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.
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 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.
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.
One potential side character could be Cilan. He could come and visit Ash and Dawn, bringing new recipes and ideas for Pokemon food. Dawn's friends from the Pokemon contests, like Zoey, could also be side characters. They might come to visit and share stories of their recent contest experiences. And then there's Ash's mother. She would be overjoyed at their marriage and could be constantly checking in on them, bringing home - made treats for them and their Pokemon.
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.
Charizard is another one. A Charizard shapeshifter can bring in a lot of cool elements. It can be a powerful and intimidating presence, and when it shapeshifts, it might gain new abilities or appearances that are both fascinating and fearsome.
Misty and Brock are also main characters in many of these fanfictions. After Ash's sacrifice, they might be the ones to comfort Pikachu and the other Pokémon. They could also take on Ash's unfinished tasks, like continuing to explore the Pokémon world, helping other trainers, and fighting against any emerging threats. Their friendship with Ash gives them the motivation to keep going in his absence.
Obviously, Aria herself is the main character. Her family members like her mother and father could be important characters, offering support or having different opinions. Her best friends would also be likely to be involved, providing emotional support or getting into some friendly arguments about how she should handle the pregnancy.