In fanfiction, a Pikachu as a starter (in some cases) could be shown as very energetic and curious. It might be constantly getting into little scrapes because of its inquisitive nature. A Torchic could be portrayed as a brave but a bit naive starter. It has big dreams of becoming strong but doesn't always understand the full scope of the challenges ahead. And an Eevee as a starter could be the very adaptable and mysterious one. It has the potential to evolve in many ways, and this could be a central part of its character arc in the fanfiction, with the trainer trying to figure out which evolution path is best for it.
When it comes to different starter pokemon in fanfiction, a Chikorita could be characterized as gentle and kind - hearted. It might be the one that tries to make peace between feuding pokemon or humans. Cyndaquil could be the shy but determined starter. It has a lot of power hidden inside it, but it takes time for it to come out of its shell. Treecko can be the cool and confident one. It moves with grace and has a natural leadership quality, which might make other pokemon look up to it in the story.
You can make it unique by changing the personality of the starter. For instance, if it's a Pikachu as a starter (which is not a traditional starter), make it really grumpy and hard to please at first. Then, as the story progresses, the trainer has to find ways to bond with it.
If Ash chose a different starter, the story would change drastically. For instance, if he had a water - type starter, his travel routes might be different as he'd seek out areas with more water - type Pokémon to train and catch. His battles against Team Rocket would also be different. A water - type starter could easily put out their fire - type Pokémon like Arbok and Weezing. This would force Team Rocket to come up with new plans to steal Pikachu (if Pikachu was still with him).
If Ash had a different starter like Bulbasaur in fanfiction, his team composition would change significantly. He might focus more on grass - type Pokemon to complement Bulbasaur's abilities. His travels through different regions could be affected as well. Maybe he would be more drawn to areas with more plant - life where Bulbasaur could thrive. Also, his interactions with other trainers having fire - type starters would be more interesting due to type advantages and disadvantages.
Maybe the fanfiction could focus on a trainer who gets a starter that's considered weak, like a Caterpie. But through hard work and a series of adventures, the Caterpie evolves into a Butterfree with extraordinary powers. The story could be about how they overcome the prejudice against starting with a 'weak' Pokemon and prove themselves in battles and Pokemon contests.
Well, you could have a fanfiction where the starter pokemon are in a competition not just against other pokemon but also against each other for a special prize. The trainers are caught in the middle, trying to balance their loyalty to their pokemon and the desire to win. Another interesting angle is a story where a starter pokemon is misidentified as a different type at first. Say, a Bulbasaur that looks like a Charmander because of a rare genetic mutation. This causes all sorts of confusion and challenges for the trainer. Also, a plot where the starter pokemon are reincarnations of ancient pokemon heroes and the trainer has to unlock their past memories to help them reach their full potential.
If Ash had chosen Bulbasaur as his starter. His connection with nature could be even deeper. He might have explored more grass - type Pokémon habitats. In battles, he could use Bulbasaur's status - changing moves like Sleep Powder to gain an edge. And his rivals would need to adjust their strategies when facing a grass - type starter - led team. This would change the whole dynamic of his Pokémon journey.
She can be characterized as very perceptive. Since she doesn't have glasses, her other senses might be heightened. For example, she could be great at detecting the presence of hidden Pokemon just by listening intently or feeling vibrations. This makes her stand out among other trainers.
To write a compelling Eevee starter character, first give it a distinct personality. For example, it could be a curious Eevee that is always getting into trouble because it wants to explore. Second, give it a unique backstory. Maybe it was abandoned by its previous trainer and is now a bit shy but determined to prove itself. Third, show its growth throughout the story. As it faces challenges, it becomes more confident and learns new skills.
If Ash had a Bulbasaur as his starter instead of Pikachu in the fanfiction, his team would likely have a more balanced early - game. Bulbasaur's Grass - type moves would be useful against Rock - type gyms early on. He might then focus on catching pokemon that can cover Bulbasaur's weaknesses, like a Fire - type or Flying - type pokemon to complement it.
Well, you can make it unique by changing the entire personality of Ash based on the starter. For instance, if he starts with a Eevee, he could be more indecisive at first about which evolution path to choose for Eevee, adding an interesting character arc.