Gently, he lowers his head to the others chest, and silently listens to the beating within. This isn't the first time he's heard it, nor is it the first time he has done this. Despite this, it amazes him as much as it did the first time.
In the humid dark of a summers night, Horikawa is still, eyes closed as he listens to a heart beating like a drum, loud enough to drown out the singing of cicadas. Kanesada sleeps soundly through the sound of both. Like this, Horikawa wonders how.
The beating beneath his skin is strong and constant; but somehow, unless he is like this, he fails to hear it. His own is likely the same, but he finds it harder to think of himself. Thinking of the other comes to him more naturally than breathing.
He thought of him before he thought of himself.
It's a second chance he never dreamed of having. Having being summoned into this form, the decision as to how he would spend his time came without a second thought. With his first breath, he had asked after him, only remembering to introduce himself with the second. For weeks, he had spoke of nothing but him, gently teased all the while for it, and he had waited.
There were, are, a lot of uncertainties. He finds they grow day by day. He thinks about them most at night. In the weeks he had been waiting, it wasn't certain that Izuminokami Kanesada would show up in the citadel at all. There were no guarantees that they would be reunited. The nights he remembers from that time were much longer than those he experiences now. The uncertainties bothered him more back then, too.
None of the swords are certain as to how they recognise each other at first sight; only that they do. It was the same when Izuminokami Kanesada first arrived. Before either of them had said anything, he had known it was him.
As time goes by, uncertainties increase. Historical record debates his own authenticity, whether or not he was a real Kunihiro blade, whether he still exists, where he might be. What is certain, however, is that he was, is, Izuminokami Kanesada's partner. For him, this certainty is enough to drown out the rest. With the appearance of his partner in the citadel, he dedicated himself to this truth, and to assisting Kanesada in everything that he could.
No one knows how long they will be together like this. No one in the citadel can be sure how long the battles will continue, how long they will be needed, or what will happen after. He can't be sure how much time they have, nor how much longer he can be teased for over-indulging Kanesada, when really it's him who is being indulged. Even the teasing is a blessing on borrowed time. He never tells them how much he enjoys being relied upon by the other, or how much he needs to feel needed. It's likely he never will. It's likely he'll never tell Kanesada the same.
He's not sure if he needs to; there are many uncertainties, but he feels sure that the other already knows.
The strong, rhythmic beat beneath him is interrupted as the body it's attached to stirs. Kanesada starts to fidget, and in making a move to roll over, his body turns faster than Horikawa can react. The resulting effect is Horikawa headbutting him in the chest, and he awakes with a start. With Kanesada, this involves some startled screaming and flailing of limbs, while he tries and fails to get himself upright in his panic. Despite himself, Horikawa can't help but laugh at the reaction, and the sound serves to help ground the other into the realisation that he isn't being attacked. Kanesada struggles to focus so soon after having awoken, eyes wide and confused. Horikawa is more accustomed to the darkness than the other. He finds this fitting; it doesn't suit Kanesada anyway.
"Kunihiro?" he realises. "What are you doing?"
"Ah, Kane-san," he begins, staring at the other as he struggles to adjust to the darkness and failing to find an answer that makes sense to him. He opts for a simple fact; "I was listening for your heartbeat."
"Why?"
"Why?" he repeats, and finds he doesn't have an answer. Kanesada fills in his silence with assumptions, and Horikawa can't help but crack a smile.
"What, did you think I'd do something uncool like die in my sleep?"
He's not even sure if they can simply die in their sleep, like ordinary people. It fascinates him sometimes how easily they slip into saying such things.
"That'd be a pretty lame end for a cool, strong sword like you, wouldn't it?"
"Right?" he agrees, easily forgetting that Horikawa hasn't given him an answer. "Don't take me so lightly."
"Sorry, sorry," he apologises, removing himself from the other's side to slip away back to his own futon. "Well, now that I know you're alive and well, I can sleep easier, right?"
Kanesada grumbles under his breath, before settling back down to sleep. His breathing steadies shortly afterwards into gentle snoring. Horikawa lies awake and listens to this too.
No one knows how long they will be together like this, or what will happen after the fact. The other swords in the citadel discuss it from time to time. He, too, wonders about this. This he thinks about most nights. Will they return to the places they historically ought to be? To individual owners, to be treasured? To museums, to be displayed, to be appreciated? If that were true, Kanesada would be the latter.
How wonderful, if that were true. He hopes it to be so. It doesn't matter if he himself will return to darkness, to depths where daylight does not reach. It doesn't matter that he will not be found, that he will rust into nothingness, that he will be forgotten.
If he is safe, nothing else matters. Until the day comes when they are forced to part, he will continue to support his Kane-san in any way he can. For him, this is enough. With it, he's sure he can rest easily, even in the deepest, darkest depths of the ocean. For now, it's enough to allow him rest here, in this citadel, while listening to the soft sounds of the other's breathing.