Jin started by exploring the potential uses for the Dragon Leather harvested from the Ancient Dragon.
He decided to run his initial tests using scraps.
"Hmm. It seems that simply imbuing it with mana only makes it stretch taut and harden."
It yielded the same results as his experiment aboard the Capricorn 1—this method wouldn't work for skin.
"To use this for her skin, I either need to give the flow directionality, or perhaps use a different type of magic altogether..."
Saki’s logic was sound. "The Ancient Dragon moved around just fine, so there has to be a way."
Jin and the others spent some time testing various theories.
They ultimately confirmed that while imbuing the material with raw mana made it rigid, applying mana waves—specifically, mental waves—transformed it into a flexible, incredibly tough material.
"Right. In other words, it has an incredibly high affinity for magic and magi."
Because the affinity was so extreme, it could only react to one type of magic at a time. This meant the leather would effectively ignore any magic cast by others.
Their conclusion: Ancient Dragon leather was inviolable to any magic other than what was currently flowing through its fibers.
This was precisely why a living Ancient Dragon was invincible, yet its shed skin became vulnerable. However, once that shed skin was infused with a new flow of mana, it could be reinforced once more. From then on, it would only respond to that specific signature.
If Jin used this for Reiko’s skin, she would be virtually untouchable.
But Jin had no intention of stopping there.
"Jin-nii, what are you planning?" Elsa asked.
"I’m going to try that idea we discussed while we were on the Capricorn 1."
By "that," he meant the discussion regarding what truly determined a golem's strength. He intended to overhaul her skeleton and musculature as well.
"Reiko’s skeleton is made of hyper-adamantite. To keep her light, the bones are hollow with a foam-like internal structure."
He had heard that the bones of large birds were similar—hollowed out with a truss-like internal structure to maintain strength while minimizing weight.
"What would happen if we filled those foam sections with these offcuts?"
Elsa was the one to answer. "...Reinforcement via magic."
"Exactly. If I treat them the same way as the skin, I should be able to achieve a whole new level of reinforcement."
"That certainly sounds incredible..." Saki remarked, finally grasping the scope of the project.
"And then there are the muscles. If I use the scraps for those too..."
Ancient Dragon leather possessed an extraordinary reaction to magic. This made it a superior material for artificial muscles. Jin set to work, weaving the new material into the existing magical muscles to reconstruct them.
He then recalibrated the magi reactor to harmonize with the new skin and muscles. Finally, he performed a meticulous review of her control core’s finer settings.
He made careful adjustments to ensure her weight remained unchanged. After about two hours of work, Reiko had been restored to her original form. All that remained was to wake her up.
"Hey, Jin, can I ask you something?" Saki asked as he performed the final checks.
"Hmm? What is it?"
"Is there a specific reason you keep Reiko-chan in this form? I feel like if you wanted to maximize her strength, an adult physique would be more practical."
"Ah, that."
Jin paused, turning his attention away from his work to address Saki and Elsa directly.
"Her appearance was chosen by the Predecessor. I don’t know why she picked this specific form, but Reiko calls her Mother and calls me Father. I want to honor the Predecessor’s original intent."
He paused for a moment before continuing.
"...I know how that sounds. I can’t put it into words very well, but that’s just how it is. On the inside, she’s already become something else entirely because of my own selfish tinkering. But her appearance is the one thing I refuse to change. If you want to call that self-righteous, then so be it. If it seems strange, feel free to laugh."
Having said his piece, he turned his back to them and sat down in his chair.
A brief silence followed. Then, Elsa stepped forward quietly and gently wrapped her arms around Jin’s neck from behind.
"...It’s not strange at all. It’s just Jin-nii’s kindness."
"Yeah... you’re right," Saki added. "Jin, I’m sorry for asking such a weird question."
＊ ＊ ＊
"Alright. Reiko, 'wake up.'"
"Yes, Father."
Reiko’s eyes fluttered open. She tried to sit up, but immediately tumbled forward.
"Ah!"
As she braced her hands against the floor to stop her fall, she punched holes straight through the reinforced chert.
"I-I am terribly sorry..."
She tried to stand up in a panic, but this time she overcompensated, snapping her torso back so hard she flipped over. When the back of her head struck the floor, more cracks spiderwebbed through the stone.
