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.
In a fanfiction where Todoroki turns into a girl, his friendships at school would be transformed. His female friends like Uraraka might be more open and friendly, inviting him to do girl - only activities. The male classmates, on the other hand, might be a bit more awkward around him at first. Some might develop crushes on him, which would make things really complicated. And Todoroki himself would have to re - evaluate his relationships, learning to navigate the new social cues and expectations that come with being a girl in a hero - training environment.
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.
Well, Natsu's relationship with Gajeel could be really different. Gajeel might be more respectful towards Natsu as a girl, or he could be more competitive in a different way, like trying to prove that he's the better dragon slayer in a more gentlemanly manner. Natsu's relationship with the guild master, Makarov, would also change. Makarov might be more protective of Natsu as a girl, and Natsu might have to fight harder to be taken seriously in guild matters. In general, all of Natsu's relationships would be reshaped, with new levels of complexity and humor added to the story.
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 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.
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.
His relationship with his family would be different. His sister Shuri might be more protective of him in this new form. His mother could treat him more like a young girl, with different expectations. His friends and warriors would be confused at first. Some might not know how to act around him. His relationship with his enemies could also change. They might underestimate him as a teenage girl, which he could use to his advantage. For example, he could trick them into revealing their plans more easily.
His relationship with Kagome would change significantly. Kagome might see Inuyasha in a new light, perhaps more as a sister - like figure at first, but then as she gets used to the new form, there could be a shift in their romantic undertones. With Miroku, Miroku would be more respectful in a different way. He might be less likely to tease Inuyasha in the same way he did before. Sesshomaru, on the other hand, might initially be disgusted or confused by his half - brother's new form, but it could also lead to some interesting developments in their rivalry as Sesshomaru tries to figure out if this new Inuyasha is still a worthy opponent.
For Naruto, he might become more protective of Sasuke as a girl. Their friendship could take on a different dynamic, with Naruto being more chivalrous perhaps. Sakura might initially be jealous but then they could form a stronger bond as girls sharing common experiences. Kakashi would probably have to adjust his mentoring style to account for Sasuke's new situation.
His relationship with Annabeth would be really interesting. At first, Annabeth might be shocked and a bit unsure how to act. But they could also develop a deeper understanding of each other as Percy now experiences things as a girl. With his male friends, like Grover, there could be some awkwardness, but also a chance for them to see Percy in a new light and strengthen their bond in different ways.
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.