especially on AO3, and the emotional core really hinges on trauma bonding and mutual protection. Tanjiro gets a ton of focus, but Daki and Nezuko are the two who've been turned into demons against their will, forced into monstrous roles they never asked for. A lot of writers explore that shared violation as a foundation for a really dark, tender understanding. They're not just enemies; they're mirrors.
You see a ton of 'enemies to reluctant allies to something more' arcs built on that. Daki's vain, performative cruelty covering up her own deep-seated insecurities, and Nezuko's silent, fierce will to protect her humanity—they bounce off each other in ways that can be cathartic. A popular theme is Daki being fascinated by Nezuko's purity, something she can't corrupt, and Nezuko, in her muted way, seeing the trapped girl beneath the demon. It's less about romance in a fluffy sense and more about two broken things finding a jagged fit.
The fics that hit hardest for me are the ones where their connection is almost non-verbal. Nezuko can't speak, Daki communicates through layers of artifice, so their intimacy is built on gestures, shared silences, and fighting back-to-back. It's a specific kind of emotional language that feels unique to this pairing, born from their circumstances in the 'Demon Slayer' universe.