Obviously, Neji himself would be a very important character. His actions and words would have a great influence on his little brother. Also, other members of their clan could be main characters, like their parents or other relatives who play roles in the little brother's life and ninja training.
The friends of Neji could also be main characters. For example, Naruto. He might interact with Neji's little brother, either giving him advice based on his experiences with Neji or helping him in some tough situations. And Hinata could also be there as she is related to Neji, and she could offer support to the little brother in different ways.
I can't say for sure without reading it. But there could be his friends, like a nerdy sidekick who helps him with his studies or a sporty friend who encourages him in extracurricular activities. There might also be a strict teacher who at first doubts him but later becomes his mentor.
There could be other Pokémon like Pikachu. Pikachu could be their friend who helps the little brother adjust to the new world. He could also offer some comic relief in the story. Another possible character could be Ash. Ash has a lot of experience with Pokémon, and he could offer some guidance to Mewtwo on how to take care of his little brother.
Well, it could be about the little brother following in Neji's footsteps in terms of ninja training. Maybe he struggles to live up to Neji's reputation at first but then finds his own unique way to shine in the ninja world.
The big brother and the little brother are the main characters. The big brother could be a confident and responsible character, while the little brother might be more naive and in need of guidance. There could also be friends of the brothers in the story that interact with them, but the focus remains on the relationship between the two brothers.
Luke, on the other hand, would probably be a more experienced and perhaps a bit jaded character. He could have seen a lot in his adventures and might be a bit overprotective of Percy at times. There could also be other characters from the original source material that interact with them, like their friends or mentors who play important roles in shaping their relationship and their individual growth.
Denmark could be one of the main characters. He is often shown as Norway's friend/rival in the Hetalia series. He might come around to check out the new little brother and end up getting involved in their daily lives, maybe trying to show the little brother his own Danish culture, like Lego and Danish pastries.
A friendly bat family could be important characters. They could take in the little bat when it gets separated from its own family. They teach the little bat the ways of their bat colony, like how to find the best food sources and how to communicate with other bats through echolocation.
It might also be a term of endearment for a younger male friend or acquaintance. In some groups, people use 'brother' in a friendly way, and 'little brother' could be a specific person within that group who is no longer interested in stories.
There's not enough information to say exactly who the little brother is. He could be an ordinary boy in a family who is the youngest among his siblings.
Obviously, Naruto and Hana are the main characters. Naruto, the well - known ninja from the Naruto series, and Hana, who might be a new or less - known character in this fanfiction context where she has the role of Naruto's older sister - like figure.