You can develop Ash by highlighting his bond with his Pikachu in new ways. Maybe Pikachu gets sick and Ash has to find a rare cure, and Astrid helps him. This shows his caring side. For Astrid, give her a signature Pokemon that has a special bond with her. Describe how she trains it and how it reflects her character. You can also include how she and Ash learn from each other's training methods, which would develop both characters.
Ash could become more responsible as he has a unique starter that might need special care or training. He'd have to learn new ways to bond with it, which would make him more empathetic towards Pokemon in general. For example, if the starter is shy, Ash would have to be patient and gentle, thus developing his patience.
Ash is that energetic trainer we all know from Pokemon, always eager to take on new challenges and befriend more Pokemon. Astrid in Pokemon fanfiction can be seen as a fresh addition to Ash's world. She may be designed to add new dynamics to the story. Maybe she has a different approach to Pokemon training compared to Ash, like focusing more on Pokemon's natural instincts rather than just battle strategies. And she and Ash could start off not getting along but then gradually become good friends as they share experiences in the Pokemon world.
Often, it starts with Ash having an internal struggle or realization about his gender identity. He might notice that he doesn't feel like he fits the traditional male role he has been in. Then, he begins to explore this new side of himself, perhaps trying out different appearances or behaviors that are more in line with his true self.
At first, Meloetta might be shy around Ash. But as Ash shows kindness and respect for Meloetta's power and uniqueness, they start to bond. Ash could take Meloetta to different Pokemon contests or battles. Meloetta, in turn, uses its music to cheer on Ash's Pokemon. Over time, they become inseparable friends, relying on each other in tough situations and having fun together during their travels.
Ash could develop by learning from Sakura's more methodical approach to Pokemon. For example, he might start to study Pokemon habitats more carefully. Sakura, on the other hand, could become more adventurous like Ash, taking more risks in battles.
A more complex plot could be set in a world where there is a threat to all Pokemon. Ash and Astrid, along with their Pokemon friends, have to gather a set of ancient artifacts to save the Pokemon world. Along the journey, they have to overcome their own personal doubts and differences to succeed.
To develop the characters, give them unique goals and dreams. One might dream of becoming the best Pokemon trainer in the world, while the other wants to open a Pokemon sanctuary. Their relationship can grow as they support each other in achieving these goals. Also, show their flaws. No one is perfect. One could be a bit stubborn, and the other might be too trusting. These flaws can create conflict and make the story more interesting.
He could develop more patience. Since rare pokemon are often hard to find and catch, he would need to wait for the right moment. This could also teach him to be more observant of his surroundings and the behavior of pokemon. And his bond with his existing pokemon could also strengthen as they work together to catch the rare one.
Often, in 'pokemon ash and harem fanfic', Ash becomes more confident. With the support of the characters in his harem, he might take on tougher challenges. For example, if a girl in the harem is really good at a certain type of Pokémon battle, Ash could learn from her and become better at that too.
The story could also take a darker turn. If the Pokemon was abandoned in a dangerous area, it might face hardships that turn it bitter towards Ash. When they meet again, the Pokemon could be hostile, and Ash has to try to make amends while also facing the consequences of his past actions.