I'm a longtime reader who's a little puzzled this pairing became so prominent, honestly. Fred and Hermione share maybe a dozen lines in the books? The appeal has to be rooted in a very specific kind of extrapolation. Authors using them are often less interested in a traditional romance and more in exploring the concept of a partner who's also your co-conspirator, your chaotic best friend first. They build a world where Hermione's rigidity gets softened by Fred's anarchy, but not in a way that diminishes her. It's not him 'fixing' her seriousness; it's him giving her a safe space to be unserious. A lot of fics start with shared trauma after the war, where Hermione's grief finds an unexpected echo in Fred's absence of a twin, and they rebuild something playful together from the ashes.
What I find most compelling is how this dynamic inverts the 'Golden Girl & Troublemaker' trope. Hermione isn't dragged into pranks; she becomes the mastermind, the planner who provides the brilliant framework for Fred's explosive creativity. Their friendship feels earned, a slow-built trust where she learns the value of joy for its own sake, and he learns the strategic power behind the mischief. The sibling-like element comes from that deep, platonic familiarity—the kind where you can bicker over spell diagrams but also sit in comfortable silence while tinkering with a new joke product. It often feels more like a partnership of equals discovering a new facet of themselves than a grand passion, which is a refreshing take in a fandom full of epic, destiny-driven romances.
My favorite example is a fic where they run a joke shop together after the war, and the romance is almost an afterthought to the scenes of them inventing things, arguing over safety protocols, and finding a quiet, shared humor in the mundane. It feels real in a way more dramatic pairings sometimes don't.