It could also be a new character created specifically for the fanfic. Fanfic writers often introduce original characters to add new twists to the story. This new character could be someone who meets Ash during his travels and they gradually fall in love without the other main characters knowing.
Maybe it's a new character created specifically for the fanfiction. A girl from a far - off region who Ash meets during one of his journeys. She could be a powerful trainer or have some special abilities that draw Ash to her, and they start a secret relationship.
It could be Misty. They have a long - standing relationship in the Pokemon series. They've traveled together and shared many adventures, so in a fanfiction, it's not a stretch to imagine her as a secret girlfriend.
Another possibility is that it's a Pokémon in human form, which is a common trope in some fanfics. This Pokémon - turned - human could have fallen in love with Ash during their adventures together, and now they have to keep their relationship a secret from the world of Pokémon and humans alike.
He might use his Pokemon adventures as a cover. Since he's always traveling to different places for battles and to catch new Pokemon, it would be easy for him to meet someone secretly. He could say he's off on a solo Pokemon - hunting trip when in fact he's meeting his girlfriend.
Well, it could also be about the challenges they face in keeping the relationship under wraps. Such as dealing with jealousy from other people who like Ash or his girlfriend. And there could be a plot where they almost get caught, but they manage to come up with a clever cover - up at the last minute.
It could be a powerful Pokémon trainer. This trainer might see potential in Ash and decide to take him under their wing.
Misty is a very common character. She was one of Ash's earliest companions. In fanfiction, she is often portrayed as his girlfriend because of their long - standing friendship and the chemistry they had during their travels. They bickered like an old married couple at times, which could easily be developed into a romantic relationship in fan - created stories.
In some fanfictions, it could be a character from another magical school. Like a girl from Beauxbatons or Durmstrang. She might have met Harry during an international magical event and they've been secretly seeing each other ever since.
It could be a new character, a powerful trainer who is jealous of Ash's natural talent and secretly tries to undermine him at every turn.
I have no idea who the author of 'ash's secret fanfic' is. There's not enough information given to determine that.