“I am Kanae Kocho’s younger sister, Shinobu Kocho.” Shinobu studied the boy standing in front of her.
He couldn’t be older than a teenager—far too young for this kind of work.
‘Why would my sister recommend him as a doctor?’ she wondered.
“Yes,” Tetsuya replied.
Facing Shinobu’s scrutiny, he remained perfectly composed, acting neither haughty nor overly humble.
“My name is Tetsuya Kamihara.” Even Akaza didn’t know his real name, so he had absolutely nothing to worry about.
The only people who knew “Tetsuya Kamihara” was a demon were Kanae, Tamayo, and Yushiro.
Just as he expected, Shinobu’s expression didn’t shift in the slightest upon hearing it.
“Age?” “Sixteen.” “Where are you from?” “The Ikebukuro district in Tokyo.” …
After running through the basic background check, Shinobu’s expression turned entirely serious.
“When exactly did you meet my sister?” “About four months ago, in a small coastal town.
Miss Kanae saved me from a demon…” Tetsuya had rehearsed this story down to the letter.
With Kanae personally feeding him the details, there was no way Shinobu could poke any logical holes in his narrative.
Shinobu gave a slow nod.
Four months ago, Kanae had indeed been dispatched on a mission to a coastal town.
Without needing any further prompting, Shinobu automatically filled in the blanks for him.
‘My sister saved this boy, realized he had professional medical knowledge, and recommended him to the Butterfly Mansion.’ “This is an examination paper,” Shinobu said, pulling out a printed sheet.
“I need to properly evaluate your professional medical knowledge.” Tetsuya accepted the test.
Thanks to Kanae’s recommendation letter, he had already earned her baseline trust.
The Demon Slayer Corps didn’t exactly have strict recruitment restrictions to begin with; their ranks were mostly filled with tragic survivors whose lives had been ruined by demons.
This exam was just standard procedure.
The two moved into the inner room.
Shinobu set up a small desk for him to work at, then sat off to the side with a book.
She kept one eye on him to proctor the exam while simultaneously diving back into her toxicology studies.
The room fell completely silent, save for the rapid, continuous scratching of Tetsuya’s pen against the paper.
He was blazing through the questions so quickly that Shinobu couldn’t help but glance over repeatedly.
A flicker of genuine surprise passed through her eyes.
She hadn’t even looked at his answers yet, but his sheer writing speed was definitely startling.
A little over half an hour later, Tetsuya set his pen down and handed the completed exam over.
Shinobu took the paper, her eyes rapidly scanning the lines of text.
Within moments, she had already formed a solid assessment.
“Well, Miss Shinobu?
How did I do?” Tetsuya asked.
“You scored above a ninety,” she replied.
However, Shinobu’s delicate brows furrowed together.
Some of his answers were just a little too perfect.
There was no mistaking it—her sister had absolutely leaked the exam questions to this boy.
‘Haa…’ Shinobu let out a long, weary sigh.
Yep.
This was exactly the kind of reckless, impulsive thing her older sister would do.
“Welcome to the Butterfly Mansion,” Shinobu announced.
Even though she knew for a fact Kanae had helped him cheat, she had no intention of turning him away.
Even if his actual medical knowledge wasn’t quite up to par, he was still young.
They could train him.
Besides, her sister had gone out of her way to leak the answers just to ensure he got in!
How could Shinobu possibly bear to refuse her sister’s dying wish?
“Thank you!” Tetsuya smiled warmly.
It was going exactly as he had planned.
Having Kanae working from the inside was downright terrifying—walking into the Butterfly Mansion felt as easy as walking through his own front door!
It perfectly proved the old saying: the easiest way to destroy a mighty fortress is from the inside out.
Even answering the questions too perfectly had been Kanae’s idea.
She knew her younger sister better than anyone.
Shinobu would be exasperated by Kanae’s sheer stubbornness, but in the end, she would always cave and clean up her mess.
“By the way, why haven’t I seen Miss Kanae around?” Tetsuya asked innocently.
Since he was playing the role of an oblivious human, he shouldn’t know anything about her current situation.
Hearing his question, Shinobu’s delicate frame went rigid.
She recovered a second later, though a deep, lingering bitterness clouded her eyes.
‘So he really doesn’t know.’ “My sister…
sacrificed her life during a mission two months ago.” “No…
how could this happen?!” Tetsuya gasped, perfectly acting the part of a devastated friend.
He forced a look of absolute heartbreak, even squeezing out a few crocodile tears for good measure.
Inwardly, however, he frowned.
He had just received Shinobu’s official approval to join, so why hadn’t the damned system’s notification popped up yet?
“For the next few days, you’ll shadow me and learn the mansion’s workflow,” Shinobu instructed.
Tetsuya’s lips twitched.
It seemed merely gaining Shinobu’s acceptance didn’t technically count as ‘officially joining’ the Demon Slayer Corps.
He was still just a demon using a Blood Demon Art to masquerade as a human.
Without the Immunity Physique, the moment the sun touched him, he would instantly burst into ash.
His brain shifted into overdrive.
“Actually, since I just moved all my belongings here, I have a lot of things to sort out outside during the day.
I might only be able to come in and shadow you at night.” “That is not a problem,” Shinobu replied.
Since he was highly recommended by Kanae, she didn’t find the request particularly suspicious and simply nodded in understanding.
Leaving the Butterfly Mansion, Tetsuya couldn’t help but curse the system in his head.
What exactly did he need to do to trigger the completion?
His moving excuse would only placate Shinobu for a little while.
Over time, his strictly nocturnal schedule would definitely raise red flags.
But for now, he just had to bite the bullet and push forward.
Days flew by.
Tetsuya spent his days hiding entirely out of sight, only venturing into the Butterfly Mansion after nightfall to begin his medical training.
“This is Aoi Kanzaki,” Shinobu introduced.
“She is a swordsman of the Water Breathing school, but she’s currently helping out as a nurse here.” “And this is Kanao Tsuyuri, my adoptive sister.
She is currently training hard to become a fully qualified swordsman, though she occasionally helps out in the wards.” …
Shinobu guided Tetsuya around the mansion, introducing the staff to him one by one.
Tetsuya casually scanned the room.
There were way more people working here than he had expected—around twenty or so established doctors and nurses.
In the original timeline, only a handful of them had ever made an appearance.
The rest were likely just background characters completely irrelevant to the main plot.
He already knew every single one of their names and quirks thanks to Kanae’s detailed briefings, but he still had to put on a humble, polite facade and listen attentively to Shinobu’s introductions.
“Hello!” “It’s so nice to meet you!” Both sides exchanged warm greetings.
After all, they were going to be working closely together from now on.
The only exception was Kanao Tsuyuri.
She simply stood perfectly still, offering a polite, empty smile like a delicate porcelain doll.
“That’s just how she is.
Please don’t mind her,” Shinobu explained with a fond smile.
Tetsuya gave an understanding nod.
Afterward, Tetsuya shadowed Shinobu into the wards, observing the patients and actively soaking up advanced medical practices.
Desperate to get his hands on that Immunity Physique as quickly as possible, Tetsuya secretly activated his Sharingan during the lessons.
With his photographic memory, he memorized every ounce of medical theory instantly, while his visual tracking allowed him to perfectly replicate Shinobu’s surgical and practical techniques.
His progress was nothing short of miraculous.
Within days, his skills were already catching up to the veteran doctors who had been practicing for years.
Even Shinobu was secretly astounded by his flawless performance.
His learning speed was downright terrifying.
‘No wonder my sister was willing to leak the exam answers just to get him in,’ Shinobu thought.
‘A prodigy like this is a once-in-a-lifetime find.’