A/N: Written for the February round of Bleach flashfics at LiveJournal.

This fic made me do the following:

1.) Ship platonic Shunsui Nanao. Hard.

2.) Hero-worship Nanao and want to hug her so badly it hurts.

3.) Cry a river.

(Before anyone asks... this fic doesn't reflect my own past, which has been very happy for the most part, but still. Ouch-bloody-ouch.)

Title is Latin. Contextual meaning is 'something which is necessary, or cannot be done without.' Literal meaning is 'without which not.'

Enjoy!

-Eia

xxxxx

Sine Qua Non

xxxxx

Nanao has always been a quiet girl.

At least, as far as she can remember, she's fairly sure she has been. Perhaps before her death she had been outgoing, lively, an engaging individual to have at parties... but she rather doubts it. People don't change like that in the moments spent traveling the neck of the hourglass. Old patterns don't drop away with memory, old habits remain steadfast. Sometimes people have minor revelations upon discovering that they are dead and that there is nothing left to fight for, but Nanao is fairly certain she hadn't been one of them.

She can't be sure. She doesn't remember. She doesn't even know how many days she lived in the dusty streets of Rukongai before her memory began to work again. It's possible that it was only a day or two, but she wouldn't be surprised if she had gone months, walking through one moment and forgetting it the next.

Certainly when she had finally woken up and realized that she was a person, breathing and walking and hungry enough to die of it all over again, she had been very thin and bruised and tired, and the wound on the right side of her forehead was old and messily healed into a jagged whitish scar. That suggested logically that it had been some period of time since she'd acquired it, but since it was still tender to the touch, not long enough for it to have occurred before her death. Fresh wounds don't translate onto the soul, otherwise people who had died violent deaths would come through the veil in the same horrific state they had departed in.

From these facts, she has deduced with reasonable certainty that she was unconscious but physically functional for at least a week, but not more than a month, because she did not die of hunger as someone with her level of reiatsu should have past that period of time.

This is what she does between moments of distraction-- remember what she can, and wonder about what she can't. Ise Nanao is a rational person who likes to have all the facts, and it bothers her that this unknown period is blank and featureless in her mind.

What happened? It is likely that she will never truly know, but on nights like this one when sleep eludes her, she has nothing better to do than turn old facts over and over again in her mind.

Nanao doesn't sleep often, because she most of the time she doesn't dream. All there is waiting for her behind her eyelids is a familiar, awful darkness and endless silence. It makes her sick.

She stays awake and wonders because it's more restful than sleeping.

xxxxx

All the way through her years at the Academy, Nanao avoids making friends. Friends are troublesome. They come too close, they touch, they discover things. She doesn't want to answer questions about the scar on her forehead because she simply can't, and there are few things she hates more in the world than not knowing the answer to questions. So she styles her hair to cover it and fastens it in place with two steel pins so that it will not come undone and reveal her, and speaks only when spoken to.

She is an excellent student. Despite all her efforts to be beneath notice, her precision and flawless executions of the demon arts and her cool, rapid thought processes during sparring draw attention to her consistently. By her third year it's widely accepted that she will make a ranking position in the Thirteen Squads upon graduation, and everyone forgets about her. Nobody even looks at the top rank on written tests because they know which name will be written there, every single time, no matter the subject.

For all her brilliance, she does her best to come across as utterly ordinary, unnoticeable. Most people buy it after the first little while. They grow bored of her stiff, paper-dry facade and unbending iron spine and stop inviting her to weekend parties and dances. She is glad for that. She is happy to be overlooked. Shadows are much more comfortable places to her than limelights. Most people see this and are smart enough to ignore her.

Nanao, later, remembers the first person to completely miss this cue.

He is a captain, has been one long enough to wear the white coat easily, long enough to treat it without any sort of reverence. He wears a silly straw hat and a sillier pink-and-floral overcoat, his chin is rough with stubble, and somehow he avoids looking ridiculous. His name is Kyouraku Shunsui and when he looks at the student named Ise Nanao, he does not see a soul made of paper and books, but her actual self.

It unsettles her deeply enough to make her run.

