What Yuiko Tsugikuni had been called in her previous life no longer mattered.
In this life, her name was Yuiko Tsugikuni.
She was the eldest daughter of the Tsugikuni clan.
Above her were two older brothers—twins.
The elder, Michikatsu Tsugikuni.
The younger, Yoriichi Tsugikuni.
That's right. She had transmigrated.
Into the world of Demon Slayer: Kimetsu no Yaiba.
The age before the Pillars.
Before the Demon King ever drew breath.
Even before her death, Demon Slayer: Kimetsu no Yaiba had been everywhere—
so popular that even people who didn't watch anime had seen clips of it online.
Yuiko Tsugikuni had fallen for it too,
drawn in by the breathtaking animation and glowing sword arcs.
But…
she must have arrived too early.
She had only finished watching the first season before her death,
and most of what she knew came from short clips on her feed—
especially that one, the most famous of them all:
"What do you take life for?!"
If she wasn't mistaken, then her "second brother" —
the so-called ceiling of Demon Slayer, Yoriichi Tsugikuni himself —
was a man who wouldn't be born in the main story for several hundred years.
Which raised a very practical problem:
how on earth was she supposed to survive long enough to see the plot begin?
"Sigh…"
A wooden flute hung around her neck as she propped her cheeks on both hands,
gazing down from the veranda.
Below, her eldest brother, Michikatsu Tsugikuni,
was training with his master, wooden blades clashing in rhythmic arcs.
Yuiko's bare feet swayed absent-mindedly in the air,
the picture of a girl lost somewhere between two worlds.
"Yuiko, don't sit like that. A young lady must keep her posture straight."
The gentle voice of her mother, Junai, drifted from behind.
Yuiko turned around just in time to see her second brother, Yoriichi,
leaning against Junai's side like a spoiled child.
"Mother," Yuiko said quickly, shifting from her careless sprawl
into the proper seiza position she still hadn't quite gotten used to.
In her previous life, her mother had been a lazy gambler—
the kind who lost every coin the family owned and couldn't be bothered to care.
A woman Yuiko had found impossible to respect… or to love.
Which was why the softness in Junai's smile now left her completely defenseless.
Once she was sitting properly, Yuiko exhaled in relief
and waved at her brother clinging to their mother's sleeve.
"Yo, Yoriichi."
So this was the legendary prodigy of Demon Slayer fame—
as a child, nothing more than a little mama's boy.
Before she could laugh, a sharp thunk landed on her head.
Junai had flicked her forehead with perfect accuracy.
"How many times have I told you? You must address him properly—'Elder Brother'!"
"Yes, yes—Elder Brother," Yuiko groaned, rubbing the sore spot.
She didn't notice the flicker of pain that crossed Junai's eyes.
Her mother was gentle by nature—
but when she was angry, it was always frighteningly sudden.
Faced with his sister's careless antics, Yoriichi only smiled softly.
A gentle mother.
A kind brother.
A mischievous little sister.
They were Yoriichi's most precious treasures.
He helped Junai to the veranda, where Yuiko sat beside her, chattering quietly.
Then he walked to where his elder twin, Michikatsu, was training.
Their instructor—a retainer of their father—noticed Yoriichi standing silently at the edge of the yard.
Amused, the man offered him a wooden sword with a chuckle.
"Why don't you give it a try?"
Drenched in sweat and aching from the blows of his father's retainer,
Michikatsu Tsugikuni was finally allowed a brief rest.
"Mother. Yuiko."
He stepped toward the veranda, lifting his head to look at them.
"Brother~!" Yuiko called with a cheerful wave,
then turned her gaze back to the courtyard, eyes intent on the scene below.
Michikatsu frowned—barely, almost imperceptibly.
He followed her line of sight.
There stood his younger twin, Yoriichi.
Michikatsu didn't believe the frail boy could last even a few strikes against their father's seasoned retainer.
After all, despite all his own effort and talent,
he himself had never once managed to win.
But what none of them expected—
was that after learning the proper grip from a single sentence,
Yoriichi Tsugikuni moved.
Like a gust of wind.
In the blink of an eye,
the retainer—towering, seasoned, and sure of himself—was struck down.
For a moment, Michikatsu's eyes held nothing but disbelief.
Junai's lips parted in silent shock.
Only Yuiko watched with a knowing look, as if to say, of course.
No wonder he was called the ceiling of all swordsmen—
even at seven, his strength was something no ordinary human could imagine.
