Rin allowed himself a small, private smile as he turned toward the tunnel entrance.
He didn't say anything further about the secret passage or about Sumireko's two years of confident familiarity with a building that had been keeping something from her the entire time. There was no particular satisfaction in pointing out the gap between what someone thinks they know and what they actually know. He simply turned and stepped into the dark, leaving the observation to make itself.
He had sensed the energy the moment he walked into the library. Not loudly, not with the aggressive pulse of something hostile, but with the quiet, persistent presence of something old that had been waiting in the same place for a very long time, patient the way deep water is patient. He had followed it through the stacks the way you follow a sound through a building at night: pausing, adjusting, following the direction where it grew slightly stronger. The book that triggered the mechanism had been the last piece, and it had taken him longer to find it than he would have preferred to admit.
The tunnel received him in its cool, mineral-scented dark, the stone walls narrowing slightly from the staircase's width. Somewhere below, that faint source of light was growing incrementally more distinct with each step. The air tasted of centuries. Whatever was down here had not been disturbed in a very long time.
Behind him, the sound of a long skirt navigating stone stairs.
Then: "Ah!"
He turned in the same motion, already moving, one arm extending outward before he had consciously processed what was happening. Sumireko's weight came into his arm at an angle that suggested she had missed a step rather than tripped on level ground, and he adjusted his stance to absorb it, his feet finding purchase on the worn stone without difficulty.
She recovered quickly. Composed herself with the particular, slightly flushed dignity of someone who is embarrassed but has decided not to be visibly embarrassed about being embarrassed. Her hand found the wall.
"Thank you very much, Rin-kun," she said, with the measured warmth of someone who means it and is also regulating how much of the other thing she lets show.
Rin set her back on her feet and released her without ceremony. He looked at her for a moment, and then looked at the darkness ahead, and the energy coming from that darkness, and the long skirt she was wearing that had nearly sent her down a flight of ancient stone steps.
"Go back," he said. "It gets worse further down. You don't need to be in here."
Sumireko shook her head. Her chin was up and her gaze was steady, the look of someone who has made a decision and is not interested in revisiting it. "I'm not leaving you in there alone. It's too dangerous."
"I'm not in danger."
"You don't know that."
He looked at her.
She looked back.
The firmness in her eyes was real. Not bravado, not performance. Something genuine, the specific stubbornness of a person who has decided that a principle is involved and is going to hold the line. He could have argued further. He could have pointed out that he had faced things considerably more dangerous than whatever was at the bottom of an old tunnel, and had done so without assistance, and had returned from all of them. He could have said any number of accurate and persuasive things.
Instead he exhaled, turned back toward the dark, and said, "Stay close and watch the steps."
Behind him, Sumireko made a small, pleased sound.
The tunnel descended further than the staircase had suggested. The stone walls pressed closer as they went deeper, and the light from above had long since thinned to nothing, leaving them in the particular darkness of underground spaces, deep and complete and warm in an odd way, like being inside something rather than being in the absence of something. The energy Rin was tracking grew steadily stronger with each landing, each turn of the passage, each stretch of corridor where the walls bore the faint, worn suggestion of markings that might have once been deliberate.
Sumireko kept pace behind him in silence for a while, which he appreciated. She was navigating the long skirt with more care now, gathering it slightly at the side with one hand. The other hand occasionally found the wall when the footing changed.
Then the other hand found his arm.
Not tentatively. Both hands, closing around his right arm with the uncomplicated grip of someone who has decided that a situation requires an anchor and has identified the nearest available one. The darkness ahead had thickened past the point where even residual light helped, and Sumireko Sanshokuin, who had been maintaining a creditable performance of being entirely fine, had arrived at the honest limit of it.
Rin stopped walking.
He looked down at his arm. At the hands around it. At the particular arrangement of things that had just occurred to him.
A small, slow smile crossed his face before he could quite stop it.
"It's fine," he said. "Though I should mention, the binding does rather change the texture of the experience."
Silence.
A very specific kind of silence, the kind that happens when a sentence has arrived at its destination and is being fully processed.
Then: "Ah!"
The sound Sumireko made was small and involuntary, the sound of someone who has been caught completely off guard by something they absolutely should have anticipated. The flush that had visited her cheeks during the staircase incident had apparently stored up its resources for a second attempt, because the one that arrived now was thorough and comprehensive and visible even in the dark, a warmth that radiated outward from her face with the committed energy of something that had decided to make an impression.
Her teeth found her lip.
"I had no idea," she said, with the precise, careful diction of someone constructing a sentence under pressure, "that Rin-kun was this sort of person."
The words came out half indignant and half something else entirely, something that shared real estate with laughter and hadn't quite decided which direction to go.
Rin had already turned back toward the darkness ahead. His expression, in profile, was composed. Entirely composed. Almost suspiciously composed, for a person who had just said what he had said.
"The passage continues," he said pleasantly. "Watch the floor."
Sumireko Sanshokuin gritted her teeth. Her hands, after a moment of consideration, did not leave his arm.
The tunnel received them both, and the old energy pulled them deeper, and the light at the bottom grew slowly, steadily brighter.