He gives her three days to compose herself before he comes to find her again, and this time he corners her so that she can't run again and asks her to be his lieutenant when she graduates.

Nanao is startled. His reasons are unclear, which makes her uneasy, and she hates how he seems to read right through her defenses to the truth of her without so much as a by-your-leave... but other than her personal discomfort with him, she has no reason to decline such a generous offer so early in her career. She accepts despite her misgivings... and instantly regrets it when he hugs her and calls her 'Nanao-chan' like an elder brother who has known her for years.

From her swift annoyance with this, she deduces later that night that she likely had an older brother before her death. She writes this down in her black leather-bound journal in small, painfully neat characters, then puts it out of her mind. What she had before death has no bearing on her life now. Reflecting on what she may have lost is an exercise in futility and she knows it.

Still, the next time she sees him she manages to find a smile for him-- her first in living memory.

xxxxx

The day she graduates and becomes his lieutenant is the day before he gets too close. It is the morning after that everything falls apart.

Accustomed to the Academy, where her roommates knew better than to come into her room, she is incautious. He is waiting for her when she emerges from the shower, hair wet on her forehead and failing to adequately hide much of anything.

He greets her joyfully with his usual cry of 'Nanao-chaann!' and smiles broadly at her. Then his eyes find her forehead and his smile dies.

She can feel the questions coming, can't take it, and runs, one hand held hard to her forehead to hide the mystery there. She is naked except for her towel and doesn't care. She would rather face a hundred questions about why she is stumbling through the streets of the Court in a scrap of white terrycloth than face even one question from him about this.

He catches up. She is good at flash-step but not nearly as good or as practiced as him. He scoops her up into his arms like she weighs nothing, not even pausing before turning around and returning them to the Ninth Squadron quarters.

Nanao lies quiescent across his chest, resigned, until they make it back and he dumps her unceremoniously on the couch and turns away to let her rearrange her silly towel to cover anything valuable. She does, but not because she's modest, just because it's something to do with her hands.

"Who gave you that?" he asks, without preamble. His ever-present smile is nowhere in evidence.

Nanao has avoided this question and others like it for nearly half a decade. It feels like losing a battle to have to answer it now, but there is nothing left but the truth. She has never tried, but she suspects she would be a terrible liar. "I don't know."

"What did he look like? Or she, I suppose, I don't want to sound sexist."

"I don't know."

"You didn't see their face?" he asks. He is merciless.

She has never seen this side of him before but it doesn't surprise her. Someone truly as carefree and shallow as he affects to be would not have made captain, or remained one for any length of time. "I don't..." She hesitates. She has never told this to anyone. It has always been her secret, her painful mystery to deal with by herself. Sharing it feels weak to her, but he won't rest until he has the truth so she takes a deep breath and does it anyway. "I can't remember."

"Head injury, trauma-induced amnesia," he says to himself as if confirming something, then turns to her. His eyes are grave. "Get dressed."

Nanao wonders what his plan is, learns fairly quickly when she finds herself sitting before the quiet solemn eyes of the Fourth Squadron captain and her lieutenant.

Four years of hiding it, and now three people know in one day. She wants to retreat to a safe room with a lock and stay in it for weeks until she feels secure in herself again. This exposure is painful to her, but he won't let her run.

Unohana Retsu is tranquil and lovely. She makes Nanao feel safer just by her presence, and she relaxes a little despite herself. Her fingers are cool on Nanao's forehead, gentle and unintrusive. "I can heal this," she says, "but only if you wish me to. It stays because you are attached to it, not because it must. Wounds on the soul work differently than those on a physical body-- they heal clean, unless the bearer wishes to keep them."

Nanao opens her mouth to protest, but closes it again whens he realizes that Unohana is right. The scar is the only evidence she has to mark the dead days she can't remember. Releasing it would mean letting go of whatever happened in those days and its effects on her. "I don't know," she whispers.

"Are you afraid to remember?" asks the healer, so gently it makes Nanao want to cry.

She doesn't cry. She is a warrior. "Yes," she admits. "I am. Not knowing is troublesome to me, but I am afraid."

