What is cognition?
Cognition is the process by which humans, through perception, memory, thinking, and other mental activities, acquire, process, and apply information—it covers our capacity to understand and adapt to the objective world.
In short: everything you see and hear, everything you think and feel and live through, woven into a complete system.
Roy had watched Demon Slayer in his past life, stepped into its characters' shoes, and lived their joy, sorrow, resentment, and rage. That's why his world of cognition bears a deep footprint from it.
Pensive, he rolled up from the snow. A small cloth pouch slipped from his chest, a few candies tumbling out. That jogged his memory—today was the full-month celebration for his youngest sister, Kamado Hanako.
Because of his intervention, Tanjiro was no longer the family's eldest son, and the youngest child from the original story, Rokuta, hadn't been born yet. In other words… the plot was still two or three years away.
Not only was their father, Kamado Tanjuro, alive, even their grandmother—the white-haired old woman who often held Tanjiro while stargazing—was still with them.
That was curious. In reality Roy is the Zoldyck family's eldest son; in his cognitive world, he's likewise the Kamado family's eldest. Maybe that "fit" is exactly what let him push open the Demon Slayer door.
Sensing the connection, Roy bent to pick up the candies one by one, then began checking his own condition.
First, "Nen"…
He tried to commune with the aura within his body, channeling it toward his right index finger. A milky halo wavered there—faint, not easy to see against the snow, but undeniably real.
He couldn't help exhaling in relief.
After all, like the Hunter × Hunter world, this wasn't a gentle place; it was a land prowled by man-eating demons. Whether he had Nen mattered enormously—especially before he'd learned a Breathing Style and obtained a Nichirin Blade.
Next came "assassination arts."
"Silent Gait," "Rhythm Echo," and "The Snake Awakens"…
Silent Gait: the Zoldyck clan's assassin footwork, built on four elements—light, floating, sly, and phantasmal.
When he moved he was light as a swallow; when it turned uncanny, it was dreamlike. Roy set one foot down and drifted forward like a ghost—now east, now west—his figure threading through the old forest. You couldn't pin down his silhouette, and yet he didn't leave so much as a single footprint in the snow. That… flat-out stunned a certain little brother who'd ventured out at dusk, worried for his nii-san's safety.
Because it was Hanako's full-month celebration, their father Tanjuro had sent the eldest, Rōichirō, down the mountain early to buy supplies. Near nightfall, with no sign of him, Kie—their mother, and Rōichirō and Tanjiro's mom—dispatched Tanjiro, whose nose is like a hound's, to look for him.
Tanjiro did find him—and was left dumbstruck.
Wrapped in a thick black-and-green checkered coat with a pale-purple scarf at his neck, the naïve, straightforward boy stood blankly in the blowing snow. His lovely amber eyes reflected Roy's flitting, phantasmal figure; his mouth gaped open, the word "big brother" stuck in his throat.
One thought kept looping in his head—Was that really his big brother Rōichirō?
Yes—one hundred percent, it was.
Eyes can lie, but scents don't. Tanjiro's nostrils flared; once he confirmed the scent was Roy's, he quietly ducked behind a big tree, only his head poking out.
Having just run through Silent Gait to reacquaint himself with his body, Roy didn't pause. He moved straight on to "Rhythm Echo."
This assassination technique, in short, is the advanced application of Silent Gait—adding a layer of deception to bewilder the opponent.
Roy drew a deep breath and stepped out. From the spot where he stood, a ring of afterimages bloomed in an instant.
Killua had used this very move on the airship during the Hunter Exam when he and Gon were snatching the ball from Netero—only Netero's level was far too high for a rookie who couldn't even use Nen to touch.
But that was the strongest human, Netero—not the greenhorn Tanjiro…
Tanjiro's eyes went wide again at the sight. He started counting on his fingers, trying to work out how many big brothers there were—when a sharp "tssk" sliced the air.
A razor hand-chop slid past his ear and buried itself in the very tree he was hiding behind!
The Snake Awakens: temper the hand into a blade that cuts anything. At a refined level it pierces rock; at mastery it can pierce refined iron; at perfection it can sever weapons forged with Nen.
Roy had seen Silva cleave a fine katana with a single hand-chop.
He himself was still at the rudiments—able to sink a few centimeters into solid wood at best. Beyond that would take time and grind… unlike a sense for killing intent, which you can pick up a little of just by osmosis.
Indeed, the moment Tanjiro found Roy, Roy had already noticed Tanjiro.
He pulled his hand from the hole he'd punched through the tree, smiled at this brother who was both familiar and brand-new, and said, "Mother sent you to find me, didn't she? Sorry—I fell asleep for a bit and made you worry."
Tanjiro looked at Roy's face, then glanced at his right hand. He was clearly breathing, but his wits hadn't fully returned—he nodded dumbly, then shook his head like a rattle.
Roy chuckled and ruffled his hair. "Got a lot of questions you want to ask?"
Tanjiro shook his head… then nodded.
Roy's smile widened.
Gently he said, "Today is Hanako's full-month celebration. Let's celebrate her first—then we can talk, okay?"
Tanjiro nodded hard.
Roy patted his shoulder and didn't add more. He flicked up the hoe with his toe and caught it, slung the basket onto his back, and strolled toward home.
Tanjiro followed step for step, head buzzing, until the moon climbed the treetops and stretched their shadows long…
Only then did they stop.
From here, through the wind and snow, they could already make out a few familiar wooden huts.
From the east-facing one, a thin thread of smoke curled up; you could faintly hear children's laughter and adults' coughing. Add a few barks and cat-scrabbles, and you'd have a perfect, cozy mountain-village scene.
"It's just a shame… there really are demons in this world. In two or three years, this place will be gone…"
Roy murmured.
"Demons?" Tanjiro looked up at him, puzzled.
Clearly his big brother didn't want to explain more. In the next glance, Roy was already striding toward the house.
Tanjiro blinked, tightened the scarf around his neck, and hurried after him.
Hearing movement, the adults and children inside poked out their heads. Seeing Roy and Tanjiro, the kids broke into cheers—
Nezuko, Takeo, Shigeru, and the tiniest one, Hanako, still in swaddling and cradled by a white-haired old woman…
"Onii-chan, you're back!"
"Mm… I'm back."
Roy answered with a smile, his eyes meeting those of the man sitting at the corner of the veranda.
The man coughed a few times, fiddling with a set of Japanese ceremonial robes. Seeing Roy look his way, he smiled back just as gently.