Disclaimer: BLEACH belongs to Kubo, Viz, and probably some other people who aren't me.

Universal Constant

A Bleach Fanfic

Chaos Theory AU

Chapter One: Yin

If fear was plucky, and globes were square,
And dirt was cleanly and tears were glee
Things would seem fair,—
Yet they'd all despair,
For if here was there
We wouldn't be we.

-e.e. cummings

One month after the Battle of Fake Karakura Town

The bustle and hum of the Fourth registered only lowly from Isane's room; this was something of a welcome relief to Yumichika, though he'd never admit it.

In keeping with her personality, the space was painted in soft blue, the tatami neat and the floor cushions in front of the table patterned with dainty flowers embroidered into the pastel silk. The walls bore both ink paintings and slightly-less-traditional watercolors, each square with its selected spot, and the fat hibiscus blooms in the vase on the window perfumed the entire space only faintly.

Isane herself sat across the table from him, not bothering to round her shoulders forward to look shorter. Or perhaps only forgetting to. Her cup of tea sat far enough away not to interfere with her work, steam still curling from the delicate ceramic she'd promptly ignored as soon as she'd poured it. She was frowning at the canary-yellow feather held carefully in her left hand, the small scissors in her right ajar but motionless.

Of course, Yumichika appreciated the care she was showing the task, even if he thought she'd do just as well not to worry so much.

"Cut too large first, then trim," he suggested, already following his own advice.

She hummed, brows furrowing a bit, then nodded. "Sorry. It just seems like I should be really careful, considering where they're going." She glanced up, sheepish.

Yumichika couldn't quite get over how different she was—in person, so to speak—from how she was as fukutaichō of the Fourth. All that confidence and certainty just seemed to melt right off her. It was quite the interesting change, though frankly he found it unnecessary and often told her so.

"I promise I won't let them anywhere near my eyelashes if they aren't to my liking," he replied archly.

This, for whatever strange reason, seemed to be the kind of thing that reassured her, and she went back to work with a little more gusto this time.

It wasn't more than another few minutes before the rasp of her blades paused again, though. "Any particular reason you're changing these, by the way? I thought you really liked the ones you have."

Yumichika shrugged, more offhandedly than he really felt. "I do. That doesn't mean I want to keep them forever."

Isane made a small noise in the back of her throat, like she didn't quite agree. She was much more sentimental than he was—most likely she read more into the statement than he'd really meant to convey with it.

"I guess… I just figured that now would be the sort of time to hold onto the things that haven't changed. Er—not that this is a big deal or anything." She waved the hand with the feathers in what was probably meant to be a conciliatory gesture. A few pieces fell away from where she'd been trimming, fluffing out into the air and drifting down onto the tabletop like snow. She made a face and cleared her throat, trying to gather them back together.

Yumichika suppressed a laugh by pursing his lips tightly together for a moment. "You're not wrong, I suppose," he said, making a few more delicate cuts of his own. "But I tend to save all that effort for the important things."

He'd never been one to hold onto things clearly on their way out the door, so to speak. And the old Gotei 13, his old place in it—that was already gone, like it or not. He was far too much a realist to pretend otherwise. The best thing to do now was adapt in the way the new reality demanded. Someone had to, and frankly the entire rest of his division was terrible at that sort of thing.

Isane seemed to follow even the unspoken parts of his train of thought. She was quite good at that, he'd discovered.

"How's Ikkaku-san holding up? And the rest of the division?"

Yumichika grimaced for just a moment before smoothing away the ugliness of the expression. "Not well, in either case," he said, expelling a sigh from his nose. "I doubt this will surprise you, but the Eleventh does not do particularly well at self-direction. Nor at processing emotions." The death of their captain and the ongoing absence of their vice-captain had hit everyone hard. As it would, when they'd all built Zaraki-taichō up in their minds as the invincible fighter who could not fall. The goal to be striven towards, and the only person worth serving under in the meantime.

Yumichika did not believe in idols, but even he'd admired the captain a great deal. Mostly for the purity and simplicity of the way he lived. It had been a particularly beautiful sort of minimalism of purpose. Almost ascetic, though he doubted anyone on either side of the comparison would welcome it.

