Honestly? The 'immortal god gets bored and adopts a mortal chaos-gremlin' trope has its claws in me. There's something about Junko, with all her frantic, messy, human desperation for despair, being the one thing that Izuru Kamukura can't predict or grow bored with. He's seen everything, but her brand of volatile, emotional self-destruction is a novel data set. I love fics where he's not in love, per se, but obsessed—like a scientist who found a fascinatingly flawed specimen. The tension is never about romance winning, but about whether her despair can finally make him feel something, or if he'll just dissect her psychology until the novelty wears off. It's a horrifying dynamic that's weirdly compelling. That one fic 'A Study in Entropy' captured it perfectly; Izuru just watches her schemes like they're a particularly complex ant farm.
I'm less sold on the 'they were made for each other' takes. It flattens them both. The appeal is the fundamental mismatch: the ultimate hope who feels nothing, and the ultimate despair who feels everything too intensely. Writing them as a power couple who rules the world misses the tragic, destructive core. It's more interesting if their connection dooms them, or if Izuru's apathy is the one thing Junko can't corrupt into despair, which just makes her try harder. That push-pull, where she's the only person he bothers to acknowledge and he's the only person utterly immune to her, is the engine.