Perhaps it was because he finally saw a hope of catching up to Yoriichi—or perhaps because of the brave swing of her blade back in the mountains—that Michikatsu's attitude toward his little sister, Yuko, had softened considerably.
Michikatsu tilted his head slightly. "Did I ever say I wouldn't teach you?"
Yuko's face lit up. "I knew I could count on you, big bro!"
"Heh."
After her training in Sun Breathing, Yuko began her practice of Moon Breathing under Michikatsu's guidance.
Moon Breathing was born from Sun Breathing—a derivative—but still far stronger than the five major Breathing Styles, standing alongside Sun Breathing as one of the original styles.
Even though many believed Moon Breathing didn't deserve that title.
Its difficulty was still far beyond what ordinary people could hope to master.
Fortunately, Yuko was not among the ordinary.
With the creator of Moon Breathing personally teaching her, Yuko progressed at incredible speed, quickly grasping the fundamentals of the form.
However…
After watching Yuko finish a full set of Moon Breathing forms, Michikatsu shook his head. "You're not suited for Moon Breathing."
"Phew—"
Yuko slowed her breathing and sheathed her blade. She didn't argue; she knew herself better than anyone. She could use Moon Breathing, but her ceiling would never reach Michikatsu's heights.
Just like how, despite both using Water Breathing, Tanjiro was simply not as gifted as Giyu.
The best style is always the one that suits you.
"Instead of butting heads with Moon Breathing, you might have better luck tinkering with that 'Star Breathing' of yours."
"I have nothing left to teach you."
Yuko went speechless.
Star Breathing…?
She hadn't even made a folder for that yet!
"Brother, Yuko."
At that moment, Yoriichi approached with two Nichirin blades wrapped in white cloth.
"Your Nichirin swords are finished."
Unlike future generations—where one had to master a Breathing Style and pass a test before becoming a Demon Slayer—this era was different.
Before Yoriichi arrived, Demon Slayers merely used pure swordsmanship with Nichirin blades to hack off demon heads, resulting in horrific casualties.
The Corps was always desperately short on manpower.
Even now, things had improved but not by much.
As long as someone learned a Breathing Style, they could immediately be deployed. There were no elite-selection trials like in later generations.
"They're finally ready?" Michikatsu eagerly picked up his sword.
He drew it, and the blade visibly turned a pale purple.
"How unique," Yoriichi said with admiration. "No other Nichirin blade in the Corps has this color."
Yuko picked up her own sword with great curiosity.
She hadn't requested any specific customization; the guard, handle, and sheath were all plain.
Shing!
The sword slid free. Yuko eagerly watched for its transformation.
The sound of the blade drew both brothers' eyes.
A pure white color slowly climbed from the base to the tip.
It was the color of snow.
In Yoriichi's eyes, it was as if faint snowflakes drifted past. His lips curved in gentle praise. "Such a beautiful color—just like pure white snow."
"Indeed."
Even Michikatsu, usually sour and jealous, agreed for once.
"This level of purity in a white Nichirin blade is unheard of…" Yoriichi paused, tilting his head, puzzled as he looked at Yuko.
Yuko lifted her blade horizontally, braced her left hand over her right wrist, and shouted:
"Bankai! White Haze Punishment!"
Michikatsu, Yoriichi: "..."
Whooosh…
A soft breeze passed.
Yuko coughed lightly and forced a smile. "Just kidding."
The moment she saw the blade's color, she understood everything.
Of course.
Yoriichi's blade was black. Hers was white. Perfect opposites. No wonder she had no talent for Sun Breathing.
Snapped out of his momentary confusion, Yoriichi spoke again. "A pure white blade… I believe it may be more compatible with Water Breathing or Wind Breathing."
Yuko nodded. She thought so too.
As the founder of the Breathing Styles, Yoriichi could teach all five major branches—but he was often away on missions and couldn't guide her consistently.
So Yuko had no choice but to seek out the current Pillars for instruction.
Michikatsu, blessed with extraordinary talent, created Moon Breathing and quickly became a pillar within the Corps—second only to Yoriichi.
Yuko alone was still searching for a style that truly belonged to her.
Sun Breathing was a universal masterpiece, but Yuko simply couldn't use it; forcing it would destroy her body.
Moon Breathing worked, but its compatibility was ultimately lacking.
After officially joining the Demon Slayer Corps, Yuko naturally received missions as well.
Fortunately, demons of this era were far weaker than in the future—the Twelve Kizuki hadn't even formed yet.
As long as she didn't run into That Bastard Boss, she could handle herself.
During her free time, she visited swordsmen of all five main Breathing Styles: Water, Flame, Thunder, Wind, and Stone.
She even chose the order based on her personal preferences.
Originally, she had planned to learn Flame Breathing as her third stop since its effects most closely resembled Sun Breathing—and she loved that.
But given her blade's color, she felt Water Breathing was more suitable and postponed Flame Breathing a bit.
Stone Breathing, due to its nature, was usually paired with heavy two-handed weapons Yuko couldn't use. She only learned the breathing principles, not the sword forms.
Unexpectedly, she displayed excellent talent in all five major Breathing Styles. For the first time in her life, she felt she wasn't being overshadowed by her two genius brothers.
She—Tsugikuni Yuko—was a prodigy!
Of course, this was also because none of the Breathing Styles had fully developed forms yet.
Unlike the future where forms easily went up to the Ninth or Tenth Form, the current five styles only had a handful each.
While Yuko adapted well to all five, Water and Wind ultimately suited her best.
So she focused her training on those two.
By the time she mastered all five major Breathing Styles, two years had passed.
Yuko was now twenty. The Tsugikuni twins were twenty-one.
Yoriichi and Michikatsu had solidified their positions as the two strongest members of the Corps, constantly out on missions.
Yuko, now thoroughly familiar with Breathing theory, began attempting to merge Water and Wind into a new style of her own.
One day, Yuko was adjusting her breathing under the corridor walkway when a crisp female voice suddenly called out:
"Yuko! A demon has appeared in the western mountains. Please depart immediately for extermination."
Yuko looked up.
A kasugai crow circled overhead—named Suzume, the one assigned to her when she joined the Corps.
Receiving the mission, Yuko sheathed her Nichirin blade, gathered her things, and headed out.