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.
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.
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.
The idea of Percy having a secret girlfriend in fanfiction can open up a whole new world of character development. If his girlfriend is a new character introduced in the fanfiction, she could have unique powers or abilities that complement Percy's. Their secret relationship could be a way to explore Percy's more private and vulnerable side, away from his usual heroic adventures.
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.
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.
His secret girlfriend could be in a difficult situation herself. Maybe she's a demigod with a dangerous family or past. Percy is trying to help her while keeping their relationship a secret. This could involve them sneaking around to meet, sharing secret glances in public, and having to communicate in coded ways. It would create a really interesting and suspenseful plot.
Another popular choice in fanfiction could be Sakura. Although in the official story she has a different relationship dynamic, in some fanfictions, the writers re - imagine her as Naruto's girlfriend, perhaps exploring different sides of their personalities and how they could interact romantically.
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.
Often, she is depicted as strong - willed. Since Naruto is a very determined character himself, having a girlfriend with a similar trait can create interesting story arcs where they support and challenge each other.