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 a Muggle - born witch who he meets outside of Hogwarts during the summer holidays. She's not part of the typical wizarding society he's used to, so they keep it a secret as it's an unusual situation for a wizard like Harry to be involved with a Muggle - born in such a way.
The plot could be that Percy's girlfriend is a time - traveler. She comes from the future and warns Percy about a great danger that is going to befall his world. Percy is initially skeptical but soon realizes she is telling the truth. They then embark on a journey to stop this danger. Along the way, their relationship deepens as they rely on each other's strengths. Percy uses his combat skills and the girlfriend uses her knowledge of the future to outwit their enemies.
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.
Well, it could be really interesting. Maybe Ash in this fanfiction is from a well - known series, like Pokémon. His secret girlfriend could be someone he met during his travels. It might add a whole new layer of drama and romance to his adventures, changing the way we see his character.
Well, maybe the story could be about how Harry meets this girl in a hidden corner of Hogwarts, perhaps in the Forbidden Forest during one of his adventures. They start to secretly exchange letters and meet in the dead of night. They keep their relationship under wraps because they fear how their friends and the wizarding world might react.
It could be an old acquaintance that Percy had a brief relationship with. Maybe someone he met during one of his adventures and then lost touch with, but she later discovers she's pregnant and decides to keep the child a secret from Percy for various reasons.
One possibility could be that Percy has been leading a double life. Maybe he had a relationship in the past that he kept hidden, and now the existence of the child is coming to light. The story could explore how he copes with this new reality and how it affects his relationships with his friends and family.
The mother could also be a completely new character created specifically for this fanfiction. She might be from a different realm or have some special powers. And her relationship with Percy could be a complex one, filled with secrets and misunderstandings that led to the situation of having a secret daughter.
Maybe there is a shadowy organization that is against the existence of Percy's secret life. They view it as a threat to their own plans and will stop at nothing to eliminate Percy and his secret world.