His relationship with Pikachu would be very different. They'd be more like equals now. Pikachu might even feel the need to protect Ash more. With his other Pokemon, they might be confused at first but then develop a stronger bond as they realize it's still Ash inside.
The story would become more about Ash adapting to his new form. He'd have to learn how to move, eat, and sleep like a Pokemon.
His confidence might increase a great deal. Since legendary Pokemon are often seen as very powerful, Ash in his new form would feel more capable. He might become more solitary too, as some legendary Pokemon are known to be lone creatures.
Ash's relationships with his Pokemon would likely change significantly. Kalos has its own unique ecosystem and Pokemon culture. His Pokemon would be exposed to new stimuli and experiences. For instance, the Kalosian food for Pokemon might be different, and Ash would need to figure out how his Pokemon like it. Also, new training methods in Kalos could either strengthen or test the relationships. Ash would need to be more attuned to his Pokemon's needs and feelings in this new environment.
Suppose Percy turns into a horse. His close friends might feel a sense of responsibility to protect him, especially if he's in a dangerous area. He could form a bond with other horses, and this might introduce new characters into the story. His relationship with his human family might become more complex as they struggle to understand his new state and how to interact with him as a horse.
His relationship with Midoriya might change. Midoriya could become more protective of Todoroki as a girl, seeing her as more vulnerable. With Bakugo, there could be a different dynamic. Bakugo might initially tease Todoroki more, but as he sees Todoroki still being strong in her new form, he might develop a new respect for her.
If Eragon turns into a dragon, his relationship with Saphira would be very different. They might become equals rather than dragon and rider. His human friends may be initially scared or confused. But some, like Arya, might see it as an opportunity to gain a powerful ally for the fight against evil. His enemies would be more fearful of him, seeing him as a greater threat.
If Ash becomes a female in the fanfic, his relationships with his Pokemon friends would also be different. His Pokemon might sense the change and some could be more affectionate, while others might be a bit confused at first. Regarding his human friends, his relationship with Gary would take an interesting turn. Gary might be more intrigued by this new Ash and could either be more competitive or try to help her adjust. And when it comes to Professor Oak, he might be more fatherly or have different expectations of Ash now that she is female, perhaps expecting her to focus on different aspects of Pokemon research or training.
Aang's relationship with the Air Nomads' spirits could also be affected. As a girl, she might be seen as a new form of the Avatar by the spirits. They could communicate with her in different ways, guiding her on a different path. This could lead to new alliances and ways of using air - bending that were not available to Aang when he was male. Also, her relationship with other female characters in different tribes could become more sisterly, with them sharing secrets and knowledge specific to women.
His relationship with Toothless would be very different. Toothless might be initially confused and a bit hesitant around Hiccup in his new form. But as time passes, they would develop a new kind of bond based on Hiccup having to adapt to being a dragon. With Astrid, it could be strange at first. Astrid might be both curious and a bit jealous of Hiccup's new dragon form. She would have to see Hiccup in a whole new light.
If Ash were to turn into a Pokemon in a fanfic, his personality would surely be affected. He's used to being in control as a trainer, but as a Pokemon, he'd have to learn to follow others' commands or work within a group. His once outgoing and friendly nature might become more cautious. He'd have to be more aware of predators and threats. And his competitive spirit might take on a different form. Instead of competing in trainer battles, he'd be competing in Pokemon - on - Pokemon battles, which could make him more aggressive or strategic depending on the Pokemon he turns into.