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.
Well, one interesting plot could be Ash as a girl having a completely different approach to catching Pokémon. Maybe she'd be more empathetic towards certain Pokémon types and form different kinds of bonds. For example, she might be more in tune with the fairy - type Pokémon and use her charm to befriend them more easily.
In many fanfictions, Ash achieving the status of a Pokemon Master often involves him having extraordinary battles. He might defeat powerful legendary Pokemon that no one has ever tamed before. His Pikachu could evolve into a more powerful form, which gives him an edge in battles. Also, his bond with all his Pokemon, not just Pikachu, becomes so strong that they can execute complex strategies without any communication barriers.
One idea could be a fanfiction where a girl joins Ash on his journey. She might have a unique Pokemon that helps Ash in a difficult battle. For example, she could have a psychic - type Pokemon that can sense the opponents' next moves and help Ash strategize better.
Well, in such a fanfiction, Ash might experience the life of a Pokemon in a very different way. He could understand their thoughts and feelings more deeply. Maybe he'd have to learn how to use Pokemon moves in his new form, which would be a really strange and exciting adventure for him.
It could be a powerful and evil Pokemon trainer who wants to capture Ash in his new Pokemon form. This trainer might be after Ash because he believes that a human - turned - Pokemon has some special power or value.
His Pokemon could be his potential allies. They've been with him through thick and thin and would likely still support him even with his new vampire state. As for enemies, there might be those in the Pokemon world who fear or dislike vampires and would see Ash as a threat. Maybe some powerful and evil Pokemon trainers who want to use his new situation against him.
In some fanfictions, the girl's clothes are related to her Pokemon evolution journey. Maybe when her starter Pokemon evolves, she gets a new set of clothes that symbolizes their growth together. It could be a more elaborate and powerful - looking outfit. Also, there are often stories where she dresses up to impress Ash or to show her confidence during a difficult battle. This could involve a flashy, battle - ready costume with bright colors and strong - looking boots.
His Pikachu would probably be very supportive. It would stay by his side as he studies, maybe even help him in small ways like fetching medical tools. It trusts Ash completely and would be happy to see him pursue something new.
A frequent theme in such fanfiction could be adventure. Even as Ashley, the character would still be involved in Pokemon - related adventures. But now, with a different gender, there could be new challenges and opportunities. For example, interactions with other characters, especially of the opposite gender, might change. Also, the theme of self - discovery is quite common. Ashley would need to figure out who she is in this new form and how she fits into the Pokemon world.