Yet while the others stared in awe,
Yoriichi only gazed blankly at the bamboo sword in his hand.
He disliked it—
the feeling of striking another's flesh.
Yoriichi dropped the bamboo sword and quietly returned to the veranda.
Michikatsu, still stunned, hurried over and sat beside him.
"How did you do that?" he asked, voice tight with disbelief.
Yoriichi lowered his gaze, his tone calm and almost absent-minded.
"When he was about to attack, his lungs moved—
rapidly.
If you watch closely, you can see the direction of the bones,
the contraction of the muscles,
and the flow of blood.
That's all."
"…"
Michikatsu could only stare at him, dumbfounded.
He had no idea what his little brother was even talking about.
Junai's eyes shimmered faintly.
This child, she thought—
Yoriichi could see the gods.
Beside her, Yuiko's eyes lit up like twin lanterns.
She knew this one!
For swordsmen—for any warrior—
this was the ultimate state of mastery.
Transparent World.
The ability to perceive every weakness and movement of an opponent,
to hide one's own presence completely.
Like seeing through the body itself.
"Let's stop talking about swordsmanship," Yoriichi said softly.
"I'd rather play sugoroku or fly kites with Brother and Yuiko."
Once, his dream had been to become the second-strongest samurai in the land.
But the discomfort of striking another's flesh had made him abandon that dream completely.
Michikatsu lowered his head.
His hands, resting on his thighs, tightened into fists.
Yuiko patted his shoulder in jest, her tone playfully old-fashioned.
"Don't stay wound up all the time, Brother. You should relax your body once in a while."
He brushed her hand away without a word,
and without even bidding their mother goodbye,
walked off in silence.
Yuiko let out a long sigh.
She knew it.
Before she'd been reborn, she hadn't followed the full story of Demon Slayer,
but she'd seen enough clips online to recognize Yoriichi's older brother—
the one who would one day become Muzan Kibutsuji's strongest demon.
Upper Moon One: Kokushibo.
As for the internet calling him "Six-Eyed Lemon Boy,"
she hadn't understood it back then—
but now, she was starting to get it.
All right then.
She was going to fix this guy's twisted heart if it was the last thing she did.
Yuiko was sure she could handle it.
Come on—she'd been a full-grown college student in her previous life.
How hard could it be to teach a little kid a few life lessons?
…
But before she could even begin to "fix" Michikatsu's personality,
a few nights later—
"Yuiko."
Yoriichi's voice woke her from sleep.
She slid open the door, rubbing her drowsy eyes
"What is it, Yoriichi?" Yuiko yawned, blinking sleepily.
"Don't tell me you're scared to go to the bathroom alone?"
Her teasing tone fell flat.
Yoriichi's expression was as calm as always—
but this time, there was sorrow in it.
"Mother has passed away."
"…Eh?"
The drowsiness vanished instantly.
Junai…
Yuiko's lips parted,
but no sound came out.
"You should ask Mother's attendants for the details," Yoriichi said quietly.
"For now, I have to say goodbye to Brother."
"Goodbye…?"
"I'm leaving for the temple."
Only then did Yuiko notice the small travel bag resting beside him.
Their father was a superstitious man.
Among the twins, Yoriichi—born with strange, mottled markings—
had never been favored.
If not for Junai's protection,
he would have been killed by his own father the day he was born.
The difference in how Yoriichi and Michikatsu were treated
was so vast it could hardly be called anything less than that between master and servant.
From food and clothing to education,
the two lived in entirely different worlds.
As the forsaken child,
Yoriichi was destined to leave for the temple at the age of ten—
to live out his life as a monk.
But… they were only seven now.
And Yuiko was only six.
Yuiko suddenly remembered the scene of Yoriichi defeating their father's retainer.
Their father wanted an heir strong enough to uphold the family name.
Yoriichi's overwhelming talent had already begun to shift the balance between the twins.
Had he chosen to leave early—
to become a monk—
only so that he wouldn't outshine Michikatsu?
While Yuiko stood frozen in thought, Yoriichi's quiet voice reached her.
"Yuiko, I'm leaving now."
"Take care of yourself, okay?"
With that, he picked up his travel bag and turned to go.
"Wait."
The word slipped from Yuiko's mouth before she even realized it.
Yoriichi paused and turned to look at her.
It was her turn to speak—
but her mind went completely blank.
She had no idea what to say.
Ask him to stay?
He was only seven…
and yet he was already strong enough to defeat grown men—
so much stronger than she could ever be.
Her silence was met with a gentle smile.