Another way is to explore her relationships. If she's a girl, she might have a different dynamic with Ash (assuming it's set in the Pokémon anime universe). Maybe she's more of a confidante to Ash, listening to his problems and offering her own cute, Pokémon - perspective advice. Also, she could interact differently with male Pokémon, perhaps showing a bit of shyness or coyness at times.
You can also develop her through her abilities. As a girl, she might have a special ability related to her gender. For example, she could have a stronger connection to electric - type moves that are more about healing or support rather than just attack. This would make her a more unique and interesting character in the fanfic, with a role that goes beyond the typical Pikachu we know from the main series.
For Pikachu, it can be developed by showing its loyalty. For instance, when Chikorita is in danger, Pikachu doesn't hesitate to use its Thunderbolt to protect it. For Chikorita, its growth can be shown. Maybe it starts off a bit shy but as the story progresses, it becomes more confident in using its Razor Leaf attack to defend Pikachu.
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.
One way is to give her a dark past. For instance, she could have been betrayed by someone she trusted in the past, and this has made her cynical and distrustful of others. As a result, she starts to act out and become a bad girl. So, the writer can explore this backstory and how it affects her present actions.
She could be initially a cold - hearted person who doesn't like animals much. But when the St Bernard licks her, it stirs up some long - forgotten emotions. As she spends more time with the dog, she starts to show kindness and empathy, which are new aspects of her character that are developed through her relationship with the St Bernard.
You could develop her character by giving her new powers or abilities. This could change her perspective and how she interacts with others. Say she gains the ability to communicate with animals. This would lead to her having new friends and new ways of gathering information. And it would also show her growth as a character as she learns to control and use this new power effectively.
Their relationship can be developed through shared experiences. For example, when they encounter a difficult Pokémon battle together, Pikachu can protect Lillie, and Lillie can in turn use her skills to support Pikachu. This mutual reliance will strengthen their bond.
They could also have common goals. Let's say they both want to build a really cool treehouse in a particular forest area. They would need to gather materials, plan the structure, and work together. During this process, they'll learn more about each other's strengths and weaknesses. Emolga could use its flying ability to carry light materials to the treehouse site, while Pikachu could use its small size to fit into tight spaces to set up the interior. This kind of cooperation will deepen their bond.
The Ghost Phoenix could be developed by giving it a unique personality. For example, it could be a very proud and noble being, with a sense of honor that guides its actions.
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.
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.