Okay, so, I gotta admit I've always been a bit skeptical about this pairing on paper. Like, the canon interactions are minimal, right? But that's kind of the point where the tension builds in fanworks. It's not about what's there; it's about the immense gulf between them. Obanai's bitterness, his almost pathological devotion to Mitsuri, versus Tanjiro's boundless empathy and sunshine disposition—they're oil and water. Writers who handle it well don't force romance; they force confrontation. They'll put Tanjiro in a situation where his kindness becomes a mirror, forcing Obanai to examine his own resentments. The tension is less 'will they kiss' and more 'can this jaded, closed-off person even tolerate being understood'.
I read one story that was basically a series of quiet moments after a battle, injuries being tended to. Tanjiro just... doesn't react to Obanai's sharp words. He acknowledges the pain behind them. And you see Obanai getting increasingly frustrated, not because Tanjiro is annoying him, but because the usual social armor isn't working. The emotional payoff isn't a confession; it's Obanai, exhausted, maybe offering a curt piece of advice about snake breathing, a fragment of his guarded world. It's that begrudging, hard-won inch of respect that feels more significant than any grand declaration.
The best fics use the supernatural elements to heighten this. Tanjiro's sense of smell picking up on the complex cocktail of Obanai's emotions—the bitterness, the hidden shame, the undercurrent of protectiveness he'd never voice. It creates a unique, almost invasive form of intimacy that Obanai can't block, which is a fantastic source of tension. You're watching someone be seen against their will, and the struggle is internal, quiet, and deeply compelling in a way fluffier pairings can't match.