In some OC fanfictions, it could be a new witch created by the author. For example, a shy witch who meets Harry in the Forbidden Forest while he's on one of his solitary walks. She might be secretly helping him with some of his struggles against the dark forces without him knowing at first, and gradually a romantic relationship develops.
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 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.
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.
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.
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.
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.
She could be reborn as a Hogwarts student. Maybe she uses her modern knowledge to introduce new things to the wizarding world, like different muggle games or study techniques. She might also try to change the course of some events in the original story, for example, preventing certain characters from getting into dangerous situations.
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.
A character like Seraphina. She could be an old witch who has traveled the world studying different forms of divination. Her character could have a very mysterious aura, with a pet raven that she claims can also see glimpses of the future. She might wear long, flowing robes with strange symbols embroidered on them, representing different divination methods she has mastered.