You could give her a distinct physical feature. For instance, instead of the normal brown stripes on a Pikachu, she could have blue ones. This would immediately set her apart. Also, her personality could be more shy and reserved compared to the typical Pikachu, making her reactions to situations different.
One way to develop her character is through her relationships. Besides Ash, she could have a special friendship with other Pokémon in Ash's team. Maybe she is like a big sister to the younger Pokémon, teaching them things and protecting them. This would show her nurturing side and develop her character further.
They could start by sharing their favorite things, like Ash sharing his Pokemon training experiences and the female Pikachu sharing her favorite places to play in the wild.
The 'ash meets pikachu fanfic' often creates new scenarios for their adventures. For example, they might end up in a different dimension where the rules of Pokémon battles are completely different. This allows for a fresh take on their relationship and how they adapt to new situations.
I'm not sure if there is a specific 'ash no pikachu fanfic' out there. There are countless fanfics in the Pokémon fandom, but this particular one might be hard to find. You could try searching on dedicated fanfic websites like FanFiction.net or Archive of Our Own and use the relevant keywords to see if it exists.
In a fanfic about Ash and a female Pikachu, the story could be set in a new region. The female Pikachu could be a rare color variant, like pink or purple. Ash would be intrigued by her from the start. They could start training together, and the female Pikachu could be very intelligent and learn new moves quickly. Their journey might include trying to protect the female Pikachu from trainers who want to catch her because of her rarity, and in the process, they grow closer and become an unbeatable team.
Another idea could be a more emotional story. The female Pikachu gets separated from her group for some reason. Ash finds her and takes it upon himself to reunite her with her family. During this time, they have to endure harsh weather conditions and dangerous terrains. As they spend time together, they develop a deep connection that is based on mutual care and the shared goal of getting the female Pikachu back to her loved ones.
Create a story around their shared interests. Since they both love Pokémon battles, you could write about how they train together. Ash could come up with new strategies and the female Pikachu could practice implementing them. You can also add in other Pokémon friends that join them in their training sessions, making it a fun and family - friendly adventure.
To create a unique Detective Pikachu OC, start with their appearance. Give them a distinct look, like unique hair or clothing style that sets them apart. For example, they could have a long purple coat with Pokemon - themed patches. Then, think about their backstory. Maybe they grew up in a family of Pokemon trainers but decided to become a detective instead. Their motivation could be to find a missing Pokemon that was once their family's pet.
For Ash, you can develop his character by showing how his views on Pikachu change or expand because of the female fan. Maybe he starts to see new aspects of Pikachu that he hadn't noticed before. For the female Pikachu fan, you could give her a backstory of how she became such a big fan. For example, she had a sick Pikachu as a child that was healed by a mysterious Pokémon trainer, and that's what sparked her love.
One idea could be that the female Pikachu has a special power that only Ash can help her control. For example, she might be able to communicate with a certain type of Pokémon that is usually very difficult to reach. Ash, with his experience and kind heart, guides her in using this power for good, like helping to resolve a conflict between different Pokémon groups in a forest.