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Ever since Senku was a child, soulmates had been a scientific mystery—one that refused to align with logic, equations, or the laws of physics. It was an anomaly, an unexplained phenomenon that taunted his rational mind. If he could be the one to solve it, to categorize it, to give it a solid, evidence-based foundation, then maybe it would finally make sense.
A connection between two people across space and time, written in ink on flesh. It wasn’t telepathy, nor was it a trick of the brain. The markings physically appeared, reacting instantaneously to touch and movement. That meant there had to be a mechanism behind it—something measurable. A chemical process, a neurological response, quantum entanglement, maybe even some kind of biological imprinting.
The universe followed rules, and Senku refused to believe this was an exception.
Byakuya had told him about it when he was young, sitting beside him on the couch with that ever-present grin, eyes alight with nostalgia. They had been watching a documentary about human evolution when the topic naturally arose.
“You can talk to your soulmate by drawing on your body,” Byakuya explained, rolling up his sleeve to reveal faded ink smudges near his wrist. “They will always be there for you. Most romantic thing in the world.”
Senku, perched beside him with a notebook in hand, had merely scoffed, arms crossing over his chest. Romantic? Secondary at best. The actual phenomenon, though—that was interesting.
“Eh, I just wanna know why it happens. What makes it work?”
Byakuya only chuckled, ruffling the boy’s wild white hair, a fond glint in his eye. “Senku, your soulmate isn’t a scientific experiment.”
The boy wrinkled his nose, swatting his father’s hand away. “Everything’s a scientific experiment if you test it enough times.”
Byakuya laughed, leaning back against the couch. “Well, can’t argue with that.”
Still—
After their conversation, Senku wasted no time putting the theory to the test. The moment he was alone, he grabbed a marker from his desk and scribbled onto his arm in neat, calculated strokes:
“What’s your name?”
Then, he waited.
Seconds passed. Then minutes. Then an hour.
Nothing.
He frowned, tilting his head.
Had he written too small? Did the ink not work? Maybe the phenomenon only functioned under certain conditions, like a specific time of day or a biological response triggered at puberty. That would explain why children rarely reported receiving messages from their soulmates.
Or, maybe… his soulmate just wasn’t writing back.
The thought left an odd, hollow feeling in his chest.
“They might be in a different time zone, so don’t worry too much about it,” Byakuya reassured him later that evening, laughing as he caught sight of the lingering ink on Senku’s arm. “I mean, Lillian was from another country. It took her some time to understand what I was writing down. Plus, the age gap didn’t help. I had to wait a while before she even wrote back!”
Senku huffed, tapping his fingers impatiently against the desk. “Well, she better hurry, because I wanna solve this mystery as soon as possible.”
Byakuya only smiled knowingly, ruffling his hair again.
“You’ll understand someday.”
Senku didn’t respond.
He wasn’t interested in soulmates for the sentimentality of it. He didn’t care about romance or fate or the idea of some preordained connection. But if this was a universal experience—one that nearly every human being shared—then he had to understand it.
He had to know why.
–
"Senku! My soulmate wrote to me!"
Taiju came bursting into the small lab like a human wrecking ball, grinning so wide it looked like his face might split in half. His chubby fingers trembled with excitement as he shoved his arm forward, practically bouncing in place. His cheeks were flushed, his whole body vibrating with energy, like he’d just discovered fire itself.
The writing on his skin was neat, elegant—too precise for someone like Taiju to have scribbled himself.
"Her handwriting is so amazing! Can you believe it?!"
Senku, who was in the middle of a rudimentary experiment with baking soda and vinegar, barely looked up. The bubbling reaction fizzled in its dish, but compared to Taiju’s explosion of enthusiasm, it was practically underwhelming.
He twirled a pen between his fingers, unimpressed. "Great, big oaf. Hope you two enjoy your sappy soulmate bonding. Now hurry up and bring me the supplies I asked for."
Taiju pouted, his excitement momentarily dimming. "But Senku, I don’t know how to read it!"
That got his attention.
Senku’s brow twitched as he finally turned to look at his best friend, who was still holding his arm out like it was some ancient artifact. The writing was fresh, the ink crisp against his skin. Taiju had never been great at reading—he always left that kind of stuff to Senku.
Sighing, Senku adjusted his goggles before squinting at the letters. His sharp gaze skimmed the text before he read aloud, "'Hello! My name is Yuzuriha. What’s yours?'"
