After leaving the Demon Slayer Corps, Hiru began teaching Yoriichi how to capture the ashes left behind when a demon dies.
Despite his considerable talent, Yoriichi failed to succeed even once over the course of two years.
Hiru eventually realized that Yoriichi was rejecting his experiment in a gentler, more subtle way. So he stopped bringing it up, though he continued researching the medicine to restore humans.
Yet, whether by fate or some other force, Hiru only managed to create several types of medicine that were prohibitively expensive to mass-produce—yet exceptionally effective against demons. This setback struck him far harder than Yoriichi's initial refusal of his Primordial Oni experiment.
Hiru shut himself inside his box for a full half-month before Yoriichi finally coaxed him out.
"Speaking of which," Hiru murmured, perched on Yoriichi's shoulder beneath the star-filled sky, "have you ever considered passing down your own Breathing Style?"
"Why bring that up now?" Yoriichi scanned the surrounding traces. "Besides, haven't I already passed it on?"
"I'm not talking about those derivative Breathing Styles. I mean your own, original one." Hiru poked Yoriichi's cheek. "Wouldn't it be a shame if such brilliant and powerful sword techniques were never passed down?"
"Those who exhaust the Way return to the same place." Yoriichi walked at a steady pace. "Hiru, in the eyes and hearts of the world, I'm not as important as you think.
We're merely fragments of history—transient passers-by. Perhaps even now, somewhere in the world, a child with greater talent than mine is being born. Someone will inevitably reach my level, or even surpass me.
So, it's best to simply let things take their natural course."
"Is that so…" Hiru's gaze lingered on the faint smile at the corner of Yoriichi's mouth before shifting away. After a pause, he changed the subject. "Hmm. Speaking of which, we'll be heading to distant lands to slay demons soon. Should we write to Tetsushiro to say goodbye? After all, unless something unexpected happens, we'll probably never meet again."
"It seems you hold Mr. Rengoku in high regard, Hiru."
"Yes." Hiru swung his legs gently. "Though that owl swordsman is rather noisy and bad with words, I like people who are open and straightforward. More importantly, he truly acknowledges my brother.
Unlike the other swordsmen, he genuinely respects you—perhaps even admires you."
"Yes, Mr. Rengoku is a kind man."
"Brother, do you have someone you want to say goodbye to?"
Yoriichi paused slightly. "Indeed… there is someone I want to bid farewell to."
"You mean that overly enthusiastic charcoal burner, right?" Hiru sighed. "That guy is just plain stupid—utterly stupid!
He saw me eating in my demon form, yet after thanking me for saving him from the bear, he asked if I had some strange illness and suggested I rest at his house! How was I supposed to respond to that?!
And seriously—why did he get chased by a bear while cutting wood just because he found honey? Then he climbed a tree like a monkey and hid so well!
But the most baffling part—Brother, you saw him, didn't you? You definitely did it on purpose back then!"
"Hmm, I just didn't look up," Yoriichi said with a small laugh. "But Mr. Sumiyoshi really was frozen in shock when he found out you were a demon."
"He deserved it! And then later, when he was walking home at night after selling charcoal, he actually started feeling sorry for demons! What kind of place produces such do-gooders?" Hiru pressed his hand to his forehead. "He's lucky he didn't die—thank goodness he brought the anti-demon medicine with him!"
"Yeah, Hiru's medicine really works well."
"I'd rather it didn't…" Hiru muttered, slipping into gloom again. He slid down from Yoriichi's shoulder to sit on top of the box, curling into a ball. "Not getting what you want is torture."
"…I'm sorry, Hiru."
Hiru immediately understood what Yoriichi meant. Standing atop the box, he patted his brother on the back of the head. "It's fine. Just remember to watch the sunrise with me.
Honestly, whenever I recall my human life now, it feels so distant. Thinking about it, even as a demon, dying at that moment would have been a peaceful end.
And it would be incredibly romantic—on a high cliff, with the cool morning breeze and dew, finally turning to ash under the first rays of dawn… No, I can picture it now! I have to sketch this out!
Beautiful moments should be preserved!"
Then Yoriichi heard the sound of quick footsteps and the creak of the box opening.
