Two days into their stay at Sasori's base in the Land of Birds, Finn had run out of things in the ninjutsu library worth his time.
He stood in the middle of the laboratory holding a scroll open, reading the contents with the expression of someone who had ordered a meal and received something considerably less interesting than expected. The base itself was exactly what he had imagined a genius puppeteer's private workspace to look like: organized around function rather than comfort, every surface given over to some ongoing project or material storage, the kind of space that a person inhabits as a tool rather than a home.
Kakuzu had lasted two days before the itching of inaction became intolerable and he had left to find work. The war in the surrounding territories was generating exactly the kind of environment he found professionally stimulating: chaos, casualties, and clients willing to pay premium rates for results during instability. He had not asked Finn to come. The work was ordinary enough that he didn't need backup, and Finn, for his part, trusted Kakuzu's self-assessment on questions of danger. The Earth Grudge Fear made him extraordinarily difficult to kill in any scenario that didn't involve specific, coordinated efforts by multiple high-tier shinobi. He would be fine.
That left Finn and Sasori in the laboratory, which had been more productive than Finn expected and less satisfying than he wanted.
"This is the same material I already have from Kakuzu," Finn said, tossing the scroll onto a side table. "Common techniques. B-rank at the ceiling. Where are the actual secrets? A-rank, S-rank, forbidden techniques? Anything that requires real preparation to use?"
Sasori, at the main table with Uchiha Kagami's body open in front of him, did not look up. His hands were working with the careful attention of someone who found the task genuinely interesting and resented interruption on principle. "I don't collect ninjutsu for its own sake. The things in this library are what I needed to understand puppet mechanics and chakra string theory. I didn't acquire what I didn't need." A pause. "Human puppetry. I invented it. If you want a forbidden technique with practical application, I can teach you that one."
He looked up briefly. "I would want Haki in exchange."
Finn looked at him for a moment, then at the body on the table, then back at Sasori. The exchange request was not the problem. He had no particular attachment to keeping Haki exclusive to this world; it would not destabilize anything he cared about, and the knowledge leaking into the ninja world's power ecosystem had no meaningful impact on his own position. He had given Kakuzu the same training freely enough. The question was whether the transaction was worth anything at all from Sasori's side, given the specifics of what Sasori actually was.
"The issue," Finn said, "is that you have one functioning biological component left. Everything else is mechanical. Haki is cultivated through the body, through the will expressed through a living physical system. Kakuzu has his own complications, but he still has biological hearts and biological tissue doing most of the heavy lifting. You have a regeneration core." He crossed his arms. "I'm not saying it's impossible. I'm saying I genuinely don't know, and I don't want to promise something that your current structure can't support."
"Kakuzu practices," Sasori said.
"Kakuzu is arguably still a person. Your situation is more like..." Finn tried to think of an equivalent from his own experience. There had been a man once, in the Marine's peripheral orbit, whose circumstances were structurally similar, a person progressively converted away from biological existence through external modification rather than internal development. Bartholomew Kuma. Whatever Vegapunk had done to him by the end had raised similar questions about where the threshold of the system was. "I'd want a second opinion from someone who understands the relevant engineering before I committed to an answer."
"Then let's try and see," Sasori said. "You lose nothing if it doesn't work."
This was a reasonable point. Finn reached into his coat and produced a small scroll from one of the inner pockets, tossed it across the space between them. "Training outline is in there. The concepts, the stages, the indicators to watch for. Work through it and we'll see where it goes."
Sasori caught it without looking and set it to one side with the particular precision of someone who would not lose it and would return to it when they chose to.
He produced a second scroll from his own workspace and slid it across to Finn. Human puppetry. The complete methodology, or enough of it to constitute a genuine technical transfer. Finn picked it up, noted the density of what was inside from the weight of the scroll, and set it down with the intention of reading it eventually. He had no particular interest in learning the art. But the principles behind it, the specific questions it had forced Sasori to answer about how biological material could be modified and preserved and interfaced with chakra systems, might be useful from angles he couldn't fully anticipate yet.
He walked over to the main table and looked at Uchiha Kagami.
Kagami had died two days ago, in his sleep, without pain, in the way that Sasori had chosen to handle the logistics of his project. The body had been treated in the interim. It did not look like a corpse. The skin retained its color and texture, the joints their flexibility, the overall impression one of a man who was unconscious rather than dead. Whatever Sasori used for preservation, it was thorough.
"Is the Sharingan going to work?" Finn asked. "The Third Kazekage's Magnetic Release carried over into the puppet. Can an organ-based kekkei genkai do the same?"
Sasori's hands did not slow. "I don't know yet. That's what I'm trying to determine. Magnetic Release is a chakra-nature kekkei genkai. It exists in the chakra network, in the pathway structures, in how the cells process and express chakra attributes. When I preserve and puppetize a body carrying that kind of kekkei genkai, the network structures are intact and can be reactivated through chakra input. The Sharingan is different. It exists in specific physical organs. The eyes themselves are not just windows into the technique; they are the technique. If the eyes remain viable and the neural pathways connecting them to the chakra center remain intact, perhaps it functions. If not..." He set down one instrument and picked up another. "We find out."
"On the way here," Finn said, "Kakuzu told me you killed the Third Kazekage."
"Yes."
"He was considering whether to sell that information to the Sand Village."
