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.
In the fanfic, Pikachu's character can be deepened by exploring its feelings. It could be shown that Pikachu sometimes feels lonely because it misses Ash (assuming it's the Pikachu from the anime). Chikorita can then comfort it. And for Chikorita, its character can be developed by showing its adaptability. When it enters a new environment with Pikachu, it quickly learns how to survive and thrive there.
Pikachu's character could be developed through its mischievous side. It might play some small pranks on Chikorita at first, but then realizes that it's not nice and apologizes. Chikorita's character can be developed by highlighting its determination. When they are trying to reach a far - away place, Chikorita keeps going even when it's tired, inspiring Pikachu to keep up.
Perhaps it's a fanfic that focuses on a day in the life of Pikachu and Chikorita. They could start their day by looking for food in the forest. Then, they meet some other Pokémon friends and play games together. Later, they end the day by watching the sunset and reflecting on how much they enjoy each other's company.
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.
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.
For Pikachu, you could develop his curious side more. Maybe he's always eager to explore new areas in the fanfiction, which sometimes gets him into trouble. Sylveon could be shown as the more cautious one, using its ribbons to hold Pikachu back when he's about to rush into danger.
The relationship can be developed by having them face difficult challenges together. For example, in a tough Pokémon battle where they are outnumbered. They have to rely on each other's skills and instincts to win.
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.
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.
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.
For Sakura, her character can be developed by showing her growth in dealing with someone as unpredictable as Hidan. She could start off being cautious and a bit scared of him due to his violent nature. But as the story progresses, she could become more confident in handling him. For Hidan, his character can be developed by having him question his own beliefs when interacting with Sakura. Maybe her scientific approach to things makes him think about his religious fanaticism in a new light.