"Yachiru's… not well, either," Isane replied, her lips pursing. "Unohana-taichō is looking after her as much as she'll allow, but she spends most of her time alone these days. It's hard to know what to say to her."

He nodded. That tracked with his observations, when he'd gone to visit her. Somehow, he doubted the fukutaichō would be returning to the Eleventh. Her life had been symbiotic on Zaraki-taichō's; though there were other people who she liked and would probably eventually draw comfort from, he doubted she'd wish to resume those duties. Besides, vice-captains were appointed by captains, and they still didn't know what would be happening in that regard.

Though they were far from the only division with the problem, they certainly had a unique version of it. No one that Kyōraku-sōtaichō could put in the position would ever really be accepted, because that person would not have felled the previous captain in a fair fight. That would make matters… exceedingly difficult. Yumichika was trying to hold things together organizationally in the meantime, but it was proving quite difficult when the rest of the officers were just non-functional. Perhaps it was time he lit a fire under some of them and tried to at least work through the positively disgusting backlog of paperwork.

"And yourself, Isane-san? The Fourth seems to still be in working order, for the most part." Yumichika resumed his cutting, slowly accumulating a pile of tiny pieces of feather.

"Well, there's been a lot of work, as I'm sure you can guess." Isane shifted in her seat, finally setting the scissors down to take a sip of her tea. "We've honestly only just been able to release the worst-injured of our patients. Poor Ishida-san was in awful shape. And, well, healing Ukitake-taichō is always complicated."

It wasn't the first hint Yumichika had ever come across to this effect, but he didn't press her for more information. It didn't really concern him, and was no doubt sensitive information besides. Isane took her divisional duties every bit as seriously as Yumichika took his, even if what those duties boiled down to was wildly different.

Glancing between his pile and Isane's smaller one, Yumichika set down his scissors as well. "I think there's enough here. You said there was some kind of living world invention that would make this feasible?"

Isane visibly brightened. "Yes! It's called eyelash glue, I think. Rangiku-san found it in the real Karakura Town. I can use a kaidō to set it faster and make it hold better, but it should help with the application, anyway." She picked up something from the floor next to her and rolled it over the surface of the table towards him.

Yumichika caught it deftly, raising a finely-arched eyebrow at the label. "The living world is quite the place," he said. He was almost envious.

Isane huffed, rising from her seat only to settle herself back down next to him. "Maybe hold off on saying that until we know if this works," she pointed out, adding her collection of feathers to his. "You want both lash-lines, right? Upper and lower?"

"Mhm."

Isane had the steadiest hands of anyone he knew, and Yumichika could swear that her eyesight was flawless. The result of this combination was a precision that probably helped her a great deal in her profession, and also meant she was the only person he'd trust to do this kind of delicate task right next to his eyes. She wore an intent look of concentration as she worked, the occasional glow in his peripheral vision indicating the use of the promised kaidō.

When she was finished, she handed him a small mirror. Yumichika blinked a few times to make sure everything was holding, then raised it to eye level.

Yellow had been an excellent color selection. Isane's work, as ever, was quite impressive; she'd concentrated the feathers at the outer corners of both eyes, and blended them seamlessly with his actual eyelashes.

"You, my dear Isane-san, are wasted on healing people."

She snorted, an unladylike sound if ever there was one, and rolled her eyes at him. "My true calling being…?"

Yumichika smiled slyly at her. "Art."

He was entirely unsurprised when he made it back to the division that night only to find Ikkaku drinking alone. Well, alone in that he was the only one still awake, at least.

At any other time and for any other reason, Yumichika might have rolled his eyes at this discovery, but he was inclined to let this one pass. Few people would understand the real nature of Ikkaku's grief, but he was one of those few.

So instead, he stepped neatly over a few unconscious lower seats and settled himself to the other man's left. Plucking one of many bottles of sake off the ground, Yumichika took hold of the untouched dish next to Ikkaku's knee and poured himself some as well.

