Not related to Down the Road. Makes a reference to it, however, because it is not canonically stated who raised Ryuko before Soichiro/Isshin's "death".
Contrary to popular belief, Kiryuin Satsuki and Matoi Ryuko had met once in their relatively short lives before their "first" encounter in Honnouji Academy.
It was in secret, as all things were when backstabbing and plots of overthrowing an international clothing conglomerate were involved. Satsuki was scarcely more than one and a half years of age when her father had crept close to her relatively unadorned crib and pressed into her mushy hands a small bundle of cloth and squirming flesh. She had rejected it at first, startled at the writhing mass of limbs, but had eventually found comfort in shared warmth when their bodies drew close and they huddled together. Not even an hour had passed after they had slipped into slumber before Soichiro had gently extracted the newly-revived newborn from Satsuki's soft grip, preparing for departure and regretfully cutting the time the siblings spent together short.
That night, when he was to leave the Kiryuin manor with a sedated Ryuko in tow, he had cut her hair and placed it into a clear glass vial for later storage. And after he had passed her on to trusted associates for temporary rearing, he had kept it in his lab coat pockets for safe storage, secretly palming it in his hand during long days and nights of research while the Revocs CEO's decisions grew more questionable and her behavior erratic. It kept him sane, kept him grounded. It did him good to remind himself of his second daughter's life, proof that if she could endure Ragyo's cruelty, he could too.
When Satsuki became five years of age and it was becoming increasingly clear on the Kiryuin matriarch's end that he was rapidly outliving his worth, he had surreptitiously passed that same vial to her, claiming he had managed to get something to remember his child by before Ragyo had disposed of the corpse. He whispered that it was time for him to pass it on and that she should keep it secret, keep it safe. That same day, he had withdrawn a plain silver-handled black katana from the vault he kept his earlier life fiber-related experiments in, sheathless and without edge.
Its previous adornments had been all but stripped away from it, leaving it to its new recipient to adorn and assign it meaning herself. Its older sister was a set of twin white blades with a golden habiki and tsuba, and a menuki stylized in the form of an equally colored long, flowing dragon. It had been lost to the ages, very much like the five year old's younger sibling, and it was assumed by both father and child that Ragyo had clandestinely stolen it for herself and re-purposed it, envious of its beauty.
"Satsuki," he instructed, pressing the flat of the blade into her hands and kneeling down so that he could meet his child properly eye-to-eye, "when I leave today, I will never return. Take up this sword. Hone it into a weapon to be feared by all. Let it guide you when you are lost. And when you come of age, you will know what to do."
And then he was gone, leaving her alone, his life supposedly stolen away in a fatal car accident. But Satsuki knew better, catching sly, satisfied smiles on her mother's face in between somewhat exaggerated shows of sorrow. The butlers were instructed to go about their business as usual, Soroi included, who was a special favorite of Soichiro's and was the first non-family member to be introduced to Satsuki. No picture of her father was placed on the family mantle, nor were traditional grieving traditions observed. His death was never mentioned, nor was the death of her baby sister no matter how many times she asked the servants.
Often, she wondered what she would have been named. It kept her up at nights when it was too quiet and there was no schoolwork to be done. She would have liked to believe they would have shared everything together, singular meals and fears about their mother included. They would have fought. They would have taken turns kissing cuts and sloppily treating wounds left from falling down trees in ill-made attempts to scale them. They would have done everything normal siblings did under the sun, if not more.
So when she turned twelve, she took down the blade from its mount on her wall and fingered it lovingly. She named it "縛斬" and placed within it her hopes and dreams of avenging the two lives Ragyo had callously stolen from her, whispering renewals of promises long-made with every stroke of the sharpening stone. It was the first time she had allowed herself to indulge in such a "frivolous display of sentimental emotions" after her father had exposed all of Ragyo's lies and entrusted her to lead the charge against the life fibers, knowing his time was rapidly growing short. She liked to believe it was specifically forged in memory of her sister, who was never given a name. She thought the blade black because she favored white and believed her sister to have preferred the dark color.
She took the contents from the lovingly protected vial and weaved them into newly-obtained sageo's fibers, careful to wrap them under layers so they would neither slip out nor be seen. Years passed. She assembled her elites and built her bastion, a shining beacon of rebellion disguised as a city-state. Nobody, not even Inumuta with his propensity to stick his nose into territories where it certainly didn't belong, knew Bakuzan's true meaning and why she had placed a high importance unto it. They had assumed it merely a tool for her to cut down her mother's empire with one fell swoop, for her reasons were as cloaked as the blade was within its sheath- dark ambitions masked in the guise of purity.
