Yumiko was a Sunagakure spy.
She belonged to the covert intelligence mobile unit under Sunagakure elder Ebizo, known simply as the "Covert Division."
The purpose of her transfer to the eastern front was that every major nation and shinobi village understood that, with Konoha's current forces, it was impossible for them to wage war against four countries at once.
Had the Third Raikage not exhausted Iwagakure's forces earlier that year by fighting ten thousand Iwagakure shinobi, indirectly destabilizing Kumogakure from within and forcing the Fourth Raikage Ay to temporarily withdraw from the Third Shinobi World War to consolidate his rule—
The front along the Land of Rain would likely have already pushed into the Land of Fire, perhaps even reaching Kikyo Castle and forcing the Third Hokage to take the field personally.
At this moment, Konoha's main forces were simultaneously resisting Iwagakure and Sunagakure on the western front. With both sides locked in a stalemate, Konoha would inevitably struggle to deal with Kirigakure's assault on the eastern front.
Thus, The crucial turning point that would shape the course of the entire Shinobi World War lay on the eastern battlefield.
Both Sunagakure and Iwagakure hoped that Kirigakure could land a fatal blow against Konoha on the eastern front. To that end, they had sent large numbers of spies like Yumiko to operate behind the eastern lines, waiting for opportunities to aid Kirigakure and weaken Konoha.
Before becoming a Sunagakure spy disguised as a "Medical Ninja," Yumiko had truly been a Medical Ninja under Granny Chiyo of Sunagakure.
She knew very well how Medical Ninjas were graded. She could also identify the talents of Konoha's various Medical Ninjas, and she understood even more clearly—
What kind of devastating chain reaction the emergence of a "Legendary Sannin"-level Medical Ninja could bring.
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