Post-mortem Nen. As the name implies—Nen that can only activate after death.
Take the Kakin Empire's Second Prince, Camilla Hui Guo Rou. Her ability "Cat's Name" triggered passively after death, killing her murderer to bring her back to life. Or the cat-girl Neferpitou, who after being killed by the blackened Adult Gon, activated post-mortem Nen to prevent Gon from reaching the Ant King—using "Doctor Blythe" to control her headless corpse until it was completely destroyed.
There was also the post-mortem Nen Chrollo collected—"Sun and Moon." Hisoka's active resurrection.
In short, post-mortem Nen had flexible applications. It could save oneself or inflict revenge on others, possessing characteristics of intensity, obsession, and willpower.
But the one before them was clearly different. First, Nanno Hiroshi was just an ordinary traveling merchant, not a Nen user. Second, this was the Demon Slayer world within Roy's cognitive ocean. As a local "native," without Roy's guidance, he couldn't possibly access Nen or perform actions beyond his cognitive world.
So there was only one possibility—he was a remnant soul.
Soul: the supernatural, non-material component of humans. The materialized expression of human spiritual will transcending the body.
This became clear when Tanjiro tried to help Nanno Hiroshi up, only for his hands to pass straight through.
'A demon... he's another kind of demon.'
Having faced demons like Sato Takeichiro and Dohihara with their powerful physical forms, Tanjiro stared at his hands passing through Nanno Hiroshi, momentarily unsure what to call him.
Nanno Hiroshi himself was quite optimistic. Or rather, as a traveling merchant who'd wandered north and south for years, he'd long developed the skill of speaking appropriately to both people and demons. He smiled at Tanjiro. "Benefactor need not mind. Having perished at a demon's hands yet fortunately seeing the world one last time, I'm already extremely lucky."
As he spoke, the remnant soul visibly thinned, blown by cold wind.
"He's fading!" Tanjiro panicked.
Tonight he'd seen too many bones. He couldn't bear to watch another "life" wither away.
But was Nanno Hiroshi truly still a living being?
Roy pondered thoughtfully. Silently, Roy coated his right hand with a layer of Nen and, like Tanjiro, attempted to help Nanno Hiroshi up.
To both Tanjiro and Nanno Hiroshi's surprise, he actually managed to lift the barely remaining soul. As if Nanno Hiroshi had gained "substance" in that instant—incomprehensible.
Nanno Hiroshi looked at Roy in amazement and bowed again. "Thank you for your rescue, benefactor. I feel much more comfortable now."
Indeed. Tanjiro circled around Nanno Hiroshi curiously. "He's really become much more solid."
Roy said nothing. He could directly feel his Nen aura being consumed. In other words, Nanno Hiroshi's existence and brief maintenance of his current state were built on the foundation of Roy continuously supplying Nen.
Once he withdrew this supply, only one outcome awaited Nanno Hiroshi—dissipation.
Ultimately, Nen abilities relied on the six major categories, products condensing essence, aura, and spirit into one. A reflection of all "imaginary quantities" that measured "humans" as creatures—emotions, willpower, personality, and so on.
Summarized in one sentence—it was the "power of my thoughts" based on values and mechanics.
'Because of Nen, I can touch souls?'
Roy had an epiphany. The status panel flickered twice and appeared. Below "Nen Ability Shape Transformation," a new column opened—"Nen Ability Nature Transformation."
Roy was stunned. Was this a reward for slaying the demon?
He only remembered the book his great-great-grandfather gave him mentioned "nature transformation" once but didn't introduce specific training methods. Perhaps the old man felt that having just opened his aura nodes without systematically developing Nen abilities, delving too deeply into nature transformation was premature, so he'd only given him half the content.
However, what Maha wouldn't expect was that Roy's Nen ability was "Gate of Recognition." From the first dream, he'd been continuously exploring and developing his Nen ability in depth.
So the question arose—'After returning, should I ask him for the second half about Nen Ability Nature Transformation?'
Roy pondered for several seconds and ultimately decided—no rush. Food must be eaten bite by bite, and roads walked step by step. Currently, priority should be improving his own values and learning how to achieve Total Concentration with Sun Breathing.
Many thoughts flashed through his mind in an instant. Roy came back to his senses and spoke frankly to Nanno Hiroshi. "No need to rush thanking me. You'll ultimately still disappear."
Nanno Hiroshi wasn't a Nen user, and his body had been destroyed. There was absolutely no possibility of using post-mortem Nen to revive.
This barely remaining remnant soul was maintained by Roy's Nen, and Roy couldn't possibly support an unrelated person forever.
"I said being able to see this world once more is already extremely fortunate." Nanno Hiroshi smiled.
"But disappearing means truly being gone." Seeing his calm demeanor, Tanjiro couldn't help but worry for him.
Roy glared at him. "Shut up!"
He looked calmly at Nanno Hiroshi, honestly admiring this traveling merchant's accepting mindset even in this situation. "You truly are an optimistic person. If you have unfinished business, speak freely. We two brothers will try our best to fulfill it if it is within our capability."
"I can help too." Tanjiro blurted out, then quickly covered his mouth, only daring to steal glances at Roy.
Fortunately, Roy didn't hold it against him this time.
"There is one matter." Nanno Hiroshi looked toward Mount Sagiri along the mountain range draped in white snow. His knees weakened as he knelt again at Roy's feet. "I humbly beg benefactor to let me see my daughter once. That would content me."
"How old is your daughter?"
"One and a half years old."
"Can she call you Daddy?"
"Yes." Nanno Hiroshi's face lit up. "Not only can she call Daddy, but she has also learned to walk too. You don't know—every time I come home, she crawls onto my belly, calling 'daddy, daddy,' rubbing her drool all over my face."
His voice grew distant, wistful. "I was thinking when she grows a bit older, I'd braid her hair nicely and take her to Tokyo to look around, taste the famous rice dumplings. And visit the children's clothing store to buy her some pretty dresses and watch her admire herself in the mirror."
Nanno Hiroshi rambled on, as if he had endless words.
Roy didn't interrupt, just silently listening. Beside him, Tanjiro was already in tears.
Dohihara's dying words—"this damned world"—suddenly surfaced in Tanjiro's mind. The young man unconsciously touched the axe at his waist. The blade reflected moonlight, flashing with a cold, sharp glint.