In the end, Aoi was saved by Kanae.
Shuya had the sneaking suspicion that Kanae's real goal wasn't actually to rescue her. She just wanted to watch the frustrated little expression on Shinobu's face—the one that said she wanted to get mad but couldn't do anything about it.
True black-bellied energy really does belong to the big sister.
After that, Aoi and Kanae had another emotional reunion. The three girls left together to find Kanao Tsuyuri.
Shinobu was still a little wary of Shuya, so she didn't let him tag along. He could only shrug and stay behind in the room, happily enjoying the breakfast Aoi had made for him.
A bowl of white rice, a piece of grilled saury, a bowl of miso soup, and a small plate of pickled ginger—Shinobu's favorite.
Pretty standard Japanese breakfast. But Aoi's cooking as head caretaker of the Butterfly Mansion was excellent, so it tasted great.
Shuya finished quickly and started wandering the mansion. He soon found himself in what looked like a training hall. Inside the empty room, a small figure was swinging a wooden sword all by herself.
Her movements were still a little clumsy, but the forms were unmistakably the twelve sword styles of Sun Breathing—the strongest Breathing Style there was.
"Shuya-nii!"
The moment Nezuko spotted him, her eyes lit up. She dropped the wooden sword and ran straight into his arms.
Shuya ruffled her hair, amused. "Nezuko? What are you doing training all alone in here?"
The girl practicing Sun Breathing was none other than Nezuko Kamado, the heroine of this world.
And she was already using the real Sun Breathing.
When Tanjiro had demonstrated it the day before, Shuya had remembered that Nezuko actually had even greater sword talent than her brother in the original story. So he'd made sure she stayed and watched too. She'd listened carefully while Tanjiro explained the breathing rhythm and the matching sword forms.
That was why Shuya wasn't surprised she could use it. What did surprise him was that she was up this early and already training seriously.
"Shuya-nii, can you teach me?"
Nezuko looked up at him with big, pleading eyes, voice soft and spoiled.
"Of course I can."
Shuya smiled. It wasn't a big deal anyway. Besides, he'd come to this training hall looking for a quiet place to practice Sun Breathing himself—specifically to see if he could awaken Transparent World, one of the ultimate secrets Saeko had figured out.
"Your forms were a little off earlier," he said. "Run through all twelve styles again in front of me. I'll correct you."
Even though both he and Saeko thought the Breathing Style forms were too rigid and lacked flexibility in real fights, beginners needed to drill the basics exactly right first. Just like learning math—you had to memorize the formulas before you could use them creatively.
"Okay!"
Nezuko nodded eagerly, picked up the wooden sword, and performed the full set of twelve Sun Breathing forms in front of him.
Her movements were still a little stiff and childish, but overall pretty solid. No wonder the author said she had more natural talent for swordsmanship than Tanjiro.
After helping her fix the forms, Shuya manifested his own white oak practice sword and began practicing alongside her, trying to trigger Transparent World.
Over and over.
His movements grew smoother with every repetition. The time it took to complete the full set kept getting shorter.
"Amazing!"
Nezuko stopped her own practice to watch him, mouth slightly open in awe.
The truth was, part of why she was pushing herself so hard to learn Sun Breathing had to do with Shuya.
She already knew he was strong. But yesterday afternoon, when Shinobu had suddenly attacked him with Insect Breathing, even though he dodged it easily, that moment had hit her hard. It reminded her of the helpless feeling she'd felt when her mother and brothers were turned into demons.
Combined with the quiet expectations Shuya had shown toward her, it made her want to master something she'd never really cared about before.
Shuya had no idea what was going through Nezuko's head. He just kept moving faster and faster until—
In one single breath, he connected all twelve forms into one continuous motion.
Or was it actually a thirteenth form created by linking the first twelve together?
No. Shuya felt it in his bones—this was the true Sun Breathing.
Why would Yoriichi Tsugikuni, the greatest swordsman in history, only create twelve forms? The answer was simple: Muzan had exactly twelve vital points—five brains and seven hearts. Yoriichi had summed up the technique that struck all twelve at once and called it Sun Breathing.
Everyone else just didn't have his talent, so they had to break it down into twelve separate forms to learn it. Even then, most people still couldn't master it and ended up creating derivative styles instead.
The moment Shuya truly understood that—
Something clicked inside him.
Aura surged outward on its own, forming a perfect circular domain that instantly covered the entire Butterfly Mansion.
His En had evolved.
In that moment, he could see everything with perfect clarity—blood flowing through veins, every heartbeat, the exact way every muscle fired during movement. He could sense what everyone in the mansion was about to do next.
For example, on the other side of the mansion, Kanae and the others had just found Kanao Tsuyuri.
Even though Kanao hadn't moved yet, Shuya could clearly see her heart rate spiking, blood rushing to her brain, and the muscles in her legs tensing up.
Sure enough, a second later—just like he'd already seen coming—Kanao shouted "Sister!" in pure joy and threw herself into Kanae's arms.
Shuya smiled.
He'd done it.
He had awakened one of the ultimate secrets of Breathing Styles.
Transparent World.