Whenever she doubted herself, she stood at the helm of Honnouji Academy's tower and overlooked its construction, thinking her sister would have approved of her actions. Sometimes, she thought she could hear her voice coming from within the blade, carried to her ears by the fickle caresses of gentle breezes. And when she gripped the handle of the blade, she felt through the protection of thick white woven threads the fine hairs of her unknown sibling. She liked to believe in that way they were connected, that they were holding hands.
"Soroi," she says one day, emptying a delicate white teacup and replacing it onto its accompanying saucer. "Enough tea, please."
She reaches for her katana and steps out onto the platform overlooking the finished campus, where a growing gaggle of students was collecting. It was time to begin.
Epilogue
It came as a complete surprise when one Matoi Ryuko had shown up at her doorstep on her birthday, dressed in her usual sukajan, a white blouse, and a short dark skirt. Satsuki invited her in for tea, duly noting the fact that the younger was making a poor effort to hide something behind her back. She had grinned impishly when the elder had questioned her about it, eventually presenting it with a grand flourish and a cheesy "ta-da!" for effect. Inside a battered small black case were the fragments of twin Bakuzan, glinting dully in the sunlight. The saya was missing, the untied sageo was forever stained a murky green and the blades themselves were crudely polished. But despite the shortcomings, the fact that her baby sister found them herself specifically for her made the gift all the more welcome.
"Ryuko…"
Ryuko refused to look in her sister's eyes, a light blush decorating her cheeks and a noticeable stutter creeping into her speech.
"I... I dove down into the bay to get them… last week, I mean. Before you say anything, it's not as deep as you think, especially with how tall you built the academy, and it only took me a while to find them. …Several attempts, actually.
I know you said you put down your blade and all, but I thought it would be nice to have it as a keepsake. I mean, it doesn't have to be that you necessarily have to leave it behind after putting it down. I mean, you can… I'm not trying to tell you what to do or anything. It's just… you know… kind of like how I-"
"Thank you."
Now it was her turn to be surprised. "Eh?"
"Bakuzan," she repeated gently, turning to carefully hang the broken wakizashi in a glass cabinet and nestle the opened case containing the remaining fragments under it before facing her again. "Thank you for finding it for me."
She closed the mahogany-framed doors and gazed at the discolored cords wistfully, sighing almost inaudibly even as a fond smile briefly took hold.
"I had carried Bakuzan with purpose, wielding it for the lives mother took from me with the intention of avenging them. Ragyo's death within the last few months had taken away their purpose, as well as my own with one fell swoop. I lost myself for a while. That is, until I had resolved to Rei to remake this world for the better, as you've seen yourself. Bakuzan did mean something to me at one point, but now I have you, Ryuko. And that is all that matters."
The light deepened to a red as bright as her ubiquitous bang. "You… you dummy," the former delinquent managed to mumble as her older sister laid her calloused hands upon her shoulders.
"Perhaps one day I'll tell you Bakuzan's secret. But it's not important right now."
Ryuko mutely nodded. Her socked feet shuffled on the carpeting, rubbing against each other the slightest bit. Satsuki quirked an eyebrow.
"Ryuko? Is there something else you would like to tell me?"
The hybrid rubbed the back of her head, nervously biting a lip all the while.
"Oh, Mako said she had a day off from the clinic today. So we could go out together. Celebrate somewhere quiet, go out shopping... that kind of stuff. I mean… that is, if you want. I know you probably have plans arranged with those four dorks and I would be intruding and you probably would like to be left alone afterward and-"
"Yes," she found herself saying, a small smile beginning to decorate her face. "I would like that."
Notes: Unnamed twin white "Bakuzan"/swords- reflects Ragyo's selfish nature. Were the first life-fiber-hardened weapons and were forged as dual katanas before they were destroyed and remade into the sewing needle-shaped ones seen in series. Came in a set of two to explain why Ragyo had two blades that she claimed preceded black Bakuzan and the scissor swords. Also, I like the number.
Black Bakuzan cannot transform like the sword scissors or Ragyo's dual blades because it was not designed/re-engineered to do so. Well, at least not in non-life-fiber-hybrids/creatures' hands. Probably. Hah. Satsuki wields it at twelve because I assume she used it in her recruitment of Hoka, Ira, and Uzu before Junior High/High school.
Habiki- blade collar
Menuki- decorative handle/grip
Sageo- thick cord usually found wrapped around a scabbard/sheath (Saya)
Tsuka- handle
Tsuba- sword guard