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 Ron would definitely change. Ron might be a bit more hesitant around her at first, not really knowing how to act. But as time goes on, they could develop a deeper, more platonic bond. It would be like Ron having a sister all of a sudden.
Well, with Dipper becoming a girl in the fanfiction, his relationship with Mabel would have a new dynamic. Mabel would be all about embracing Dipper's new form. She'd be planning slumber parties and trying to give Dipper a new 'girly' identity. As for Wendy, Dipper might find himself more in tune with Wendy's interests on an emotional level now that he's a girl. Wendy might be more open to sharing her feelings with Dipper - girl. His relationship with the townspeople could also change. Some might be more accepting, while others might be a bit more judgmental. And his relationship with the supernatural elements in Gravity Falls could be different too. Maybe some entities are more attracted to his new form, while others are repelled, which would lead to a whole host of new adventures and challenges in their relationships.
With his female friends, there could be a shift in the dynamic as well. They might be more open with him now that he's a girl, sharing secrets and experiences that they previously wouldn't have. He could also become more involved in 'girl - only' activities, strengthening those friendships in new ways.
His relationship with Gray might become more complicated. Gray might be more hesitant to pick fights with Natsu now that she's a girl. And with Lucy, they could become closer as girlfriends, sharing more secrets and having sleepovers. Natsu might also attract new male friends who are interested in her as a girl.
His relationships would change drastically. His work colleagues, for example, would notice his different behavior. He might start to work at night and avoid daylight. His relationships with other vampires could also be complex. He could form alliances with some vampires who are more sympathetic to his situation, while others might see him as an outsider. In his social circle, he would become a mystery to those who knew him before, and he would have to find new ways to interact with people.
His relationships with male friends might become more complex. They could start seeing him in a different light, maybe with a new - found attraction or a sense of awkwardness. For instance, if he had a close male friend before, that friend might struggle with how to act around him now. With female characters, he could either form a sisterly bond or face some jealousy if they see him as a threat in terms of male attention. His enemies might also underestimate him, thinking that as a girl he is weaker, but he would prove them wrong.
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.
His relationship with Luffy would become more of a big - brother - little - brother dynamic. Luffy would likely play with baby Zoro a lot. Sanji might be more nurturing towards baby Zoro, which is a big shift from their usual banter. Nami and Robin would probably dote on him more, and Usopp might try to come up with little games for baby Zoro.
His relationship with Sasuke would likely be affected. Sasuke might initially be suspicious of Naruto's new form, thinking it could be a trick by an enemy. But as time goes on, they could find new ways to work together. Naruto's new Kitsune powers could complement Sasuke's skills. For example, Naruto could use his enhanced speed as a Kitsune to keep up with Sasuke in combat.