Muzan didn't dare experiment on his own cells—this was, after all, Konoha Hospital's laboratory, not some hidden den of forbidden science.
His body's cellular strength rivaled that of Hashirama Senju's, and he had no intention of becoming a test subject in Orochimaru's lab one day.
Instead, he focused on studying animal cells under the microscope.
The hospital had abundant biological samples, even a few from shinobi with kekkei genkai. After hours of research and careful note-taking, he finally became aware of a figure leaning over his shoulder.
Her presence was impossible to miss.
Especially her figure—ample enough to block the horizon.
"Ahem… Lady Tsunade, did you need something?"
Tsunade smirked, seemingly pleased with his reserved composure.
"You're studying basic biology," she said. "I left plenty of research notes in this hospital—feel free to read through them."
She found herself unexpectedly satisfied with his demeanor. Even though Muzan tried to appear expressionless, she caught the brief two-second pause of his eyes—proof, in her mind, that her Yin Seal hadn't weakened her charm.
"However," she added lazily, "I'm short on an errand boy. For the time being, you'll be the one fetching my sake."
She hated these stoic types who pretended not to look—because the fact that they tried not to only confirmed her allure. For a moment, she'd even worried her Yin Seal had worn off.
Had Muzan known what was running through her head, he'd have sneered.
After all, there was already one fool named Jiraiya who had tested that same courage—and paid for it.
...
In fact...
Running errands for Tsunade wasn't without benefit.
As the pioneer of Medical Ninjutsu, she gave Muzan detailed guidance on mastering the Mystical Palm Technique and offered insights that weren't taught in any official curriculum.
She didn't hold anything back—passing on her practical experience and secret theories alike.
Before long, Muzan had essentially become Tsunade's unofficial disciple, and his medical proficiency grew at an astonishing rate.
But beyond his medical pursuits, darker thoughts stirred.
"My cells possess an extraordinary infection rate," Muzan noted in his journal.
"Even a shinobi's body might not be able to withstand contamination."
To advance his studies, he applied for a private laboratory next door and even obtained samples of jōnin-level tissue.
Analyzing these samples gave him a clearer understanding of his own physiology.
"The stronger the host, the greater the enhancement after transforming into a man-eating demon.
A shinobi with a unique bloodline might experience an even stronger effect.
But… the risk remains extreme."
He remembered the rabbit he had once used in testing.
Its body, unable to contain his power, had burst apart instantly.
Muzan closed his notebook, his gaze turning cold.
"It's not time to expand my influence yet. When the Third Great Shinobi War begins… the chaos will bring my true opportunity."
He sealed the completed data scroll with precise hand signs and stored it away.
Then, glancing at the calendar, he realized:
"Today's the Academy graduation ceremony. The Third Hokage wanted me there—looks like he's finally found me a teacher."
Because Tsunade had refused to take him, the Hokage hadn't yet assigned Muzan an instructor.
Meanwhile, Kakashi's path remained unchanged—he had officially become the student of Minato Namikaze, the "Yellow Flash," drawing him directly into the Hokage's faction.
---
Graduation Day.
The Academy was unusually lively that morning.
Dozens of new graduates stood proudly wearing their Konoha forehead protectors.
From today onward, they were official genin of the village.
"Hey, isn't that Muzan?"
"Whoa, it really is! Been ages since I last saw him!"
"He's still wearing that same black cloak—hasn't changed one bit."
"Man, I hope I get on his team. Having a medical ninja as a teammate would be a lifesaver!"
In the world of shinobi, injury was unavoidable.
Most minor wounds could be bandaged in the field, but serious ones required treatment at the hospital—an expensive affair.
Having a medic on the team meant saving both ryo and lives, so many quietly hoped to be paired with him.
—
Among the crowd, Kakashi Hatake finally spotted his older brother and couldn't hide a spark of excitement—though his tone, as usual, stayed sharp.
"Big brother, I thought you'd fallen asleep in the lab and missed team formation."
Since becoming a genin, Kakashi had rarely been home.
He spent most of his days on missions, building merit and skill under Minato's guidance.
Muzan gave a faint smile.
"Impossible. I've been waiting for this day.
You're only here to go through the motions, aren't you?"
Kakashi's dull, half-lidded eyes didn't fool him; every move his brother made was measured, alert—a shinobi constantly on guard, even within Konoha's safety.
That habit alone spoke volumes about how much he'd matured.
At least, Muzan thought, his own interference in the timeline had delayed their father's mission failure.
This version of Kakashi hadn't yet become the cold, practical prodigy of legend.
"You're right," Kakashi admitted. "I'm just hoping my teammates aren't completely useless. It's hard being the strong."
Before Muzan could respond, a familiar voice cut in.
"Tch! With that attitude, you'll end up without teammates!"
Obito Uchiha stomped over, followed by Rin Nohara.
The Uchiha boy, still unable to awaken his Sharingan, often joked bitterly about whether he was even a true Uchiha.
Hearing Kakashi's arrogance set him off immediately.
"Don't say that, Obito," Rin chided gently.
Obito bit back his retort, his jealousy plain as he glanced at her.
One day, he swore, once his Sharingan awakened, he'd wipe that smug look off Kakashi's face.
"Maybe you two will end up on the same team," Muzan said dryly.
Their simultaneous response was instant:
"No way!"
"Like I'd team up with him! He'd drag the whole squad down—I won't risk a mission because of dead weight!" Kakashi snapped.
"You're one to talk!" Obito shot back. "You'll probably get your teammates killed!"
Their bickering filled the room until a calm but firm voice silenced them.
A tall figure stepped into the classroom.
Muzan immediately recognized the overwhelming presence.
"A jōnin," he murmured. "Looks like Konoha's taking this year's graduates seriously."
The first instructor called out a few names, escorting his team away.
Moments later, a gentle male voice spoke from the doorway:
"Hatake Kakashi. Uchiha Obito. Nohara Rin."
Every head turned toward the man with bright golden hair and a serene smile.
"Sorry to keep you waiting," said Minato Namikaze, the man known as the Yellow Flash of Konoha.
Gasps filled the room.
"It's really him—Lord Minato himself! The Yellow Flash is leading a genin squad?!"
"I'm with Rin again—but why does Kakashi have to be on our team?!"
Obito's words tumbled out in disbelief as he stared at his idol.
Minato was already a living legend—so feared by enemy nations that entire missions were abandoned at the mere sight of him.
"Hah. You think I want to be teamed with you?" Kakashi muttered.
"What was that?!"
As the two immediately fell back into their argument, Muzan felt a chill run down his spine.
Something about this pairing—this energy—sent a faint, instinctive sense of foreboding through him.
And just as that thought crossed his mind, a raspy, unfamiliar voice echoed from behind him…