Shunsui hits her on the back with the flat of his hand, a familial pat that nearly knocks the wind from her lungs. "It's up to you," he said genially. "You decide for yourself whether you want to remember or not. If you don't, I'll respect that and never mention it again. If you do, I'll leave the room if you want. Nanao-chan, all I ask is that you make a decision. That's all."

She stares at her knees, and the white-knuckled fists she has clenched atop them. She has killed Hollows, she has tasted her own blood in her mouth in the heat of battle, she has felt the exhaustion that comes after the last enemy falls, but this frightens her more than anything ever has. Ise Nanao is a warrior made, but a human woman born, and she is afraid.

"Aren't you tired of running?" bursts out Unohana's liuetenant, her face painfully earnest. Her name is Kotetsu Isane. Nanao remembers her as a senior in her first Academy year. She is shy, but kind and insightful, and Nanao liked her then and likes her now.

She is right and Nanao knows it, but the fear will not abate.

"Isane," murmurs Unohana in mild reproach.

The lieutenant bows and blushes and says nothing more, but she has done enough already.

"All right," whispers Nanao. "I'll try."

Shunsui thumps her proudly on the back again. "That's my lieutenant," he says approvingly. "I'll wait outside." He turns to leave.

Nanao catches his hand, not even sure why she's doing it but sure that it's the right thing to do. "No," she says. "Stay. Please."

Bewildered, Shunsui covers her hand with his and sits next to her, places one sword-strong arm around her shoulders to brace her comfortingly against whatever is coming. "All right, Nanao-chan. Whatever you like."

She wonders why he is so kind to her when she has never done anything for him to deserve it.

"By your leave, then," says Unohana, and draws her soul-slayer while laying her hand on Nanao's forehead again. "Take a deep breath and close your eyes."

Nanao does so. There is a split second of sick anticipation, and then the world drops away sideways and she falls.

She remembers.

She remembers everything-- every moment of the twenty-seven days she spent crawling around Rukongai, running mindlessly away from... from...

The memory twists, turns black.

She is screaming, crying, begging, she can hear herself distantly but it doesn't sound like herself. Nanao is a warrior, she doesn't cry like that, like a weakling with no hope left in the world.

There is a man. She sees his face, knows his features, knows his name. Father. His hair is black, his eyes are brown, but all she sees when she looks at him is the metallic red of fresh blood and the yellow-green-violet of old bruises. An entire spectrum of pain-colours. His arms are white. She can see blue veins through the pale skin.

She remembers now how she spent the last moments of her life, twisting the knife deeper into his suprised throat even as the curtain of black swept across her vision and her heartbeat slowed to nothing. Mutual homicide, murder, patricide, self-defense, desperation. Ugly, meaningless words for a lifetime of agony and hatred.

She remembers waking up next to him in Rukongai and the vomit that rises in her throat when she realizes that even here she cannot escape him, even death cannot get him away from her, and how she tries to stand and run but can't quite make it. She'd never quite made it and nothing has changed with death. His fist, ringed and heavy as iron, crashes into her forehead.

The ring is a seal, the last remnant of a failed family, once noble and wealthy but ragged and tawdry now with the coming of the modern age. It is unreasonable but he blames her for his own failure to save his heritage. She was not born a son. This is not her fault, but an accident shortly after her conception left him unable to ever sire another child, and because she is not a son it means the end of his line. Not her fault, but he cannot blame himself and who else is there for him to blame?

The ring tears her open, gashes her to the bone, dashes her blood over her face.

Nanao remembers finding her feet, though she cannot feel them, and running until her lungs give out on legs that are numb with adrenaline. She remembers hiding until she can feel him coming again, not understanding how she knows but understanding that if she doesn't run he will find her and kill her all over again, his tan-yellow hands around her throat until the black spots swallow her eyes and her breath stops in her chest. She remembers picking up her ragged body and running, remembers the slow cell-death of thirst and hunger weakening her steadily until all she can do is lie in an alley with her hair over her face and pray he won't recognize her if he finds her.

She remembers a kind child giving her food and water, and waking up shortly thereafter to continue running on new strength, no longer knowing why but conscious enough now to secure more sustenance and continue surviving.

