flavor in Ashley's form.
A long - lost relative of Sirius Black could be a shapeshifter. Since the Blacks had many secrets and were involved in a lot of dark magic in their family history, it would be believable for a hidden relative to have this unique ability.
Obviously, Ash and Pikachu are the main ones. But we could also see Misty and Brock, Ash's old friends from the beginning of his journey. Maybe Gary too, as he has been Ash's rival for a long time.
There could be a lone traveler who Ash meets while he is alone. This traveler might have a unique way of training Pokemon that Ash has never seen before and could teach him some new tricks.
Obviously, Ash would be the main character. His Pikachu would also play a major role as always. Then there could be Professor Oak, who might help Ash understand his new identity. Also, Gary Oak could be involved, maybe as a rival who is initially skeptical of Ash's new - found status.
There could be a new trainer who has a very different style of training Pokémon compared to Ash. Maybe a young girl who uses music to communicate with her Pokémon. Another potential character could be an old Pokémon sage who has a vast knowledge of ancient Pokémon lore and can offer Ash some valuable advice on his journey.
Potential characters might include a new group of Pokémon thieves that Ash has to deal with. Also, there could be a local Pokémon healer who becomes an important part of the story. There might be some young Pokémon enthusiasts who look up to Ash and join him on his adventures. Another possibility is a reclusive Pokémon trainer who has some rare Pokémon and Ash tries to learn from them.
One potential father figure for Ash in fanfiction could be Professor Oak. He has been like a mentor and a guiding figure in Ash's life. He has given Ash his first Pokémon and has always been there to offer advice, which could easily be developed into a father - like relationship in fanfiction.
There could be a new rival for Ash. This rival might have a very different approach to Pokemon training, like focusing more on the beauty and grace of Pokemon rather than just battle strength.
There could be a female trainer who becomes Ash's rival while he's disguised as a girl. She might be initially dismissive of him, thinking he's just another girl trying to make it in the Pokemon world, but later be surprised when she discovers his true identity.