I think Brock and Misty could be main characters too. They travel with Ash in Kanto. Brock with his knowledge of Pokémon care and Misty with her own skills as a trainer could play important roles in Ash's journey to become the champion in this fanfiction. Maybe they offer advice, support, or even join in some of the battles.
In addition to Ash, Pikachu, Brock, and Misty, there could be some of the gym leaders as main characters. They are the ones Ash has to defeat to become the champion. For example, Brock was a gym leader too. Each gym leader has their own personality and Pokémon team, and they would be important in the story as they present challenges and obstacles for Ash on his path to the Kanto championship.
It could be about Ash achieving the ultimate goal of becoming the Kanto champion. Maybe it details his journey, the battles he had to overcome, and the growth he experiences along the way to claim the title.
Ash can be developed further by showing his insecurities as a champion. Maybe he starts to doubt if he really is worthy of the title, and this makes him train even harder. He could also develop new relationships with other trainers in Kanto, learning from them and teaching them in return. Another way is to show his growth in handling the political side of being a champion, like dealing with the Pokémon League's bureaucracy.
One plot idea could be Ash facing a new and powerful villain who has a vendetta against all the previous Kanto champions. This villain could be trying to rewrite the history of the Kanto region's Pokémon battles. Another idea is Ash having to defend his champion title against trainers from other regions who believe he doesn't deserve it because he's from a relatively small town. A third plot could involve Ash finding a rare and mysterious Pokémon in Kanto that has the power to either greatly enhance his team or put them in great danger, and he has to figure out how to handle it while still being the champion.
Ash and Pikachu are the top ones. But in addition to them, Nurse Joy is quite popular in some fanfictions. She can be more than just the healer of Pokémon, like having a secret past or special powers. Officer Jenny is also sometimes a key character. Her role in maintaining law and order in Kanto can be expanded in fanfiction. And then there are the Gym Leaders of Kanto. Each Gym Leader has their own fan following, and they can be written in many different ways in fanfiction to interact with Ash and his journey.
Ash's character in the Kanto fanfiction with Lillie might develop in terms of his leadership skills. He often takes the lead in their adventures, making decisions that affect both of them and the Pokemon they encounter. Lillie's development is more about self - discovery. In Kanto, she discovers her own strengths and weaknesses. She realizes that she has a natural ability to calm down agitated Pokemon, which she uses to help Ash during battles. Their time in Kanto is filled with challenges that shape their characters in different but complementary ways.
In some 'pokemon champion ash fanfiction', Gary could be an important character. He has always had a competitive relationship with Ash, so in stories where Ash is a champion, Gary might try to challenge him or be involved in some sort of plot related to Ash's championship. Another character could be Serena, especially if the fanfiction focuses on the romantic aspect of Ash's life along with his championship journey.
The unknown champion could be a reclusive hermit - like character who has been living in seclusion in a forest. They have trained their Pokémon in harmony with nature and have a very different approach compared to the more mainstream trainers. Maybe they are an old - timer who has seen different eras of Pokémon training.
Misty is often a popular character in Ash Kanto Battle Club fanfictions. Her relationship with Ash, whether it's a friendly rivalry or a budding romance in some interpretations, can be a central focus. Additionally, some fanfictions might feature Gym Leaders from Kanto in the context of the Battle Club, and they can also be popular due to their established status in the Pokemon universe.
Probably the champion of the light is the central character. There could also be sidekicks or allies that help the champion in their quest to uphold the light. Maybe there are antagonists too, like villains who represent the dark side and try to stop the champion.
He might meet a mysterious old trainer who imparts some unique wisdom. There could also be a young and talented trainer who becomes his rival or friend.