Whenever anyone but him chose the sake, it was terrible, but he knocked the first one back in a single gulp anyway, his mouth forming a disapproving moue at the sharp taste of it. Oh well.

"Is it weird that I'm gonna miss her callin' me pachinko-head? Fuckin' brat."

Yumichika arched both brows. "She's not dead, you know."

Ikkaku's mouth pulled to the side. Obviously, he knew that, but he was wallowing so deeply right now that Yumichika wouldn't be surprised to get an earful of melodrama at any moment.

"Yeah, but she ain't ever gonna be the same. None of us are."

Hm. Not as bad as he'd been expecting. Yet.

Yumichika poured himself another drink, intentionally setting the bottle on his opposite side, where Ikkaku could neither see nor easily reach it. "That doesn't mean she'll never insult you again."

"Dammit, Yumichika, that ain't what I mean, and you know it." Ikkaku made a noise that might have almost been a snarl had he put any real aggression into it. It was disappointing, in a sense.

"I know. But you're not very good at saying what you mean." Yumichika sipped more slowly from the second cup, the pleasant hum of Isane's room and conversation already ebbing from his mood. How annoying.

A month wasn't enough time to grieve. Yumichika knew that. But really, it should be enough time to start thinking about the other things that had to be done in life. Ikkaku could not keep refusing to acknowledge the real issues in front of them, even if they were difficult to cope with. Even if they meant that the both of them would have to take a good, long look at themselves and decide where to go from here. Who to become. Yumichika knew who he wanted to be—but Ikkaku was always slower about things like this.

"I mean I dunno what the fuck we're gonna do now," he said at last, setting his empty cup down too hard on the engawa's wooden floor. It wasn't like it hadn't been scuffed thousands of times already. "Nothing's the same anymore!"

Yumichika licked a drop of sake from the lip of his dish, staring out at the practice yard. The best seats in the Eleventh didn't face anything as mundane as a garden or the edge of a forest. This was where the officers watched fights. He could see Ikkaku in the periphery of his vision, narrow eyes locked on the motion. Waiting for a response, certainly.

"We're the same." Yumichika gave in and cut a glance towards Ikkaku, lifting both shoulders.

No doubt as individuals they would change a great deal from here on out. And perhaps some of the unimportant details would shift around between them, too. But the core of what they were, as a unit, was by far the most constant thing in either of their lives. It had been the same since before Kenpachi Zaraki, since before the Gotei 13 and the Eleventh Division, and it would be the same after. Long after.

Ikkaku raised a hand to the back of his neck, rubbing his nape like he had an itch. "Yeah. I know."

"Well that's the place to start, then. Whatever you're doing, I'm doing. And whatever I'm doing, you are. Is there anything you particularly want to be doing right now?"

Yumichika knew Ikkaku liked being here. Truthfully, he did too. More than he'd liked being out in the Rukongai. But if Ikkaku decided that the thing to do was leave the Seireitei, temporarily or permanently, he wouldn't even have to debate what he'd do. Both of them led, both of them followed, and it usually just worked out that whoever felt the greater need to go one way instead of another picked the direction.

Ikkaku shook his head. "I dunno. Fuck. I thought I was gonna die under his command, you know? And fight till then, and just… fuck." Ikkaku's fist hit the floor, breaking clean through one of the boards and sending the sake dish skittering sideways. Still none of the sleeping shinigami woke.

Expelling a breath, Yumichika shifted out of seiza and crossed his legs under him instead. They both sat just outside the half-circle of warm light cast by the lantern in the window. It threw Ikkaku's face into shadow, softening the foxlike angles of his features. There wasn't much that could really be said about that—what Ikkaku said was true. Yumichika had known it was true. And the goal was now impossible. The posts removed, so to speak.

"This is what's left of him," he said slowly, gesturing over at the unconscious officers and then out towards the barracks more generally. "Why not die serving all of this instead?"

It was hardly the same. The division writ large didn't have the same magnetism, the same strong sense of purpose. The same minimalistic appeal. They weren't especially unified by anything except a penchant for violence and an admiration for a dead man who'd lived a life soaked in it. Zaraki-taichō had been a lot of things, but he wasn't a leader, exactly. He just got followed. Yumichika could make the distinction relatively easily, but he didn't think that it was obvious to everyone.