"He probably has by now," Sasori said, without any particular affect. "The Sand Village is selecting a Fourth Kazekage. The information about what happened to the Third will reach them through one channel or another. I left no evidence, but the evidence's absence is its own kind of evidence eventually. Kakuzu selling it now simply accelerates the timeline." He paused. "I don't care."
Finn thought about the Fourth Kazekage and what he knew about the succession. Rasa, with his gold sand Magnetic Release, a different expression of the same attribute family as the Third's iron sand. Capable enough. Would stabilize the Sand Village through the post-war period on the strength of his kekkei genkai's economic applications as much as its military ones.
He thought, very briefly, about gold sand as a puppet material. Then he thought about what that thought implied for Rasa specifically. Then he decided not to mention this to Kakuzu on the grounds that some ideas were too practically efficient to release into the world without considering the downstream consequences.
He kept this thought to himself and moved on.
"Walk me through the classification again," Finn said. "You started to explain it and then I had questions. Finish the taxonomy."
Sasori looked at him with mild surprise, then shrugged internally and continued. "Kekkei genkai fall into two fundamental categories. The first is chakra-nature based: Magnetic Release, Melt Release, Ice Release, Storm Release, Wood Release. In each case, the bloodline limit represents the successful fusion of two chakra nature types into a new combined attribute. The mechanism is understood in general terms even if it's extremely difficult to replicate. Hashirama Senju confirmed publicly that Wood Release is water and earth combined. Everyone knows the theoretical input. No one else has ever successfully produced the output."
"Because knowing the ingredients isn't the same as being able to cook," Finn said.
"The two requirements beyond the nature combination are a chakra volume sufficient to sustain the technique at any meaningful scale, and a physical constitution capable of channeling that volume without the body destroying itself in the process. Hashirama met both requirements at a level that has no recorded parallel. Someone like Yamato met neither at the level required for real Wood Release capability. He has the nature combination, inherited through Hashirama cell transplantation, but the output is proportional to what he himself can support. Which is not much, relative to the original."
Finn was quiet for a moment, turning this over. "So Wood Release, theoretically, is achievable for anyone who can fuse water and earth chakra and has the physical and chakra foundation to support it."
"Theoretically. In practice, the combination alone is extremely rare, and the foundation requirement eliminates everyone who achieves the combination. The technique exists as an artifact of one specific person."
"And the second category?"
"Organ-based kekkei genkai. Sharingan. Byakugan. The Shikotsumyaku of the Kaguya clan. In these cases, the bloodline limit is expressed through specific biological structures that differ from ordinary human anatomy. The eye structure of the Sharingan is not a standard eye doing something unusual; it is a fundamentally different organ that ordinary eyes cannot replicate through training or chakra manipulation. These cannot be created artificially by combining inputs. They are present or absent."
"And the puppet question is whether those organs remain functional outside the original body."
"Correct. The chakra-nature variety transfers cleanly because the relevant structures are distributed throughout the body's chakra network, which I can preserve and reactivate. The organ variety requires the specific physical structure to remain viable and connected to a functional chakra input pathway." He made a small adjustment on the table in front of him. "I am finding out."
Finn looked at the ceiling for a moment. He was processing the first category of kekkei genkai against everything he knew about his own current situation: six months of ninjutsu foundation, three attributes functional, two still developing, a chakra volume that was expanding at a rate Kakuzu had found deeply unsettling, and a physical constitution that had been built by decades of Marine training in a world where the power ceiling was set by people who had survived things that would have ended most ninja careers before they started.
Water and earth.
He had both. He had chakra volume in quantities that Kakuzu had compared, with visible discomfort, to Tailed Beast territory. He had physical vitality that the world itself had found difficult to suppress when he first arrived, and that suppression had since faded almost entirely.
Orochimaru had said, in a laboratory on the other side of the Land of Fire, that someone with physical vitality comparable to Hashirama Senju could integrate the First Hokage's cells without the yin-yang balance problem, because their native biology would match the proliferation pressure rather than being eroded by it. He had said this was a fantasy because no such person existed. He had been speaking from a framework that did not include the information that a former Marine Admiral from a different world entirely had recently arrived in the Land of Rain and was now thinking very carefully about Wood Release.
"So," Finn said slowly, thinking through the implications of everything Sasori had just explained, "if the attribute-combination type of bloodline limit can theoretically be created by anyone who meets the requirements..."
He paused.
Then the thought arrived, and it was not subtle. It was very clear and somewhat alarming in its directness and very obviously the correct conclusion if you followed the logic Sasori had just laid out to its natural end.
"Hashirama Senju is buried somewhere in the Land of Fire," Finn said. "Konoha's founders have a burial site. If we go and dig him up, and you make a human puppet from his body, you would have access to Wood Release at the full original scale. Which means you would have access to Tailed Beast-level suppression capability. Which means..." He stopped. "Actually, that makes you more or less untouchable in most combat scenarios, doesn't it."
The work at the main table stopped completely.
Sasori turned and looked at Finn with an expression that the puppet's face managed to convey with some precision: the look of a man who has explained a technical concept to someone and watched them immediately use it to arrive at a conclusion that the explainer had not intended, had not considered, and found simultaneously unreasonable and logically airtight.
"That," Sasori said, after a pause, "is not what I was suggesting."
"I know," Finn said. "I'm suggesting it."
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