It's possible that the author might introduce some new characters in this fanfiction. They could be classmates or rivals that are unique to this 'different second year' story. These new characters could bring new dynamics and conflicts to the story, making it more interesting.
I'm not sure without reading it, but it could be the same main characters from the original work just in a different second - year scenario.
The main characters might also be characters that were minor in the original but are given more prominence in this fanfiction. Maybe a sidekick in the original becomes a central figure in the second - year story, with their own story arc and development.
Percy is the main focus. There could be other demigods that are either intrigued or threatened by Percy's difference. Some might want to be his allies, while others might see him as a threat to the status quo at Camp Half - Blood. And his family, like his mother, would also play an important role in dealing with the changes in Percy.
There could also be Sasuke. Since he and Naruto have a complex relationship in the original series, it would be interesting to see how it's developed further in this fanfiction. And Sakura might also be a main character, maybe with her medical ninjutsu taking on a new role.
I don't know as I haven't read the fanfiction. It could be a young adventurer who is intrigued by the cloud and sets out to explore it. Maybe there's a wise old character who gives advice about the cloud's significance.
There could be other Fairy Tail members as main characters too. They might play important roles in Natsu's new adventures. For instance, Lucy could be there to support Natsu with her celestial spirit magic. Gray might also be involved, adding his ice - make magic to the mix and having some interesting interactions with Natsu's new dragon - related situations.
I don't know for sure, but it could be the 'bird of a different feather' itself as the main character.
Most likely Naruto would be one of the main characters, since it's in the title. There might also be his friends like Sasuke and Sakura as they are often involved in Naruto's storylines in fanfictions.
There might be a group of misfit students who band together to support the champion. They could be from different houses and have different skills, like one being good at potions, another at charms, and they use their combined knowledge to help the champion in the tasks. So this group of misfits could be considered main characters in a way.
I'm not sure specifically as there are many 'Second Life in a Different World' web novels out there. But often, there's a protagonist who is the one getting the second life. They could be an ordinary person from our world who then becomes extraordinary in the different world. There might also be side characters like a mentor figure who helps the protagonist learn about the new world, or an antagonist who opposes the protagonist's goals.
Obviously, Vernon Dursley could be a main character. We could explore his thoughts and feelings more deeply, especially if the fanfiction takes a different turn on his personality, like making him more sympathetic.
Well, of course, Harry Potter is still likely to be one of the main characters. His character would be central even in this different destiny story as the series is named after him.