Pokemon Hunter J is the central figure for sure. But depending on the plot of the fanfic, there could be other trainers or hunters who cross paths with her. For example, there might be a young and idealistic trainer who tries to stop her from hunting Pokemon. And of course, there could be some unique Pokemon characters with their own personalities that are important to the story, like a powerful and intelligent Pokemon that she has a hard time capturing.
Obviously, Pokemon Hunter J would be one of the main characters. There might also be some Pokemon that play significant roles, either as her targets or as ones that oppose her.
It could be about the return of Pokemon Hunter J in a fan - created story. Maybe it involves her new hunts, encounters with different Pokemon, or her interactions with other characters from the Pokemon universe.
Obviously, Ash is one of the main characters. He is the Pokémon trainer on his journey to become a Pokémon Master. Hunter J is also a main character, a notorious Pokémon hunter who often causes trouble for Ash and his friends.
Obviously, Greninja is one of the main characters as the title suggests its return. The trainer who was associated with Greninja before would also be a main character. They are likely to be central to the story's development.
A cool Pokemon Hunter J fanfic could focus on her facing off against a powerful trainer who is trying to stop her illegal activities. The battles between their Pokemon could be really intense, with the trainer using unique strategies. It could also show her slowly realizing that there might be more to life than just hunting Pokemon for profit.
In some cases, there could be a new antagonist in the fanfiction. This character could be a powerful trainer or a villainous Pokemon that stands in the way of Charizard and its trainer. Their presence creates conflict and drives the story forward as Charizard and the trainer must overcome them.
An old and wise Pokemon sage. This character could live in a hidden forest and have knowledge about ancient Pokemon powers. They could play a role in either helping Pokemon Hunter J find redemption or in protecting the Pokemon from her. For example, they could give J some cryptic advice that makes her think twice about her actions.
There might be a dragon rider as one of the main characters. If the dragon had a rider in the past, perhaps that person or their descendant is involved in the story when the dragon returns. Another possible main character could be a villain who wants to use the dragon's power for evil purposes.
Another character that could be important is Ging. Since he is Gon's father and has a mysterious past and a lot of influence in the Hunter x Hunter world. In a fanfic, he might play a role in Sakura's story, perhaps guiding her or being an obstacle she has to overcome depending on the plot of the fanfic.
Another character she could interact with is Max, her younger brother. Their reunion would be quite emotional. Max might have grown as a Pokémon trainer himself, and they could exchange tips and experiences, or even embark on a new adventure together.
Since it's a fanfic about Greninja returning to Alola, Greninja is surely a main character. Besides, the fanfic might introduce new characters who play important roles. There could be a mysterious trainer who has a connection to Greninja's past in Alola. Also, there may be Pokemon that Greninja teams up with to face challenges in Alola. These could include both common and rare Alola Pokemon, and their interactions with Greninja would be a big part of the story.