Leaving aside Ryuzetsu's disappointed departure, Karin, having received the recent intelligence gathered by the Hidden Grass Village, promptly relayed it to Shinji.
The fact that various factions, including the five great shinobi villages, were still heavily invested in researching demons came as no surprise to Shinji—he had anticipated as much.
As for the Hidden Grass Village tracking down Muku, Tanjiro (Upper Rank One), and others, Shinji remained indifferent.
As the "Lampbearer," he had long known the whereabouts of Tanjiro, Nezuko, and other Upper Rank demons lingering in the shinobi world.
His reasons for not dealing with them were twofold.
First, they had been behaving themselves, refraining from causing chaos in the shinobi world.
Second, he was currently too busy to spare the time.
With the Demon Slayer world thoroughly infiltrated by the Hegemon of Terror, all its powers, including the corruption, were suppressed by the Hegemon. As a result, Tanjiro, Nezuko, and the others could no longer spread corruption, as their "source of corruption" was about to become fodder for the Hegemon.
Under these circumstances, as long as Tanjiro and Nezuko didn't stir up trouble, Shinji was willing to turn a blind eye for the time being.
However, upon learning of Ryuzetsu's request, Shinji suddenly grew thoughtful.
He was currently short on shinobi to test "Blue Spider Lily No. 1," and after some consideration, Ryuzetsu seemed like a promising candidate.
Different shinobi constitutions would yield varying effects from "Blue Spider Lily No. 1," much like how demons ranged from weaklings to the "Twelve Kizuki."
Compared to Naruto (a Jinchuriki), Sasuke (with his Bloodline Limit), or Karin (with her Sage Body), Ryuzetsu might seem unremarkable.
But in truth, Ryuzetsu was a talented shinobi with a Bloodline Limit, skilled in Yang Release chakra and wielding the revival secret technique "Dragon Life Reincarnation."
Proficiency in Yang Release often indicated a robust physique, which could better harness the effects of "Blue Spider Lily No. 1."
With a spark of inspiration, Shinji handed a measured dose of "Blue Spider Lily No. 1" to Karin and instructed, "This is an enhancement potion that can greatly boost physical strength and vitality. It's still in the experimental stage. You can give it to Ryuzetsu and let her decide whether to use it."
"An enhancement potion?"
Karin paused, taking the vial of "Blue Spider Lily No. 1." Knowing her leader, she was certain this potion was something extraordinary.
Shinji didn't hide the truth and explained briefly, "Yes, I just developed it. Its safety has been verified through animal testing, but when used on humans, there may still be unknown side effects. If she decides to inject it, have her stay in the Land of Fire temporarily so I can monitor her physical changes post-injection."
"Understood!" Karin replied, vanishing from the spot.
Soon after, Karin caught up with Ryuzetsu, who was preparing to return to the Hidden Grass Village.
Surprised to see Karin approach, Ryuzetsu assumed she had changed her mind and exclaimed joyfully, "My lady, have you decided to rescue Muku?"
Karin shook her head. Under Ryuzetsu's disappointed gaze, she produced "Blue Spider Lily No. 1" and said, "This is an experimental enhancement potion that can comprehensively boost your physique and vitality."
Calmly, she explained the potion's effects and potential risks to Ryuzetsu.
Ryuzetsu's gaze fixed on the vial in Karin's hand. "What do you mean?"
"For the time being, I don't have time to deal with Muku and the 'Box of Paradise.' With your current strength, you're not even qualified to investigate, let alone handle it," Karin said, pausing before continuing. "This potion can make you stronger. You know the costs and risks. The choice is yours."
Ryuzetsu hadn't expected Karin to track her down for this. Caught off guard, she sank into deep thought.
Her concern wasn't just the potion's side effects but also its efficacy.
She had heard of countless so-called strength-enhancing potions, few of which were truly effective, so she worried the potion in Karin's hand might fall short.
However, since it came from Karin, Ryuzetsu decided to try it, even if its effects were limited. After all, rescuing Muku still depended on Karin, the new leader of the Hidden Grass Village.
After weighing her options, she nodded. "Alright!"
Taking "Blue Spider Lily No. 1" and confirming the injection method, Ryuzetsu didn't hesitate and immediately injected the potion into her body.
Seeing this, Karin said, "Stay in the Land of Fire for now. I'll arrange for someone to check your condition periodically. Once we confirm you're fine, you can return to the village."
With the matter settled, Ryuzetsu could only nod.
Moments later, Ryuzetsu arrived at a hideout Shinji had arranged for her.
Stepping into the underground hideout, she surveyed her surroundings. The space was sizable, well-stocked with supplies—enough for two or three people to survive underground for months.
In truth, Shinji had set up many such hideouts across the Land of Fire. This one was simply the closest to his laboratory.
As Ryuzetsu inspected the hideout, the sound of footsteps put her on alert. "Who's there?!"
"No need to be tense. I'm the physician responsible for monitoring your condition."
Wearing a plain mask and a black cloak, Shinji stepped lightly into the hideout from the entrance.
Ryuzetsu didn't lower her guard, gripping a kunai tightly and staring intently at Shinji.
Unfazed, Shinji approached a table, unsealing medical instruments from a scroll and arranging them while casually asking, "How long has it been since you injected the potion?"
"About two hours," Ryuzetsu replied, still holding her kunai.
Shinji paid it no mind. He guided Ryuzetsu to connect herself to the instruments measuring heart rate, blood pressure, and other basic vitals, then began recording in his notebook.
Noticing Shinji's diligent note-taking, Ryuzetsu gradually relaxed her guard and quietly asked, "Doctor, how's my condition?"
Shinji glanced at her. "What do you think?"
"I…"
Ryuzetsu faltered.
She could feel changes in her body, but when asked to specify, she found herself at a loss for words.
Shinji said, "You've grown much stronger—and you're still getting stronger. As for how far you'll go, that depends on whether you can fully harness the potion's power."