Divergence
She is only twenty years his senior-- a mere blink of time, in the eyes of shinigami-- but still she lords over him as if she had a thousand years on him. When they walk together, she strides ahead; when they speak, she addresses him with teasing diminutives tacked onto his name. When they sleep together, he has to fight her to top her: and sometimes, he loses the battle.
They play shunpo tag across the roofs of Seireitei, skimming with the light swiftness of dragonflies: Yoruichi constantly flitting just out of his reach in a maddening flirtation. They cross the breadth of the city in a few flutters of the heart, moving as easily as if stepping over cracks, and when Yoruichi tires of playing the elusive prey she lures him straight to her bed and comes down on him like a stooping hawk.
They do not discuss their duties, young and thoughtless as they are, but the weight of their respective birthrights crouches at the back of their minds regardless. And while the thought sends Yoruichi to prowling restlessly about the Shihouin estate like a caged cat-- sometimes jumping the wall and disappearing for days-- it drives Byakuya to spend an increasing amount of time knelt at his father's bedside: learning what he must know in order to assume a mantle that will fall to him much sooner than he had ever anticipated.
Two hundred is a young age to sacrifice self to duty, but in the Kuchiki household there is no concept of disobedience and no room for irresponsibility.
Their meetings become more infrequent as Byakuya draws away, preparing himself to replace his ailing father. More and more he consorts with duty and law instead of with Shihouin Yoruichi, and more and more he sinks himself into a persona distant, cold, and grave. No one can discern whether his stoicism denotes ready willingness or mere resignation, and not even Yoruichi can tell until the last time they sleep together: a few days after he first wears the kenseikan of the Kuchiki clan.
She has never seen his back touch the back of any chair he sits in, and now is no exception: even if all he's doing is sitting half-clothed at Yoruichi's mostly-disused vanity. Still nude, she scowls at the perfect gap left between his bare shoulderblades and the high back of the chair, and rolls over angry at his indifference. Her hand slips off the edge of the bed, brushing his discarded white undershirt, and as an advertisement of her boredom she rolls back around and flings the garment at him. Picking it out of the air like a crane picking a fish from water, he drops it calmly in his lap in favor of reaching for a hairbrush.
She rolls onto her back, craning her head back to watch him brush out his hair upside-down, and reads him in a glance. An upside-down brow quirks at what she discerns, more in frank lack of understanding than in regret or anger. She has not been blind to his change. "So that's it, then. Eh, Byakuya-bo?"
He sets the brush down, his hands rising to part and twist his hair into the bone-white kenseikan, and his eyes meet hers in the mirror. His voice is as flat and deliberate as surgery, his words carefully chosen and every syllable equally carefully enunciated.
"Let me tell you what the difference between us is, Yoruichi," he says calmly: his uncharacteristic dropping of the honorific carving a strange, almost offensive intimacy into the empty space after her name. "We both owe our families the same duty, but one of us is unwilling--" his eyes narrow cruelly in the mirror, "--or unable to shoulder it."
He turns to look at her directly. His gaze, as it always does these days, bears the watchful, contemplative disinterest of a predator that is not hungry.
She meets that gaze, and a flash sparks in her own.
She is at his back in an instant, one arm curled cagingly about his chest and pride rising hotly in her feral eyes. Her nails touch along his throat, flex briefly, and relax. Her laugh purrs into his ear, and she gives the calm line of his jaw a parting lick.
"Run along then, little master," she mocks. "Run home."
It is neither willingness nor resignation that made him into what he is. She knew that much from just one look at his cold, indifferent eyes; she knew that much from once having been in the same place. Byakuya does not yearn for his station any more than he dislikes it. It is his duty, and duties must be done. It is tradition, and tradition must be upheld. It is the rule of the household, and rules are Kuchiki Byakuya's new mistress.
Yoruichi could have gone the same way. She chose to flee instead.
Seven hundred years later, after he has worn his cold face so long it has become who he truly is, he hears of her disappearance. He allows himself to care only as much as his place and station permit him to, and tells himself he does not envy her freedom.
In the lull following Aizen's betrayal, Yoruichi walks streets she has not seen in a century: and then turns her steps towards the healing wards.
She visits with him while he is recuperating in bed, and he receives her with a gentled dignity that she presumes must be the ghost of his wife's moderating influence. A great relief has temporarily smoothed the harshness from his eyes: the cold, dogged determination she had seen in him at the bridge momentarily gone.
Ostensibly, his gaze retains that same half-lidded, sleepy intensity she knows so well: but looking closer, she realizes that beneath the outer calm are deep and profound changes. A tired, lonely sadness-- a widower's sadness-- tracks its faint prints behind the veneer of cold distance that sheathes his eyes, and the crushing burden of duty and authority weights his gaze with a deliberate and deadened stoicism. He has not broken beneath his obligations, far from it: but the toll they have taken shows in all the stress lines that spider invisibly beneath his implacable, calm exterior.
Yoruichi looks at him, and knows unconsciously that this is what she could have become had she stayed.
She reaches out, half playful and half serious, to brush his hair from his face, and he refuses her touch-- platonic though it be-- with the blind loyalty of a widower who has buried his heart with his wife.
"You haven't changed, Byakuya," she lies for a joke, her voice flippant and mocking.
He simply looks at her.
"Some of us cleave to our duties rather than flee them, Yoruichi," he replies her quietly: but there is no longer the sting of superiority to his voice, and his eyes are no longer certain.