Lillie is also a main character. She has her own story arc, especially regarding her relationship with Pokemon and her growth from being initially afraid of them to becoming more confident. And her family's secrets might also play a role in the fanfic.
Obviously, Lillie is a main character. She is often the focus as the title suggests. In these fanfictions, she can be portrayed in various ways, from a shy and timid girl to a strong and fearless trainer.
In the Pokemon Sun and Moon anime game story, the trainer (the player) is of course central. Lillie is a key figure. She starts off as a rather shy and mysterious girl. She has a lot to do with the mysterious Pokemon Cosmog and as the story progresses, she grows and becomes more confident. Hau is a friendly rival. He's always up for a Pokemon battle and has his own unique style of training. There are also the trial captains, like Mallow and Kiawe, who play important roles in guiding the main character through the trials in the Alola region.
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.
Ash is the central character for sure. Then there could be some new characters created specifically for the fanfiction. They might be the ones who instigate the betrayal. For example, a mysterious trainer who has a grudge against Ash. His friends in the Alola region play a big role too. They are the ones whose reactions to the so - called betrayal by Ash form a major part of the story. Pikachu, as Ash's most loyal pokemon, is also a key character as it has to deal with the situation where Ash is being betrayed.
The main character is the player - controlled trainer. There's also Professor Kukui who is important as he provides guidance. Lillie is another main character. She has her own story arc related to the mysterious Pokemon she's associated with. And then there's Hau, who is like a friendly rival to the player. These characters play crucial roles in driving the story forward in the Pokemon Sun and Moon anime game.
I think Moon could be a great main character. She's independent and strong - willed. In the high school au, she could be the one who challenges the traditional Pokémon training methods at school. Then there's Sun, who could be her rival or partner, depending on how the story goes. There could also be some original characters created specifically for this fanfiction. For example, a new student from a far - off region who has unique Pokémon from places not seen in the original Sun and Moon games, adding a fresh perspective to the high school's Pokémon scene.
The main trainer is a common character, of course. Also, popular Pokemon like Pikachu, Rowlet, and Litten often appear. They might be part of the 'harem' around the trainer. And sometimes, the Alolan regional forms of Pokemon, such as Alolan Vulpix, could also be included.