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.
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 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.
There are also some fanfictions where a completely new character is introduced as Naruto's secret girlfriend. This allows the writer to create a unique story line and explore different dynamics that are not bound by the pre - existing characters and relationships in the Naruto universe.
Definitely their close friends like Brock and Misty. They would be there to support Ash. And all of Serena's friends from Kalos too. Also, the Gym Leaders they've met along their journey might be invited. And of course, their Pokemon. Pikachu would be right there by Ash's side, and Serena's Fennekin would also be present.
A new, mysterious organization that is interested in controlling the legendary Pokémon. They see Latias as a key to their plan and Ash as an obstacle to be removed.
A powerful and greedy trainer who wants to capture Manaphy for its rare abilities could be the main antagonist. He might have a whole team of strong Pokemon and use underhanded tactics to try and get Manaphy.