There could be an original character created just for the fanfiction to be Harry's secret girlfriend. Writers often introduce new characters to add new plotlines and romantic interests for the main characters.
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.
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.
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.
Another scenario could be that they start off as friends or classmates and gradually develop feelings for each other without realizing it at first. Then they keep it a secret because they are not sure how the other person really feels or they are afraid of ruining their friendship. In this case, there could be a lot of internal monologues and awkward moments between them in the fanfiction.
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.
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.
Often, she is intelligent. In the world of magic, intelligence can be shown in various ways like being good at spells or understanding complex magical theories. This would attract Harry as he himself is quite smart. For example, she might be able to solve a difficult magical riddle that has stumped others, including Harry.
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.
Hermione could discover it. She's very observant. Maybe she notices that Harry sometimes seems to be looking in the wrong direction or has trouble reading spells from a distance. So, she starts to suspect something and eventually figures out he's blind.
One possibility could be a transfer student from a different wizarding school. Imagine she arrives at Hogwarts with a whole new set of magical skills and knowledge. James is intrigued by her mysterious aura and they start to get to know each other during potion - making classes. They might initially clash because of their different teaching methods from their previous schools but later fall in love.