"Reiko, calm down," Jin said soothingly.
"F-Father... I cannot... I cannot control my strength."
"Just as I suspected. The materials we used are a complete unknown. Reiko, stand up slowly. Very slowly. Let’s try... 0.1 percent output."
"Y-Yes."
Reiko understood the gravity of the situation. In her current state, even a casual touch could be fatal to a human like Jin.
Following his instructions to the letter, she suppressed her output to the absolute minimum and stood up with agonizing slowness.
"Good, that’s it. Reiko, it might take some time for you to get used to this body. There’s a chance your performance is roughly three times what it was before."
"Three times...?"
Saki couldn't help but interject. "J-Jin! Three times?! Where on earth did you get that number from?"
Elsa seemed just as stunned. "Mm....I could understand a few dozen percent. But three times... that’s an entirely different league. Why?"
"Right, let me explain." Jin nodded, gesturing for Reiko to remain seated in her chair for the time being. "First, Elsa. Can you sense any mana coming from Reiko?"
"Eh?"
Startled by the question, Elsa immediately focused her senses, trying to perceive the mana signature of the girl before her.
"...I can't feel a thing."
"Exactly. Ancient Dragon leather seals mana almost perfectly. That means the efficiency of her mana usage has skyrocketed."
The mana he referred to was essentially the same as a magi’s mental energy. In Reiko’s case, it was the mental waves generated by her control core.
Because they were waves, they possessed wavelengths, amplitudes, and waveforms. Jin explained that Reiko’s waves were identical in nature to his own, given that he was her creator. He explained these concepts to Elsa and Saki while simultaneously ensuring Reiko absorbed the information.
"She uses that mana to move her muscles and cast magic. Since that mana no longer leaks out, she’s operating at near-hundred-percent efficiency."
In reality, there were still openings like her eyes and mouth, and Jin added his usual caveat that while the word "perfect" existed, the actual thing rarely did. Nevertheless, both Elsa and Saki understood his point.
"But does that alone really triple her power?" Saki asked.
"Yes. Muscles crafted from Ancient Dragon leather are just that superior. If I had replaced every single component, she might have ended up ten times stronger."
"That’s terrifying," Saki muttered.
No. 700672 had certainly provided them with an unbelievable material.
"Of course, she can probably only maintain her absolute maximum output for a few dozen minutes unless she’s in a high-ether environment like the demon territory."
"...Even so, a few dozen minutes... Jin, I really have to wonder just who you're planning to fight." Saki looked at him with an exasperated expression.
"It’s not that. I just don't want Reiko to break. I want her to always be the finest Automata possible. I don't want her to lose to anyone, no matter who they are."
"...I see. I’m actually a little... envious of Reiko-chan," Elsa whispered, a hint of loneliness in her voice as she nodded.
As for Reiko herself, she was overwhelmed with emotion, deeply moved by the depth of Jin’s care for her.
＊ ＊ ＊
It took another half a day for Reiko to finally regain full control over her body.
The process was somewhat similar to when Aaru had first been activated. In Aaru’s case, Jin had used clay kneading to synchronize her control systems.
"Father, I have adjusted."
Jin had moved Reiko on to origami.
Just now, Reiko had finished folding a perfect paper crane. It was made of washi produced on Hourai Island, using the manufacturing process John Dini had reported.
"Jin-nii, you have to teach me how to do that later."
"Yes, me too! I’d love to learn!" Saki added enthusiastically.
The art of origami was a hit with both Elsa and Saki. That said, Jin didn’t actually know that many designs himself. He could manage cranes, servants, samurai helmets, sailboats, balloons, and boxes with square paper. With rectangular paper, he could make paper poppers and airplanes.
"I should teach Hanna next time, too..." Jin mused.
Reiko prepared both lunch and dinner that day. She insisted it was part of her ongoing calibration process.
"Mm, this is great, Reiko."
"You’re a natural at this, Reiko-chan."
"It really is delicious."
Not just Jin, but Saki and Elsa as well were full of praise. It seemed Reiko’s motor control was no longer an issue.
"With this, I can truly be of use to Father once again," Reiko said, a happy smile gracing her lips.