There's also a possibility that it's a new character created specifically for this fanfiction. This new character could bring a fresh perspective and allow the author to explore different aspects of Harry's gentlemanly behavior at the Yule Ball.
Their relationship could develop through shared experiences at the Yule Ball. For example, they might have a really deep conversation while dancing that makes them see each other in a new light.
One great 'Snape Yule Ball' fanfiction is 'A Surprising Encounter at the Yule Ball'. It shows Snape having an unexpected interaction during the ball that changes his view of the event.
In this fanfiction about the Yule Ball, there are many possibilities for Harry and Cho. They could be initially hesitant around each other. However, the Yule Ball is a special event full of magic and wonder. Maybe they find a quiet corner to talk, away from the noise and the crowd. During their conversation, Cho might open up about her feelings for Cedric, which could lead to Harry comforting her in a new and deeper way, strengthening their bond.
I think it would be really romantic if during the Yule Ball, when the music slows down, Harry takes Hermione's hand and leads her to a more secluded corner of the hall. There, under the twinkling lights and away from the crowd, he tells her how much she means to him. Not just as a friend, but as someone he has deeper feelings for. Hermione blushes and they share a tender moment that makes the Yule Ball truly special for them.
Yes, there are likely many such fanfictions. The Yule Ball is a significant event in the Harry Potter universe, and Harry's character development and his behavior as a gentleman during it can be an interesting topic for fanfiction writers to explore.
One great fanfiction could be where Harry and Ginny's relationship blossoms at the Yule Ball. Maybe Ginny gets a little jealous of Harry's initial dance partner but then they end up having a magical dance together under the enchanted ceiling.
One possibility could be that in the fanfic, Harry and Daphne meet by chance at the Yule Ball. Maybe Daphne is initially hesitant to interact with Harry as he is a Gryffindor, but as they start dancing, they find themselves drawn to each other. They start sharing stories about their Hogwarts experiences, from difficult classes to dealing with the various creatures around the castle. Harry might be surprised by Daphne's intelligence and grace, and Daphne could be intrigued by Harry's courage and his tales of facing Voldemort.
One idea could be that Snape is secretly asked to dance by a student who sees through his tough exterior. Maybe they share a moment on the dance floor where Snape reveals a softer side, reminiscing about his own Hogwarts days as a student during the Yule Ball. This could lead to a new understanding between the student and Snape.