Ikkaku seemed unsatisfied. "This your way of saying I should tell 'em about my bankai? Be the captain?" The light struck his eyes sideways, giving them a dangerous glimmer.

It didn't faze Yumichika. "Not if you don't want to. But for what it's worth, I doubt your secret is all that secret. It's unlikely anyone will try to make you a captain unless you decide you want to be one." Quite possibly not even then. Ikkaku wasn't really a leader either. At least not yet. Maybe if he decided he wanted to be one, he could, but right now the best he'd be able to do was get followed. With less enthusiasm than his predecessor.

The statement surprised him, from the way his mouth dropped halfway open. Like he wanted to say something but didn't know what. "I never told anyone but you an' Renji about that. You saying he spilled?"

"No, Ikkaku. I'm saying that the Seireitei has an entire division whose job is gathering intelligence and reporting it to people with authority. And you're not subtle."

Keeping a secret like that from the Eleventh was easy. He knew this from experience. Keeping it from the Second was likely impossible.

In the long silence that followed, Yumichika polished off his sake, allowing Ikkaku the opportunity to collect his thoughts.

"I still dunno about this. We've only ever decided this shit one way in the Eleventh. And that's the way it should stay."

The sake dish clinked gently on the ground. "So you want to find out exactly what happened, then?"

What they'd been told was that Zaraki-taichō had fallen in battle against an Arrancar, and that the Arrancar in question had in turn been killed by one of the other captains. Perhaps it would satisfy Ikkaku if the captain in question was willing to entertain a challenge. Or just accept two transfer officers. That would depend on him.

"Yeah. Yeah, I gotta know how it went. You know how we can do that?"

"As a matter of fact, I do. But you aren't going to enjoy it."

The Twelfth wasn't, in general, a place that anyone else visited unless they had to. Really had to. Experimental medical procedures like limb and organ replacement were about the only things that could drag anyone with a sense of self-preservation anywhere near Mayuri Kurotsuchi. Yumichika was sure they had to talk about the place in extremely neutral terms at the Academy, or it would never see one willing recruit.

Naturally, Ikkaku was just as displeased to be here as Yumichika had predicted. Only two things universally unnerved members of the Eleventh: the Twelfth in general, and Retsu Unohana in particular.

Fortunately, they need not venture too far into the place; Yumichika had asked Isane to put him in touch with Nemu Kurotsuchi for this, and she was waiting for them near the gate when they arrived.

She certainly had a demure appearance, though he had to wonder how useful the skirted version of the uniform really was. The aesthetic value of the choice was obvious, but became less so when considering the potential wounds it made her that much more vulnerable to. Even for shinigami, scars were forever.

Kurotsuchi-fukutaichō bowed, hands folded neatly in front of her. "Welcome," she said, voice soft, and dull to the point of nearly being toneless. "Isane-san indicated that you wished to access some of the battle footage in our archives?"

Ikkaku was still busy suspiciously scanning the grounds, so Yumichika nodded. "Yes, thank you. We're looking for anything recorded from Zaraki-taichō's time in Hueco Mundo."

She blinked, then dipped her chin. The gesture was delayed, like she'd almost forgotten to do it. "Please follow me. Mayuri-sama does not like it when his work is disturbed, so it is best if you do not linger or touch anything on the way."

"You don't gotta worry about that," Ikkaku grumbled, falling in behind Yumichika and Kurotsuchi.

It seemed that their so-called archive was an outbuilding, which minimized the number of other shinigami they ran into. The few that seemed to be about mostly kept their eyes down, hurrying to and fro with what must have been either chores to do or deliveries to make. None of them looked much like officers—but then all the seated members probably spent most of their time in the labs. Probably underground, since at surface level the only things Yumichika could see were barracks, a mess hall, and some exceedingly ordinary-looking offices. The training yard had overgrown, grass poking out from the brown dirt in a way it never would have had time to at the Eleventh. Or probably anywhere else.

