"And this is the fastest way to become stronger!"
Lock's voice was calm, but his determination was unwavering.
Shinobu Kocho looked at him, worry flickering in her violet eyes. He was still young — too young — and she feared he might lose his way if he pushed himself too hard.
Just as she was about to speak, Lock interrupted her with a faint smile.
"I know what you're thinking," he said softly. "Don't worry. I'd never become a demon. I'd rather learn from you, Shinobu."
Shinobu blinked in surprise. For a moment, she didn't know how to respond. She was indeed a few years older, but hearing Lock call her that so naturally made something inside her soften.
A warm smile bloomed across her face — gentle, radiant, and fleeting as spring sunlight.
Lock couldn't help but stare.
If it weren't for her, he would have died on Spider Mountain.
As someone who never forgot a debt of kindness, Lock silently made a vow: he would not allow Shinobu Kocho to meet the same tragic end she did in the original story he remembered.
The brighter her smile, the more determined he became.
I have to become stronger — strong enough to protect myself, and to stand beside her.
But the truth was harsh. Lock was still far from the level of a Pillar. Those who reached that height were prodigies — blessed with both talent and willpower.
Even among them, Tokito Muichiro stood out. He had become a Hashira at only fourteen, mastering swordsmanship in mere months. That record had never been broken.
Lock, too, had discovered that his body was now fourteen years old — the same age as Tokito. The perfect time to begin.
And with his extraordinary perception — his ability to instantly understand and adapt to anything he observed — Lock believed he could one day surpass even the Mist Hashira.
"Despise the enemy in spirit, but respect them in combat," he reminded himself.
Fifteen days remained until his next mission — his first true test. He would use every second to train, refine his Heat Breathing, and sharpen his blade.
Following Shinobu wasn't about admiration. It was about opportunity.
The Butterfly House wasn't just a medical ward — it was a training ground for demon slayers.
Here, even the wounded learned Total Concentration: Constant, and forged new resolve through pain.
And with so many demon slayers passing through for treatment, it was the perfect place for Lock to observe and learn from every breathing style and combat habit he could.
The Butterfly House was his first step toward strength — and the most important one.
The two of them walked through the quiet streets, sunlight filtering through the trees. Shinobu's dimples appeared as she smiled, pointing ahead.
"The Butterfly House is just up ahead. I think you'll like it here, Lock."
He looked where she gestured. A large wooden building stood surrounded by high walls and neat gardens. Though he had seen it in the anime, standing before it in reality filled him with awe.
It was no dream — he truly was in the world of Demon Slayer.
Shinobu slid the door open, and the familiar scent of medicine drifted out. Inside, patients were doing light rehabilitation exercises.
Aoi Kanzaki approached, twin ponytails swaying as she bowed.
"Kochou-sama, welcome back!"
Shinobu smiled softly. "Thank you, Aoi."
Aoi straightened and glanced curiously at Lock — a young man in white robes, carrying himself with quiet confidence.
"Shinobu-sama, who is this?" she asked.
"This is Lock. He'll be staying here for a while," Shinobu replied. Her voice was as light and calm as ever, yet somehow soothing. Turning to him, she added, "Lock, you can stay here for now. I'm often busy, but if you need anything, ask Aoi — she'll help you."
Lock nodded. "Thank you. Also, if possible… may I use the training grounds?"
"Of course," Shinobu said immediately, smiling. "I thought you might ask."
But before she could say more, a raven swooped down, cawing urgently.
"Caw! Caw! Urgent summons! Shinobu Kocho, report to Headquarters immediately!"
Shinobu's eyes widened slightly. Another mission — something serious.
She turned to Lock apologetically. "I'm sorry, Lock. I'll have Aoi show you around the training grounds. I have to leave right away."
"It's fine," Lock said with a small grin. "Just be careful. If you can't win, run. I'll be here when you get back."
Shinobu blinked, caught off guard — then smiled again, brighter than before. "I will. Thank you, Lock."
She took the raven onto her hand and, with a flutter of her haori, disappeared into the distance.
Aoi watched her go, eyes filled with admiration. "Another dangerous mission… I wish I could help her."
"You already do," Lock said gently. "You keep the Butterfly House running. That lets Shinobu fight without worry — that's a big help."
Aoi blushed slightly before crossing her arms and huffing. "Hmph. Don't think a few nice words will make me go easy on you in training!"
Lock chuckled. "Wouldn't dream of it."
After a brief exchange, Aoi led him toward the training field. Along the way, Lock took in every detail — the orderly courtyards, the shelves of medicines, and the vast open grounds where injured slayers rebuilt their strength.
It was hard to believe such a peaceful place coexisted with so much blood and tragedy.
And yet, to Lock, it was perfect. A sanctuary — and a forge.
Just then, three little girls came running down the hall, calling out cheerfully—