At first, Shion doesn't understand what he's doing. It makes sense to dream of his master. It makes sense to feel good when he thinks of him, of the strength with which he carries himself, of the memory of being saved by him. When Shion's hand drifts between his legs as he lies in bed it doesn't even catch his notice, some meaningless compulsion he doesn't tie to the thoughts of his master until later.
Hakurei's other students are younger and Shion does not often speak to any others, so he is left to realize on his own. When it begins to unsettle him, he has already learned to do more than grind his palm against himself, and he knows and recalls his master's scent, and he turns his head and lifts his forearm so he can press his lips to the scar upon it, in some small part wanting to smell or taste or feel some lingering trace of warmth from his master's blood. He is caught off guard in the middle of a lesson because he is dazed by his own thoughts of paper skin and liquid heat, and that is when it strikes him that this is something frighteningly beyond sense.
He cannot tell Hakurei for fear and shame, but Hakurei is older and sharper and wiser and Shion holds no real hope of keeping it from him. He tries regardless, without another choice; a warrior cannot afford even slight distractions, and this one looms and casts its shadow over every plane of Shion's mind. No better solution comes to him than to turn his focus to his training, to think of Hakurei's disapproval rather than the line of his shoulders, and so he throws himself into his cloths and cosmos and curls his hands like talons into his pillow at night.
Tension weaves itself into ropes along his back and shoulders; they grow so thick his master scowls at them as if Shion's robes hide nothing, a thought that prickles on Shion's cheeks. Hakurei berates him for days for allowing himself to reach such a state, but Shion can think of no solution, and so eventually, Hakurei does, calling him closer one day after a strained display of stardust.
His master grouches even as he digs his thumbs in on either side of Shion's spine; Shion manages twice to retort before the rest sound foolish in his head, and he instead occupies himself trying not to writhe desperately beneath the pleasure-pain of his muscles being worked and displaced. When Hakurei's knuckles press between his shoulderblades, Shion ducks his head and presses his thighs together, hoping it can be mistaken for the simpler shame of being scolded, and thinks, over and over, unable to help himself, that his master's skin is as dry and paper-smooth as he always imagined from the look of it, and the bits and pieces of touch he gathered during combat training. His master uses his elbows and the heels of his palms, and Shion thinks of the scar on Hakurei's forearm until he finds himself trying to see if he can discern the feel of it on his back from the touch of any other skin; he half-expects to find it warmer, though to Shion, all of Hakurei's skin seems nearly searing. He lets out a grunt through his teeth, and his master snorts at it, saying it is his own fault if it is painful. Shion means to tell him that it doesn't hurt, and instead is caught with his mouth open when Hakurei's fingers sink in low down his back, near his hips.
The sound he makes is long and raw and unmistakable, and for a moment, both master and student are still.
Somehow, Shion recovers first; he is every bit the ungrateful brat he is so often accused of being, and leaves without a word of thanks.