Pikachu is a very likely candidate. It's Ash's most iconic Pokemon and has a deep bond with him. Whenever Ash is in trouble, Pikachu often steps up to protect him.
It could be a young girl named Lily who has just started her Pokemon journey. She has a Pikachu - like Pokemon but doesn't know how to train it properly. Ash steps in and shows her the ropes, like how to train it to use electric attacks more effectively.
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.
Ash often becomes more confident in these fanfictions. He learns new ways to communicate with his Pokemon during contests, which makes him a better trainer.
His physical abilities change too. He might be stronger and faster as a werewolf. This could lead to a change in his confidence level. He might be more bold in battles, but also more cautious as he doesn't want to fully expose his werewolf side. He has to find a balance between using his new powers and still being the Ash his friends know and love.
In this fanfiction, Ash's character development might involve him becoming more empathetic. He could meet Pokemon or characters in the Lunar region who are in difficult situations. For example, if there are Pokemon being exploited in the Lunar region, Ash would step in to help. This would make him grow as a person and a trainer. Also, his understanding of different types of Pokemon and their needs could expand, since the Lunar Pokemon might have different requirements compared to the ones he's used to. He might also develop new skills in communication, if the Lunar region has its own way of communicating with Pokemon.
Ash develops by learning new strategies. In Pokemon Conquest, battles are different, so he has to adapt, which makes him more versatile. He also develops his relationships with new Pokemon, which broadens his understanding of different Pokemon types.
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 could develop his ability to understand different Pokemon cultures better. Cynthia, being from a different region, has a lot of knowledge about various Pokemon traditions. Ash could learn from her and in turn, teach her about his own experiences. This exchange could make Ash more worldly and Cynthia more in touch with the broader Pokemon community. In terms of their relationship, they could start to rely on each other more, which would be a big step for both of them character - wise.
In some fanfictions, baby Ash's character develops through his experiences with different Pokemon. For example, if he meets a scared Pokemon, his kindness as a baby might help it become more confident, and this in turn makes Ash more aware of his own ability to influence others.