With his human friends, it would be really tough. They might be scared of him at first. Astrid, for example, would be conflicted. She was attracted to Hiccup as a human, but now he's a dragon. She would have to overcome her fear and try to find a way to communicate with him. And Hiccup would feel isolated from his human friends, longing for the connection they once had.
Hiccup's relationship with the other Vikings would also be transformed. The Vikings who once respected him as a dragon tamer might now see him as a threat. They would be unsure how to deal with a dragon that used to be Hiccup. Hiccup, on the other hand, would start to see the Vikings from a different perspective, as he now has the instincts and experiences of a dragon. He might realize how much the Vikings' actions have affected the dragons and try to bridge the gap between the two species, but it would be a difficult process.
His relationship with his uncle Iroh would be deeply affected. Iroh has always been a guiding figure for Zuko. If Zuko turns into a dragon, Iroh would be worried but also proud in a way. He would search for ways to help Zuko, and their relationship would become more about Iroh protecting and guiding Zuko in this new and strange form. Zuko, in turn, would rely more on his uncle's wisdom during this time.
His relationship with Sasuke could become more complex. Sasuke might be both envious of Naruto's new power and also worried about Naruto losing his humanity in the dragon form. Sakura could be initially scared but then determined to help Naruto control his form, which would strengthen their friendship. His relationship with Kakashi would change as Kakashi would now have to mentor Naruto in a different way, guiding him to use his dragon powers for good and not let them consume him.
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.
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 friends would be both amazed and a bit scared at first. Hermione would immediately start researching ways to turn him back. Ron might be more in awe of Harry's new form, but also worried about how dangerous it could be. Their dynamic would shift as they now have to protect Harry in a different way. They might also have to hide his transformation from others who could misunderstand and try to harm Harry.
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 relationship with Annabeth would be really different. She would have to adjust to this new form of Percy. At first, she might be a bit scared or unsure, but then she would start to see the advantages of his dragon form and work with him. For example, she could use his ability to fly to reach places faster.
His relationship with Toothless would change drastically. Instead of being the rider, he becomes like a little sibling or a young apprentice to Toothless. Toothless would have to be more protective and patient with Hiccup.
His relationship with Toothless would be very different. Toothless might be more protective of him in his new form, as if sensing that Hiccup is more vulnerable as a female Night Fury. And Hiccup would have to communicate with Toothless in new ways, perhaps using more body language that is typical of female Night Furies.
Hiccup's relationship with his father (if his father is still alive in the fanfiction) would be complex. His father might be proud of him for achieving such a powerful status, but also worried about how the other tribes would react. Hiccup would also have a different relationship with the dragons in general. He would no longer be just a friend to them, but a leader, a dragon god. This could lead to some dragons being more submissive to him, while others might test his new authority.