In a lot of Shinobu x Mitsuri fic I've come across, the emotional tension feels rooted in this quiet, unspoken understanding. They're both pillars with massive public roles—the Insect Pillar and the Love Pillar—but they carry their burdens so differently. Shinobu's all coiled, smiling poison, and Mitsuri wears her heart so openly. The friction, when writers get it right, isn't about loud arguments. It's Shinobu watching Mitsuri be effortlessly affectionate with everyone, feeling this sharp, ugly little twist in her gut she can't name, and then being extra cutting in her next remark to Mitsuri to cover it up. Mitsuri, for her part, picks up on the distance but misinterprets it as dislike, which hurts because she genuinely admires Shinobu. That dance of misread signals creates a low-grade ache that can sustain a whole slow-burn.
A really effective trope I've seen plays with touch. Mitsuri is physically demonstrative, but Shinobu is not. A scene where Mitsuri, without thinking, grabs Shinobu's hand during a conversation, and Shinobu just freezes. Not in disgust, but in a kind of terrified wonder. She can't bring herself to pull away, but the act of leaving her hand there feels like a confession she's not ready to make. The tension lives in that suspended moment, in the things they don't say. It's less about will-they-won't-they and more about can-they-even-bridge-the-fundamental-way-they-inhabit-the-world. When they finally do connect, it often feels like a release of breath you didn't know you were holding, which is way more satisfying than any grand declaration.
I guess what works is that their dynamic mirrors classic 'tsundere' or 'stoic x sunshine' pairings, but with the specific, tragic backdrop of the Demon Slayer Corps. Their limited time, the constant threat—it adds a layer of urgency to the quiet tension. A good author will have that shadow hanging over every tender moment, making even a shared cup of tea feel weighted and precious.