It's that weird push-pull for me, where the power dynamic is so clear but they both keep pretending it isn't. She's the landlord and he's the tenant, literally. He has to serve her. But she's this completely ordinary, impulsive human who trips over her own feet, and he's this centuries-old, immensely powerful yokai who could obliterate her with a thought. The comedy comes from that friction – him having to obey her ridiculous commands while being utterly exasperated by her. Yet, the slow shift isn't about him learning to respect her authority; it's about him learning to respect her. Her stubborn kindness, her willingness to throw herself into danger for others. He starts protecting her not because the contract says so, but because he wants to. That's the core.
Most supernatural romances have the human being awed or scared. Nanami is just... persistently herself. She gets scared, sure, but she never stays cowed. She'll cry, then wipe her face and demand answers. Tomoe's arrogance meets its match not in another powerful being, but in a girl who simply refuses to be intimidated forever. The uniqueness is in the mundane clashing with the divine, and the divine gradually finding the mundane utterly indispensable.