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.
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 popular type of Pokemon Ash and Pikachu fanfiction might be adventure - based. For example, Ash and Pikachu could embark on a journey to a new and mysterious region filled with undiscovered Pokemon species. They might face challenges like difficult terrains and powerful, previously - unseen Pokemon. In this fanfiction, Pikachu's electric powers could be enhanced in some unique way, and Ash would have to strategize differently to overcome the obstacles.
Ash would be completely devastated. Pikachu has been his closest companion since the start of his journey. He would probably go into a state of shock at first, unable to believe that Pikachu was gone.
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.
In this kind of fanfiction, Ash might find it really hard at first. He's used to being a human trainer, and suddenly becoming a Pikachu would be a shock. He'd have to learn how to control Pikachu's electric powers which he only used to command before.
The concept of facing fears is a theme that can be explored. Ash may have fears about not being a good enough trainer, and Pikachu could have fears related to its own power or protecting Ash. Fanfictions can show how they overcome these fears together. Additionally, the theme of legacy could be present. Ash wants to be a great Pokemon master like his idols, and Pikachu is part of that journey. Their story could be seen as part of the greater Pokemon legacy, and fanfictions can play with this idea in different ways.
One possible reason for Pikachu leaving Ash in fanfiction could be a sense of self - discovery. Maybe Pikachu feels it has reached a point where it needs to explore the Pokémon world on its own terms, without Ash. It could be seeking new challenges or to find its true place among other Pokémon.
One interesting plot could be that the female Pikachu has unique powers. For example, she can communicate with other Pokémon in a special way that helps Ash in his battles. Maybe she can understand the thoughts of Legendary Pokémon, which gives Ash an edge in tough situations.
Well, in the fanfiction, Pikachu could notice Ash's Pokemon instincts when they encounter new and rare Pokemon. Ash might react in a way that indicates he has an instinctive understanding of how to approach or deal with these Pokemon. This could be because of his past experiences, but it also might be a sign of his evolving Pokemon - related instincts. Pikachu, being so attuned to Ash, would immediately sense this change in him and be intrigued by it.