"Honestly…" Yoriichi shook his head helplessly, a gentle smile tugging at his lips. "Looks like I'll be hunting demons alone today."
"Call me when you find one!" Hiru's muffled voice came from inside the box. "And I need more paint! I'm almost out of black!"
Yoriichi sighed softly. "Yeah, got it."
Yes, once the decision was made, there was no turning back.
But… he truly couldn't match Hiru's composure. It felt as though he were breaking their promise—condemning Hiru to death with his own hesitation.
[Lacking any resolve, hesitating at every turn, unable even to deliver a clear refusal.]
[Tsugikuni Yoriichi, you are too weak—so weak that your younger brother has to comfort and indulge you. How utterly, utterly pathetic.]
Yoriichi tightened his grip on the sword hilt, sorrow clouding his eyes.
[I'm sorry, Hiru, for making you bear with someone as selfish as me. But rest assured—no matter what lies ahead, whether heaven or hell, your brother will always walk beside you. I will always walk with you.]
Taking a deep breath, Yoriichi's eyes grew distant as he quickened his pace toward the direction where the demonic traces spread.
...
The next day, Yoriichi entered a town, bought paper and ink from a shop, and together with Hiru, began writing letters in their inn room.
Hiru wrote to Rengoku Tetsushiro and Ubuyashiki Asahi. His letter to Rengoku mostly contained everyday trifles, while the one to Ubuyashiki detailed how to be a proper leader, along with various new medicinal formulas and ratios he had developed.
Yoriichi, on the other hand, wrote a single letter addressed to the entire Demon Slayer Corps. It contained detailed records of his insights into the Breathing Styles and descriptions of the Blood Demon Arts of the demons they had encountered recently—all written in a formal, official tone.
After folding the letters neatly, Yoriichi rested at the inn until evening. Then he checked out, visited a Wisteria House, and asked them to have the letters delivered through the Kakushi.
When the Wisteria House staff asked for a return address, both Yoriichi and Hiru said they did not require a reply.
Without stopping to rest, they continued on their journey to Kumogakure, where Sumiyoshi's family lived.
Sumiyoshi was overjoyed to see Yoriichi again, but when he heard the reason for the visit, his expression froze.
"Eh? You came to say goodbye because you're traveling far away and won't be coming back?"
Yoriichi nodded. "Yes. You've helped me many times, so I thought I should come and tell you personally."
"Why so suddenly…" Sumiyoshi's face grew anxious. "Did something happen?"
"No, nothing." Yoriichi shook his head slowly. "It's just a simple farewell. I'll be traveling to other distant places to slay demons. And… I've long wanted to thank you, Sumiyoshi-kun."
"Huh? Thank me? For what?" Sumiyoshi leaned on his axe, puzzled.
"I wanted to thank you for accepting Hiru, even after knowing he was a demon."
"Haha! The medicine Hiru-san gave me saved my life! If anything, I should be the one thanking you more!
But I won't be sympathizing with any other demons besides Hiru-san again—last time was way too close!" Sumiyoshi laughed, his eyes a soft shade of red that radiated warmth and kindness. "Since you're leaving, why not come to my house? Let me throw you a farewell feast! You can't set off on such an important journey without a proper send-off!"
"But…"
"Come now." Sumiyoshi grabbed Yoriichi's hand and led him toward his home. "It's been a while since we last met. When you visited before, my daughter Sumire was still a baby—now she's grown and can talk!
Oh, and my second child was just born! This time it's a boy! And even though he's a boy, he's much quieter than Sumire, who's a girl! Sometimes I even wonder if the gods made a mistake, haha!"
Yoriichi tried to decline again. "But—"
"Mr. Yoriichi." Sumiyoshi turned back suddenly, meeting Yoriichi's gaze with firm eyes. "Since you came to bid me farewell, that means you already consider me a friend, right? And as friends, there's no need to be so hesitant!
Seeing a friend off is the most natural thing in the world—especially when that friend has been slaying demons nearby and keeping us safe!"
Yoriichi looked into Sumiyoshi's earnest eyes and finally gave in, nodding slightly.
"All right. I'll take you up on that."