Obviously, Hiro would be the main character. His friends like Baymax might also play important roles. They could be there to support him through this strange transformation.
It's possible that there could be some new characters introduced as well. Maybe a character who has knowledge about the origin of the horns and can help Hiro understand what's going on. For example, an old wizard or a scientist from a secret organization. And of course, Hiro's family members would also be important as they deal with his changed appearance.
Since I haven't read it, I can't be certain. But obviously Hiro is one of the main characters as the title centers around him. There might be other characters like his friends, family members, or mentors who play important roles in his journey after getting his horns early.
The fanfiction might revolve around how Hiro's friends and family react to his new horns. They could be shocked at first, but then come to accept it as part of who he is now. It could also explore how Hiro himself feels about this sudden change, whether he's self - conscious or excited about it.
In the 'hiro gets his horns early fanfiction', it could be that Hiro is part of a species where horns are a significant part of their identity. Getting them early might disrupt the normal order of things. It could be a story full of self - discovery. Hiro might struggle to understand what this early development means for him. He could face discrimination or admiration from others in his community. There might be a journey involved, where he tries to find out the reason behind this early horn - getting, and along the way, he meets various characters who either help or hinder his search.
Hinata would also play a major role. Since she has deep feelings for Naruto, she would be devastated by his amnesia. She might try to use their shared memories, like the time she confessed her love to him during the Pain attack, to jog his memory. And the Akatsuki members could also be involved in some cases. If Naruto's amnesia was caused by their actions, they might be the antagonists in the fanfiction, or they could also be trying to use his amnesia for their own purposes.
Kakashi could also be an important character. He has a lot of experience and wisdom. When Naruto gets high, Kakashi might be the one to guide Naruto through this strange state, using his own past experiences and knowledge of different ninja techniques. And then there's Jiraiya. Given his relationship with Naruto, he would surely be involved, perhaps trying to use his own unique jutsu to help Naruto come back to normal.
Obviously, Harry Potter himself is the main character as he is the one getting back at the Dursleys. The Dursleys - Uncle Vernon, Aunt Petunia, and Dudley - are also main characters as they are the targets of Harry's revenge.
Obviously, Naruto and Jiraiya are going to be the main characters. Naruto, the young and energetic ninja - in - training, and Jiraiya, the experienced and somewhat eccentric sage. There might also be some side characters like the villagers of Konoha who could react to Naruto's new relationship with Jiraiya.
In addition to the above, other characters from the wizarding world could be involved. For example, the Order of the Phoenix members. They might be suspicious of Harry at first but then come to his aid. And there could be some new characters created specifically for the fanfiction. Maybe a mysterious old wizard who has knowledge about the Dark Mark that no one else does and helps Harry on his journey to rid himself of it or at least understand it better.
The villains in the Harry Potter series, such as Voldemort or his Death Eaters, could also play important roles. Harry's anger might be directly related to their actions. For instance, if Voldemort were to target someone Harry cares about again, it could be the trigger for his extreme anger. His interactions with these villains in the fanfiction would be full of tension and high - stakes confrontations.
Obviously, Harry Potter himself would be the central character. He's the one getting the tattoos after all.