Kurotsuchi slid open the door to the outbuilding, leaving it open for the two of them to follow. Stepping inside, they were swallowed by darkness, at least until she lit a small kidō light over their heads. Various boxes and a few crates were placed against the walls of the single room; of more interest was the staircase. It proved to be narrow, forcing them to continue in single-file on the way down.

It didn't look like any archive Yumichika had ever seen. There was no more than one shelf of books; the rest of them seemed to be stacked with data discs and chips, organized in some way he couldn't really make sense of at first glance. One whole wall was taken up by a massive screen, a large command console stretched out in front of it.

From the way Kurotsuchi moved immediately to one of the shelves, he supposed she must know where the information they were looking for was kept. He squinted as she moved several disc cases to the side, labeled with what looked to be names and rough time stamps. The one she picked out of the stack said 'ZARAKI v. SEXTA ESPADA, HUECO MUNDO INVASION.'

"Cameras were deployed with the authorized invasion force," Kurotsuchi explained, removing the disc from its case and crossing to the command console. "Ordinarily, we would not have had data from the time before those individuals entered Hueco Mundo. However, Aizen's fortress had nearly universal surveillance measures as well, and Mayuri-sama salvaged parts of the data before returning to Soul Society. We've spent much of the last month sorting through it, and prioritizing certain items for the Sōtaichō."

"Pretty empty down here right now," Ikkaku observed.

"Yes," she replied. "The work was mostly completed last week. Mayuri-sama was impatient to return to his personal projects, for which he requires assistance." With a few keystrokes and what looked like a retinal scan, Kurotsuchi activated the screen. It flickered in the dark, before resolving into a still image: Zaraki-taichō grinning widely at what had to be an Arrancar, his hair a bright shade of blue and a mask fragment attached to his face near the jaw.

"This the one?" Crossing his arms over his chest, Ikkaku narrowed his eyes at the screen.

Kurotsuchi's lips thinned. "Not precisely. This is the Sexta Espada, Grimmjow Jaegerjaquez. He had a match with Zaraki-taichō prior to his death. If you wish, I can advance the footage to the end." Glancing back over her shoulder, she moved her eyes between them.

Ikkaku shook his head immediately. "No. If this is his last fight… I wanna see the whole thing."

"As you wish." Tapping a button on the console, Kurotsuchi stepped back, moving to stand back near the stairs.

Yumichika fixed his eyes on the screen as it began to move. The Arrancar, carrying Yuzu Kurosaki on his back of all things, stopped a decent distance from the captain, setting her down.

"Grimmjow. We finally gonna finish this?" Zaraki-taichō's tone was one Yumichika knew well. It had the bite of anticipation in it. Whoever this Arrancar was, they'd met before, and the captain had been looking forward to the fight.

"You got that right. But first—Yuzu." The same tone inflected the Arrancar's words, softening only slightly and only on the girl's name.

The little Kurosaki got to work on Zaraki-taichō's wounds. They didn't look like anything serious, but the fact that he let her heal them anyway suggested he was interested in starting on even footing with Grimmjow.

Yumichika glanced once at Ikkaku, but his eyes were still riveted to the screen, face set in a resting scowl. Then the battle began, and he found that he couldn't look at anything else, either.

It was intense from the start: Zaraki-taichō discarded his eyepatch within minutes. Right about the same time they seemed to come to mutual agreement that the warm-up phase of the fight was over. When the Arrancar released his zanpakutō, his form changed. Something like a cat, which made perfect sense considering the name. Pantera. He fought with no kidō, just his body, release techniques, and whatever the Arrancar version of shunpō was called.

Zaraki-taichō looked to be enjoying himself more than Yumichika had ever seen him do. A jagged grin stretched wide over his face—unwavering even as both opponents sustained injury after injury. While the captain remained largely immobile, Grimmjow was so fast that even Yumichika had difficulty tracking his motions; whenever he was still enough, it became clear that he wore a mirror-version of that bloodthirsty smile.

If this was how the captain had died, then he'd died in exactly the way he'd always wanted to.

