"Post-mortem Nen"—as the name suggests, a Nen that only activates after death.
For example, the Kakin Empire's Second Prince Camilla Hui Guo Rou's ability, "Millionfold Reincarnation Cat," passively triggers upon Camilla's death to seize the killer's life and revive her. Or Neferpitou, after being killed by an enraged Gon Freecss, invoked post-mortem Nen, using "Doctor Blythe" to keep manipulating their headless corpse to attack Gon until it was completely destroyed. There's also the post-mortem Nen Chrollo collected—"Paired Destroyers"… and Hisoka's self-resurrection…
In short, post-mortem Nen is versatile: it can save oneself or exact revenge on others, and it tends to carry traits of "intensity," "obsession," and "wish-power."
But the one before us clearly isn't that. First, Minamino Hirochi is just an ordinary itinerant peddler, not a Nen user. Second, this is the Demon Slayer world housed within Roy's sea of recognition; as a native of this world, without Roy's guidance there's no way he could access Nen or act beyond his world's framework. Which means—there's only one explanation:
He's a lingering soul.
A soul: the immaterial, supernatural component of a person—an externalization of spirit and will, independent of the body.
That much was obvious when Tanjiro tried to help Minamino up and his hands passed straight through.
'Another kind of demon…?'
With monsters like Satō Takeichirō and Doihara—flesh-and-blood demons—so fresh in mind, Tanjiro stared at his hands going through Minamino and didn't even know what to call him.
Minamino himself was cheerful—perhaps, as a traveling merchant who'd wandered for years, he'd long since learned to speak to people and ghosts alike. Smiling, he told Tanjiro, "No need to fret, benefactor. To fall to a demon's hand and still be fortunate enough to glimpse the world one last time—this is blessing enough."
As he said, his remnant soul was unstable; each gust of winter wind thinned him visibly.
"Niisan, he's fading!" Tanjiro panicked.
He'd seen too many bones tonight and couldn't bear to watch another "life" wither.
But… was Minamino Hirochi still truly a living thing?
Roy pondered.
Silently, he sheathed his right hand in Nen and, imitating Tanjiro, tried to help Minamino up.
To the surprise of both Tanjiro and Minamino—Roy actually lifted the man's remnant soul, as if for that instant Minamino had regained "substance." It was unbelievable.
Startled, Minamino bowed again. "Thank you for your aid, benefactor. I feel much steadier now."
It was true.
"Niisan, he really did get a lot more solid…"
Tanjiro circled him, fascinated.
Roy didn't speak, but he could feel his own aura being consumed. In other words, Minamino's presence—and its brief stability—depended on Roy's continuous supply of Nen. Once the "resupply" stopped, Minamino's only end would be dispersal.
Ultimately, Nen rests on the six categories and fuses "vitality," "aura," and "spirit" into one. It is the reflection of all the "intangibles" that measure a human being—"emotion," "will," "temperament"…
In a word: it's "the power of what I reckon," supported by "stats" and "mechanism."
'So it's because of Nen that I can touch a soul?'
Roy felt something click—just then his HUD flickered twice and popped up a new line beneath "Nen Shape Transformation":
"Nen Property Transformation"!
He froze. Was this a reward for slaying the demon?
He remembered Great-Grandfather's book only mentioned "property transformation" in passing without concrete training methods. Maybe the old man thought Roy had only just opened his pores and hadn't systematically developed Nen yet—too soon to dig into property transformation—so he gave him only half.
Only…
Maha wouldn't have guessed Roy's Nen was the "Door of Cognition." From the very first dream he'd been constantly exploring and developing it.
So the question:
'When I return, should I ask him for the second half on "Nen Property Transformation"?'
Roy thought for a few seconds and decided—not yet. One bite at a time, one step at a time. For now, raising his "stats" and learning how to maintain "Total Concentration" in Sun Breathing mattered most.
Countless thoughts flitted by in an instant.
He came back to himself and told Minamino plainly, "Don't thank me yet. You will vanish in the end."
Minamino wasn't a Nen user; his body was gone; a post-mortem Nen revival was impossible. This "remnant soul" persisted only on Roy's Nen—and Roy couldn't sustain a stranger forever.
"I said seeing this world again was blessing enough," Minamino replied with a gentle smile.
"But once you disperse, that's truly the end…" Tanjiro fretted at his calm.
"Shut it." Roy shot him a glare, then looked back at Minamino. In truth, he admired the man's serenity at the brink. "You truly are an optimist. If you have any unfinished wishes, speak. If it's within our power, my brother and I will try to see it done."
"Niisan's right. I'll help too." Tanjiro blurted, then clapped a hand over his mouth and peeked at Roy from the corner of his eye.
Thankfully, Roy let it slide this time.
"There is one thing." Minamino gazed toward Mount Sagiri's slope, veiled in bridal-white snow. His knees buckled, and he knelt again at Roy's feet.
"I beg you, benefactors—please take me to see my little daughter once. That alone would content me."
"How old is she?"
"A year and a half."
"Can she say 'Papa'?"
"She can." Minamino's face glowed. "Not only that, she's learned to walk. Every time I come home, she sprawls on my belly, calling 'Papa, Papa,' drooling all over my face…"
"I was thinking, when she's a bit older, I'll braid her some pretty pigtails, take her to Tokyo to try the famous rice dumplings…"
"And stop by the Mutō family's children's wear shop to buy her some lovely dresses, watch her preen in front of the mirror…"
Minamino rambled on and on.
Roy didn't interrupt—just listened in silence, while Tanjiro stood by with tears streaming down.
Doihara's dying line—"this damned world"—suddenly resurfaced in Tanjiro's mind.
Unconsciously, the boy touched the hatchet at his waist. The blade caught the moonlight and flashed with a chill gleam…