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.
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.
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.
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.
One common theme is the exploration of forbidden love. For example, when a character from a different house or with a different social status in the wizarding world falls for Harry. Another theme is the growth of the relationship through shared adventures, like fighting dark wizards together.
One common scenario is that Harry leaves Britain and goes to live in another wizarding community. He might feel that he can start anew away from the pain and memories of Britain.
One common scenario is the reunion of old Hogwarts friends. For example, Harry, Ron, and Hermione might reunite after a long time apart, perhaps after a dangerous mission. Another could be the reunion of a family member, like Sirius Black with Harry if in an alternate fanfiction storyline. There could also be a reunion of different houses' students who once had rivalries but now come together for a common cause, like fighting against a new dark wizard threat.
One common scenario is during the fight against the dark forces. Harry might realize Luna's unique perspective and abilities are valuable. So he could ask her to use her intelligence and knowledge of the magical world's oddities to find solutions to problems. For instance, Luna might know about some forgotten spells or magical creatures that can be used to defeat the enemy.
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 in the hallways. Malfoy might be with his cronies, and they're bullying other students. Harry squints as he decides whether or not to step in and confront Malfoy. It's a moment of decision - should he get involved or let it pass? And the squint is a physical manifestation of his internal debate.