Ikkaku shifted beside him, moving one hand to Hōzukimaru's hilt and resting it there. Even Yumichika's blood was up a bit by proxy, just watching this happen. He felt all his muscles lock into place when they turned to face each other after on particularly brutal pass. Even before they spoke, it was obvious that this would be the last exchange. As of yet, Yumichika had seen nothing that would cause Kurotsuchi to qualify her answer to Ikkaku's question—so what was it?

He must have stopped breathing when the final blows landed, because only after the last of the dust had settled onscreen did Yumichika notice his lungs beginning to burn and inhale again. Zaraki-taichō fell first, rolling over onto his back. A moment later, Grimmjow collapsed too, the victor by seconds, since neither of them seemed capable of again reaching their feet.

"We should do this again somet—"

The captain's words were quickly cut off by a shout.

"Bakudō #39: Enkōsen!" Kurosaki threw her hands out, the camera changing angles in enough time for the audience to see exactly what she was trying to do.

But the scythe arcing towards their captain would not be stopped. It slammed into the kidō shield and shattered it, piercing Zaraki-taichō's arm beneath it too before burying itself in his chest. The weapon withdrew, wielded by the hand of a tall, thin Arrancar with a narrow face that reminded Yumichika of some sort of insect. A grasshopper, maybe.

"That bastard!" Ikkaku had not taken the interference well; a rattling noise gave away the angry tremble in his hand. Hōzukimaru was knocking against his sheath.

"… is dead, right Kurotsuchi-fukutaichō?" Yumichika finished, looking to her for confirmation. The recording had stopped as soon as the captain vanished.

"Yes," she replied simply, moving back in front of them without any fear of the quaking Ikkaku. She removed the disc from its place in the console and replaced it in the casing she'd drawn it from. "We do not have footage of that, however."

"Who killed him, then?" Ikkaku snarled.

For a moment, the corners of Kurotsuchi's eyes pinched, like she'd nearly flinched but stopped herself before it happened. "Reports indicate that Unohana-taichō did. However, the only person who could confirm or deny this with certainty is Unohana-taichō herself."

"How come?" Yumichika asked, lifting a brow. "You said cameras were sent in with the authorized invasion team, and Unohana-taichō was among them. Shouldn't Aizen's surveillance have recorded it in any case?"

Kurotsuchi folded her hands demurely in front of her. "The camera sent with Unohana-taichō was destroyed, and no footage of the battle is available. I am sorry, but if you wish to ascertain what occurred between Unohana-taichō and the Quinta Espada, you will have to make that inquiry of Unohana-taichō. Or perhaps the Sōtaichō, as I believe he would have the details of her report."

Ikkaku scoffed, frustration weighing down his steps until he was stomping back up the stairs.

Yumichika sighed. "Don't mind him, Kurotsuchi-fukutaichō. He's been through a lot recently."

She tilted her head at him, then nodded slowly. "The death of a comrade can be difficult to emotionally process. I have taken no offense at his behavior. However, I ask that you guide him from the premises in such a way as to minimize property damage. Mayuri-sama would not approve of interruptions."

No shit.

"Of course. Thank you for your assistance, Kurotsuchi-fukutaichō." Yumichika knew how to keep professional doors open, even if Ikkaku couldn't be bothered right now. Or ever.

But right now, he had some damage control to do. Grimacing, he hurried up the stairs.

Ikkaku was the only one who could force him to make such ugly expressions.

And we're back to shortfics! This series of them will likely be quite a bit longer than the last, though, because there's a lot of stuff I need to make happen between the Winter War and the TYBW. I'm open to suggestions on any movies/filler arcs/other stuff between those two things you'd like to see turn into a fic, too, though I'd like to keep the focus on the Gotei 13 and what's up there. (There will also be stuff with the surviving Arrancar.)

I can't promise my pace lately will keep up too long, considering I'm on a break from classes, but I'll do my best to keep stuff coming in a timely manner.

Oh, and here there be shipping. Which will start to happen elsewhere, too. So you're forewarned if that's not your thing. Plot's always my main focus, but realistically, many people are in relationships and those are important to them. So there it is.