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.
Professor Snape could also be the one to discover Harry's secret blindness. Snape is very astute and has been watching Harry closely. He might notice some small signs during potions class, like Harry having trouble measuring ingredients accurately or finding the right potion bottles. Snape, although not always friendly to Harry, might be intrigued by this secret and keep it to himself at first, while trying to figure out how it could be used to his advantage or how it might be related to Harry's overall situation.
Maybe Harry has been secretly blind all along but has been using magic to hide it from others. He could have developed a special magical ability that compensates for his blindness, like a super - sensitive sense of hearing through magic that helps him detect danger and navigate Hogwarts.
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.
His best friend could discover it. Since they are so close, they might notice something different about Stiles that makes them suspect he's actually a girl.
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.
Often, it's used as a way to add depth to a character's backstory. A blind character could have become that way due to a dark magic encounter. This then drives their motivation in the story, perhaps to seek revenge on those who caused their blindness or to find a cure through magic.
It could be Draco Malfoy. In some fanfictions, the dynamic between a female Harry Potter and Draco is reimagined, and they might end up secretly married as a way to show a new kind of relationship between the two formerly antagonistic characters.
One possible secret in a 'Harry Potter and the Order of the Phoenix' fanfic could be a hidden relationship between minor characters. For example, a friendship that develops into something more between two students who were previously just acquaintances. This can add an interesting layer to the story.
It could be a former Death Eater who has had a change of heart. This person might see the error of their ways and decides to secretly train Harry, believing that he is the only one who can truly defeat Voldemort. They could teach Harry some of the Dark Arts that Voldemort uses, so that Harry can better understand and counter them.
There are indeed 'Harry Potter is secretly married' fanfictions. Fanfiction writers often take creative liberties with the characters. They might imagine a scenario where Harry has to keep his marriage a secret due to various reasons such as threats from dark forces or to protect his family. This kind of fanfiction can range from light - hearted and romantic to dark and full of intrigue, depending on the writer's vision.
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.