Romantically, if he had a love interest before, that relationship would be completely turned upside down. His love interest might be confused about their feelings, or Issei himself might find that his feelings have changed. He could also attract new romantic interests, leading to a love triangle or other complex relationship scenarios.
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 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.
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.
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 relationship with Mary Jane would become very different. Instead of a romantic interest in the traditional sense, they could become more like sisters or best friends. Mary Jane might help the girl - Spiderman adjust to her new identity.
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.
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.
In a fanfiction where Issei turns into a dragon, his relationships would be in flux. His relationship with his mentors or those he looks up to might change as they now see him as a different kind of being. They might treat him with more caution or more respect depending on how they view dragons. His friendships with his peers would also be altered. Some might see him as a weapon to be used in their battles or struggles, while others would see him as a friend in need of support. Romantically, if he had feelings for someone, that person would have to deal with the fact that Issei is no longer fully human. There could be jealousy from others who might want to be with the powerful dragon - Issei, or there could be newfound admiration from those who previously overlooked him.
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.
In this fanfic scenario, Naruto's relationship with Iruka would be tested. Iruka, who has always seen Naruto as a student and a friend, would now have to deal with a Naruto who is part - nine - tails. He would worry about Naruto's future and how society will accept him. Naruto's interactions with the other jinchuriki could also change. They might sense a kinship with him in a different way now that he has fully become a nine - tails. And his relationship with Tsunade would likely involve her trying to find a way to either reverse the transformation or make sure Naruto can control it, while also dealing with the political unrest that might come from Naruto's new state.
In this scenario, their relationship would take a 180 - degree turn. Draco would probably feel guilty about what he's done, and this guilt could drive him to be nicer to Harry. Harry, now a girl, would have to navigate a world that treats her differently. Draco could become her unexpected ally in this new situation. They might start to share secrets and confide in each other more than they ever did before. This new relationship would be full of challenges as they both have to come to terms with their new roles in each other's lives and also deal with the reactions of their friends and the rest of the wizarding community.