Taiju gasped, his entire face lighting up, as if hearing the words made it even more real. "Her name is Yuzuriha! You hear that, Senku?!"
"Yeah, yeah. That’s great." Senku waved a hand dismissively, already turning back to his experiment. "Now get to work before I revoke your love letter privileges."
Taiju, completely unfazed by the lack of enthusiasm, clutched his arm like it was the most precious discovery in the world. "Senku, you gotta help me write back!"
Senku raised a brow. "You realize that requires knowing how to write, right?"
Taiju hesitated for a moment before grinning sheepishly. "Then teach me!"
Senku sighed, rubbing his temple. "Only if you bring me the stuff I need first." He gestured toward the scattered materials piled at the edge of the lab. "Do that, and I'll teach you how to send your little love note back."
Taiju beamed. "Deal!"
And with that, he sprinted out of the lab, leaving a trail of dust in his wake.
The room fell quiet.
Senku barely noticed at first, already moving to record his observations from his earlier experiment. But then, almost absentmindedly, his gaze drifted downward—to his own arm.
Blank.
Just as it had always been.
His fingers hovered over his skin for a moment before he let out a small scoff and shook his head. Tch. Whatever. He turned back to his notes, shoving the thought away.
But for some reason, he couldn’t quite shake the feeling that maybe, just maybe—he wanted to know what it was like.
To have someone on the other end of the ink.
—
That night, after Taiju had passed out on the floor, snoring like a dying engine, Senku sat at his desk, rolling a crude ink stick between his fingers. The moonlight slanted through the window, casting pale streaks of silver across the cluttered workspace. A faint chemical scent lingered in the air, but he barely noticed.
He hesitated.
His hands were steady—always steady—but for some reason, they felt heavier than usual.
Then, in sharp, confident strokes, he pressed the ink to his skin.
"Is anyone out there?"
He waited.
Minutes passed.
Nothing.
The ink dried, dark and unwavering against his skin. Senku stared at it, tapping his fingers against the desk.
A failed trial.
But he wasn't the type to give up on an experiment after one unsuccessful attempt.
Over the years, Senku would write messages—small tests to see if anyone would respond. A simple "Hello?" at first. Then, as time passed and his patience thinned, he tried different approaches. A joke. A riddle. A chemical formula, just to see if his supposed soulmate had any knowledge of science.
No answer.
At first, he rationalized it. Maybe they hadn’t seen it. Maybe they were too young to understand. Maybe their arm had been covered up when he wrote. Maybe they just hadn’t written back yet.
So he kept trying.
Days. Months. Years.
Nothing.
Silence was all he ever received.
Eventually, he stopped.
After all, some people didn’t have soulmates. Maybe he was one of them. Or maybe—his logical mind supplied cruelly—they had died before he ever got the chance to meet them.
It shouldn’t have mattered. He wasn’t some lovesick idiot, pining for a fantasy. He had real things to focus on.
Like science and space.
And one day, making it to the moon.
So, he wiped the ink from his arm and moved on.
Because that’s what scientists did.
—
It had been twelve years.
If his soulmate were out there, they would have written back by now.
Senku had long since accepted that he was alone in this regard. No messages. No lingering ink. No evidence of a connection that so many others took for granted.
His fate wasn’t romance or some idealized love story. It was science—cold, rational, predictable. The only thing he could rely on.
And then—
The world ended.
Humanity was petrified.
And for 3,700 years, Senku was trapped in stone.
Survival became his only priority. When he finally cracked free from his stone prison, alone in a world reclaimed by nature, there was no time for sentimentality—no time to waste on something as frivolous as soulmates.
He revived Taiju first, needing manpower to rebuild civilization from scratch. But Tsukasa—brilliant, strong, and utterly opposed to the rebirth of science—had become their enemy.
The war for the future had begun.
Now, Senku, Taiju, and Yuzuriha were on the run.
Then came the explosion.
In response, a fire—a smoke signal, proof that someone else was out there.
Who?
Senku mixed a crude ink paste and, for the first time in years, wrote on his arm.
"Did you set the fire?"
He waited.
Seconds passed. Then minutes. The ink remained untouched. No response.
Senku scoffed, shaking his head. Figures.
He should have known better than to expect an answer after all this time.
But the fire wasn’t something he could ignore. He tracked its origin.
And that’s when he found her.
