The romantic tension in those stories hinges on Benimaru's distant exterior versus his latent intensity. He's a commander, often the strongest in the room, so the slow erosion of his professional detachment is the whole game. It's not about grand gestures; it's about him noticing the reader-character's quiet competence during a mission, or a stray comment that actually makes him pause mid-strategy. The 'fire' motif gets repurposed beautifully—instead of destruction, it becomes this contained warmth he reluctantly allows himself to share, like letting the reader sit close to a campfire he's tending.
A specific technique I see a lot is using his sensory perception. His heightened senses mean he's hyper-aware of the reader's presence, heartbeat, scent—stuff he'd normally ignore. But the tension builds when the narrative shows him actively tuning it out, or conversely, focusing on it too intently during a quiet moment. That internal conflict, the battle between his duty-bound focus and involuntary attention, is where the best tension simmers. It feels earned when he finally acts, because every step was a deliberate choice to let someone in.
The physical space between them is another huge factor. Stories often frame him as untouchable, literally and figuratively. The first time he doesn't flinch from an accidental brush, or initiates brief contact to guide the reader during training, carries so much weight. It's a language he speaks in actions, not words, which makes every small concession feel monumental.