Maybe an ancient Pokémon sage who has knowledge about the Sacred Fire. They guide the other characters in the story, sharing their wisdom and helping them face the various trials related to the Sacred Fire. The sage could have a backstory full of secrets and past adventures.
It could also be a Pokémon that has a connection to the Sacred Fire. This Pokémon is different from others, and its quest to discover its true nature and the significance of the Sacred Fire drives the story. Along the way, it meets different trainers and Pokémon who either help or hinder its journey.
A determined young trainer could be the main character. They have a special bond with Pokémon and are drawn to the mystery of the Sacred Fire. Their goal could be to learn more about it and protect it if necessary.
The main characters could be a young, inexperienced trainer who has just caught their first Fire - type and Ice - type Pokemon. As they journey through the Pokemon regions, they learn about the strengths and weaknesses of their Pokemon, and grow as a trainer. Along the way, they meet other trainers and Pokemon that help or hinder their progress. For example, an older, more experienced trainer who tries to convince the young trainer to give up their Ice - type Pokemon because it's 'too weak' in their opinion.
The main characters could be the Fire and Ice Pokemon themselves. For example, a Flareon and a Glaceon. They could be on a quest to find a special crystal that can balance the power of fire and ice in their world. Their interactions and growth throughout the journey would be the focus of the fanfic.
One idea could be a trainer who discovers a hidden power within the Sacred Fire Pokémon that has been forgotten for centuries. This power could be related to an ancient prophecy that the trainer must fulfill to save the Pokémon world from a new and powerful threat.
A young and ambitious Pokemon trainer who wants to save their injured Pokemon through surgery could be a main character. Also, the Pokemon doctor who is an expert in performing these surgeries.
An alchemist who is trying to study and harness the power of the dragon's fire could be a main character. They might be on a quest to find a way to use the fire for good, like curing diseases or creating powerful magic items.
The main characters could be a pair of best friends. One friend could be a very cautious and methodical trainer, while the other is more impulsive. They both encounter a double - trouble situation when they come across two wild Pokemon that have been affected by a strange energy. These Pokemon start attacking everything in sight, and the two friends have to combine their different training styles to subdue them. For example, the cautious friend might try to analyze the Pokemon's behavior first, while the impulsive one jumps right into battle.
The main characters could also include the prosecutor in Damian's case. This person would be determined to convict Damian and would present all the evidence against him. On the other hand, there could be a mysterious pokemon trainer who has some inside information about what really happened and comes forward to help Damian. And of course, Damian's pokemon could also play a role, perhaps they are taken away from him after the arrest and their fate is also part of the story.
Team Rocket could also be the focus of a reunion fanfiction. Maybe they've reformed and are having a reunion to celebrate their new lives away from their villainous ways. They could be showing off their new Pokemon or skills that they've developed.
I think Misty from the Pokemon world could be a main character in this fanfiction. Since she has a connection with water - type Pokemon, she could easily fit into the underwater world of The Little Mermaid. Along with her, a Psyduck would be there. Psyduck's clumsy yet powerful abilities could add an interesting element to the story. And of course, Ariel would be a main character. She could learn a lot from Misty about the Pokemon world and they could team up to protect both their worlds from any threats.
A new trainer and a legendary Pokemon. Imagine a young trainer who accidentally cuffs themselves to a powerful and mysterious legendary Pokemon. The trainer has to figure out how to communicate with the Pokemon and gain its trust while they are literally bound together. This would be a great opportunity for character growth for both the trainer and the Pokemon.