With his love interests, if he had any, it would be completely different. If he was interested in a girl before, she might see him as a rival now. And if there were guys who had a crush on him as Superboy, they might be confused or even more attracted to him as a girl. Also, his relationship with his family members, like his father figure, might change as they adjust to his new gender.
In a fanfiction where Natsu turns into a girl, his relationships would be turned upside down. His relationship with his friends would be filled with awkward yet endearing moments. His male friends might have a hard time adjusting at first, but gradually they would accept the new Natsu. Natsu's relationship with the female characters would also change. He might find himself being included in more girl - only activities, which would strengthen his bonds with the female members of the guild. And his relationship with the outside world, like with other guilds or villains, would also be different. Enemies might underestimate him because he is now a girl, but Natsu would prove them wrong with his strength and determination.
Well, Percy's relationships would be in upheaval. In the case of his friends, the dynamics would shift. His male friends might have to reevaluate how they interact with him. For example, they might be more cautious about physical contact or their language. His female friends could either embrace her new form and become closer, seeing her as one of them now, or they could be jealous if Percy in his new female form gets more attention. And in terms of romantic relationships, if Percy was interested in girls before, he might now find himself attracted to boys, which would completely change the nature of his love life.
His relationship with Tails would be strained. Tails would be confused and a bit scared at first. Sonic would have to hide his new vampiric urges from Tails, which would create distance between them. With Knuckles, there might be more of a distrust as Knuckles is very protective of their world. Knuckles might see Sonic as a threat now that he's a vampire.
If Naruto was turned into a monster in fanfiction, his relationship with the village would change drastically. The villagers who have finally started to accept him as a hero would once again be fearful. His friends would be divided in their reactions. Some, like Shikamaru, would immediately start thinking of ways to solve the problem. Naruto's relationship with his mentor, Kakashi, would be put to the test. Kakashi would have to balance his duty to the village and his loyalty to Naruto. And Naruto himself would feel isolated, which could further impact his relationships as he might become more withdrawn in his monster form.
His relationships with his male friends might become more complicated. Some might be more protective of him as a girl, while others might be confused about their feelings towards him. For example, if he had a rival before, that dynamic could shift completely.
His relationship with Rukia would be different. Rukia might be more protective of Ichigo in her new form, seeing her as more vulnerable. And with Uryu, he might be more hesitant to interact with Ichigo as a female hollow at first, due to his Quincy beliefs.
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 Ash were turned into a female, his relationship with Misty would likely change significantly. Misty might see her as a sister figure more than before and they could have more in - depth conversations about girl - related things in the Pokémon world, like the best places to catch certain water - type Pokémon that are more appealing to female trainers. His friendship with Brock would have a different dynamic too. Brock might be more hesitant in his usual flirting ways as Ash is now female. And his other friends might be more curious about this new Ash and how she approaches Pokémon training differently from before.
Well, with his friends, they would have to adjust to his new state. Percy would rely on them a lot more. Grover might feel an even stronger need to protect him. And with other demigods, some might tease him while others would feel sorry for him. His relationship with the gods could also change. Some gods might find it amusing and not take him as seriously, while others might see it as an opportunity to manipulate him in his vulnerable state. His romantic relationship, if he had one, would be put on hold. His partner would have to wait until he's back to normal or might develop a different kind of affection, more like towards a little brother or something.
If Trunks turned into a girl in fanfiction, his personality might become more in tune with feminine traits. He could become more empathetic. For example, he might be more attuned to the emotions of others around him. Also, his fighting style could potentially change. Maybe he would rely more on agility and finesse rather than brute strength. His relationships with other characters would also shift. His male friends might start to see him in a different light, and he might attract new friends who are female.