A girl, collapsed under a tree—bruised, battered, barely conscious. Her golden-blonde hair was tangled and dirt-streaked, strands sticking to her face with sweat. Her breathing was shallow, her chest rising and falling unevenly.
And then there were her eyes.
Piercing blue, sharp and untamed, glowing like twin embers even in her weakened state. Fierce—like a lioness cornered but refusing to submit.
For the first time in his life, Senku felt something inexplicable.
A pull.
Not a logical reaction. Not something he could explain with science or rational thought. Just a raw, undeniable connection.
He freed her from the tree’s crushing weight with a makeshift mechanism.
The girl groaned, blinking up at him. Then, despite her injuries, she smirked.
“My name is Kohaku,” she murmured, voice hoarse but steady. She exhaled sharply, locking eyes with him as if she’d been waiting for this moment her entire life.
Then, with complete certainty, she said—
“And I think I’ve fallen for you.”
Senku’s brain short-circuited.
What the hell?
—-
Ever since she was a child, Kohaku had believed in soulmates.
Ruri used to tell her stories—the beauty of two souls split apart, destined to find each other.
"The other half of your soul will always be waiting for you," Ruri had said, smiling softly as she traced patterns on Kohaku’s arm. "And when you find them, you’ll feel whole."
The village held a sacred tradition—every year, those who had yet to meet their soulmate would paint their arms with intricate patterns, marking themselves so their destined one would recognize them. Swirls of ink, constellations of meaning, each design a prayer to the unseen bond between two people.
Kohaku had tried. Many times.
But no one in the village had ever matched her marks.
She watched others find their soulmates, saw the joy that lit their faces when their ink finally had a match. But hers remained unanswered. A silent, empty space where words or symbols should have been.
Slowly, she lost hope.
Perhaps her soulmate was an outsider, someone beyond the borders of their isolated world. Or perhaps… they had never existed at all.
She had more important things to worry about, anyway. Protecting Ruri was what mattered. Keeping the village safe was what mattered.
But then—after a battle, after she lay trapped beneath the crushing weight of a fallen tree, exhausted and barely clinging to consciousness—something appeared on her arm.
Strange symbols.
Foreign, blocky letters unlike anything she had ever seen before.
Her breath hitched. Her soulmate was alive.
Why now?
Why had they never written before?
Could she make it out alive and meet them?
And then, she met him.
The sorcerer. The wild-haired man who spoke like lightning, whose eyes burned with knowledge, who she had seen die—only for him to stand before her again.
Was it possible that he was—?
She wasn’t sure.
But something deep inside her stirred, something that felt like recognition before reason could catch up.
He helped her. And she brought him to the village.
Ruri was waiting.
Kohaku had barely made it through the entrance when her sister reached for her, her delicate fingers wrapping around Kohaku’s wrist with urgency.
"Kohaku!" Ruri’s voice was breathless, a mixture of worry and excitement. "You were gone for so long, I—" She stopped, looking down at her sister’s arm, eyes widening. "This is it. We need to make sure."
Kohaku swallowed, her heart hammering against her ribs.
She allowed Ruri to guide her inside, to sit before her like she had when they were children—like when they played pretend, except this time, it was real.
With the steady hands of a priestess, Ruri dipped her brush into a bowl of dark ink, swirling it until the color was deep and rich.
Then, with deliberate care, she painted.
A universe unfolded on Kohaku’s skin.
Constellations stretched from her wrist to her shoulder, delicate star clusters connected by thin, silver-lined trails. A great sun encircled her elbow, its rays swirling outward, chasing the crescent moons that danced around her forearm. Wisps of nebulas curled over her collarbone, and comets streaked like falling stars along the curve of her bicep.
The cosmos, written in ink. A masterpiece.
Kohaku held her breath, watching as Ruri pulled back to admire her work.
"Now," Ruri whispered, voice barely above the wind. "We wait."
That night, as Senku sat in the dim glow of his lab, something tickled against his skin.
At first, he ignored it, assuming it was a stray itch, an irritation from the primitive tools he had been working with.
But the sensation didn’t fade.
It spread.
A slow, creeping sensation, like ink being absorbed into his very being.
He frowned and glanced down—
And froze.
Dark ink bled across his forearm, twisting and expanding, forming intricate shapes that had never been there before.
Stars.
Galaxies.
The sun.
His breath caught in his throat.
The ink spiraled around his arm, wrapping him in the cosmos. Supernovas and swirling nebulas, precise constellations stretching across his skin in perfect detail. It was space.
