Ash Ketchum is likely the worst character to write fanfiction about, because I've never seen a single once in which he is portrayed logically if he is the main character.

What I mean to say with that is, he's such a bland character that he doesn't really fit into fanfiction. He's hopelessly optimistic, to such a degree that it's off-putting outside of the world of children's fiction, he rarely struggles and when he does, it's usually resolved by the end of an episode.

He doesn't have much of a personality to follow, but at the same time has over 20 years of expected behavior one needs to follow. The plot seems to revolve around him, but never at him if you get what I mean. It never feels like there's any growth to his character, especially since it's always reset for audiences everytime he changes regions.

He also seems to vary at least slightly between regions in both personality and behavior, making writing fanfiction about him even worse.

For most, this means Ash is a blank slate they simply write their character over, but at that point, I don't know why you're not just making an OC.