She remembers all of it, every sensation, from the taste of dust and blood in her mouth to the itch and throb of her healing wound and the sharp smell of the murky water the child had given her, and knows she was right to be afraid.

The memory ends.

Nanao opens her eyes to find herself held tightly against a broad chest, throat raw, eyes sore, and muscles clenched so tightly she feels made of stone. Now she knows. She can't decide whether or not she regrets deciding this way. Perhaps it might have been better to leave this forgotten and simply bear the scar.

Shunsui shakes her gently and calls her name, and with the sound of his voice reality suddenly floods back in, sweet and clean and bright and smelling faintly of roses and sake. The memory retreats reluctantly to the past where it belongs, releasing her temporarily. She dissolves into him, limp and feeling empty as an unoccupied room.

Now she knows.

Her mind fills in the gap, colours it in from edge to edge so that her memory heals uninterrupted. The memory is not beautiful, but it is true and present and now she no longer has to wonder about it.

More, she realizes with a dawning lightness in her chest-- now she is free to move forward without looking back. She has faced her personal demon head-on and won. Her father is in Rukongai, scraping a meager living from the dust, and she is strong now. She is a warrior, lieutenant to a captain she respects, and there is nothing he can do. His arms are long but they cannot breach the stone walls of the Court of Pure Souls.

"Kyouraku-taichou," she croaks, not sure whether to smile or cry and deciding after a moment to do both at once. She has no idea what to say next, but his chest is warm and powerful against her cheek and she knows it's all right to say nothing.

He strokes her hair gently, protecting her as her real brother never bothered to. "That's my girl," he murmurs. "I'm going to take you home and put you to bed, all right?" He picks her up and turns to the healer. "Thanks, Retsu. I appreciate this."

"That is what I am here for," replies Unohana serenely.

Shunsui flash-steps them both home and lays her down in her bed, drawing the covers over her and prohibiting her from moving until at least tomorrow. Then he leans down and plants a careful, chaste kiss on her flawless forehead.

"Stay," she whispers, exhausted but not quite too exhausted for this. "Please."

He is still for a moment, then moves to carefully arrange himself next to her atop the covers. He props his chin on his palm and looks down at her, smiling in a way that makes her feel safer than she has ever felt. "All right, Nanao-chan. Sleep."

As if the very word was a demon spell, Nanao obeys, slipping under her own consciousness like falling into dark water.

She dreams of her future.

xxxxx

Shunsui is halfway through the bottle of sake.

Nanao takes it away and replaces it with a freshly-brewed pot of tea, ignoring his injured protests and dropping a pile of paperwork in front of his prone form. He moans, groans, rolls about like a child in a tantrum, but she knows he will do it as soon as she leaves the room, as fast as he can so he can get back to doing what he really loves-- drinking, and flirting with her shamelessly.

This is her captain-- carefree, ridiculous, allergic to responsibility but highly capable when faced with ones he cannot avoid, staggeringly kind and compassionate, eccentric and confusing sometimes but worthy of every iota of respect accorded him.

This is her, with him-- dutiful, competent, a perfect counterbalance to his flighty personality, powerful in her own right but easy to underestimate due to her unassuming facade, secure in her position and her self without the need to prove herself in battle, full of a rare inner-knowing that makes her heart and soul stronger even than her body.

This is them, together-- a perfect team, easy in their unspoken camaraderie, able to snap instantly into a seamless battle formation at a moment's notice, an odd and mismatched couple to those who do not know them but quietly affectionate in the moments no one else is privileged to see.

He keeps her secret, as do Unohana and Isane. No one knows why they regard each other with such respect and love, and no one dares to ask, sensing that it is an unwelcome topic.

Her past complete, no patches curiously missing, she no longer has reason to look back and wonder. Every night when she sleeps, there are dreams waiting for her, and she is glad to wake up after them. There are rarely horrors waiting for her within them anymore, and when there are, her captain is there before her first cry dies in her throat to wake her and chase them away.

She is still a quiet girl, but now people respect her for it.

Nanao is content.

XxxxxxxX

A/N: Excuse me while I go bawl into a pillow. TT

Hope you enjoyed it.