His space.
Senku didn’t move. He didn’t breathe.
For the first time in his life, his rational mind failed him.
He had a soulmate.
His fingers twitched, hovering over the ink, half expecting it to vanish the moment he touched it. But it didn’t. It stayed, solid and real.
His pulse pounded in his ears as he watched the final strokes appear—comets and celestial bodies linking together, completing their intricate dance.
Then, finally, the ink stopped.
The universe had been drawn.
Senku swallowed, his scientist’s instincts kicking in, overriding the dazed wonder flooding his chest.
There was an error.
The Andromeda constellation was slightly misaligned. One of the stars in Orion’s Belt was positioned just a bit too far from the others.
His lips pressed into a thin line. If this was going to be his arm now, it had to be correct.
Reaching for his own ink, Senku carefully fixed the stars, adjusting their placements, ensuring that the celestial map was astronomically accurate.
As he worked, the initial shock of the moment faded, replaced by something deeper.
A realization.
Someone had drawn this.
For him.
His fingers lingered over the last correction, his heart pounding harder than he wanted to admit.
He exhaled sharply, rolling his eyes at himself. “Tch. Ridiculous.”
But even as he spoke, he didn’t wipe the ink away.
Even as the night stretched on, Senku sat there, staring at the universe that had been written into his skin.
And for the first time in his life, he wondered—
Had she been waiting for him, too?
—
The next morning arrived with a quiet clarity, the sun rising over the trees. Senku stepped out of the hut, stretching as the crisp morning air filled his lungs. His gaze immediately sought the horizon, as if the world itself might have some new discovery waiting for him.
And then, he saw her.
Kohaku stood at the edge of the clearing, golden hair cascading in wild strands, eyes alight with something sharp, searching. But it wasn’t just her presence that drew his attention.
It was the ink.
On her arm, intricate cosmic patterns swirled—constellations, stars, comets, galaxies—woven into her skin like the blueprint of the universe itself. Identical to the markings on his own arm.
Senku’s breath hitched. He glanced down at his forearm, confirming what he already knew, yet still couldn’t fully process.
Kohaku met his gaze, her expression shifting—something unreadable flickering through her usually bold eyes. Her breath caught as she looked between him and the ink on her own skin.
Then, slowly, a smile tugged at her lips—equal parts relief and disbelief. As if she’d just solved a puzzle she hadn’t even realized she was piecing together.
After 3,712 years, he had finally found her.
Senku exhaled, tension unraveling in a way he couldn’t quite name. His mind was racing, calculating the probability, the scientific impossibility of it all—but for once, logic didn’t matter.
Because this wasn’t a theory to be tested. This was real.
He took a slow step forward, hesitating. But Kohaku didn’t.
Before he could react, she was running.
And then she crashed into him, arms locking around his shoulders in a forceful, bone-crushing hug.
Senku stiffened, his hands hovering awkwardly at his sides. His brain supplied him with approximately ten different ways to respond, but none of them felt right in the face of this sheer, unfiltered emotion. So he did nothing.
He didn’t push her away. He didn’t pull her closer either. He just stood there, letting the moment happen.
Kohaku pulled back slightly, her hands gripping his shoulders as she searched his face. Her expression was unreadable, but her voice was steady.
“…Why didn’t you write to me before now?”
The question hung in the air between them—soft, almost vulnerable. Not an accusation, but a quiet wonder.
Senku blinked. And then, despite himself, a small chuckle slipped from his lips.
"I was stuck in stone for 3,700 years, lioness."
A beat.
Then Kohaku burst out laughing. Loud and free and genuine. The sound was infectious, stripping away the weight of centuries in a single breath.
“Oh, that’s a damn good excuse,” she admitted, wiping at the corner of her eye, a grin splitting her face.
Senku smirked, the tension finally bleeding from his shoulders. Something warm settled in his chest, unfamiliar but not unwelcome.
Kohaku grinned up at him, her expression fierce, determined.
And for the first time in years, something inside Senku clicked into place. It wasn’t the cold certainty of science. It wasn’t a problem to solve or an equation to balance. It was something deeper.
Something that, despite all odds, made perfect sense.
Byakuya would’ve been happy.
Because, even in this new world, someone was looking out for him. And, against all logic, all odds—
He was looking out for her, too.
And as he held her gaze, Senku finally understood: some mysteries weren’t meant to be solved. Some things just were.