Kū

was a synthetic human created as a partial clone of Ōnoki, that was made to assist his master in replacing shinobi with synthetic humans in Iwagakure and the shinobi world.

Background
After Iwagakure was attacked by missing-nin, which resulted in multiple young casualties, Ōnoki was especially hurt by this loss as one of the victims was his grandson Kozuchi. Determined to find a means to protect the peace and keep all innocents alive, the retired Tsuchikage began working on creating the Akuta as replacement soldiers. As research was going slowly, Ōnoki was approached by a scientist with a genetic sample from a White Zetsu. Using this new material, despite the cloning techniques still being in its infancy, Ōnoki was able to not only refine his Akuta, but also produce five self-aware and independently thinking artificial humans, one of which was Kū. Uniquely, Kū was mainly based off of the Third Tsuchikage himself, using his DNA as basis of his being.

Personality
A mysterious man, he is very committed to his ambitions and will execute his tasks without hesitation, all in the goal of reshaping the world for "peace". He is shown to be fiercely loyal to his creator, Ōnoki, viewing him as his father and master while growing irritable at his sudden disappearance, striking down one of his Akuta out of anger. At the same time, he refuses to stray from Ōnoki's goal of the least amount of bloodshed possible. Unlike his fellow artificial humans, Kū is shown more well-adjusted as a person, neither questioning who he is nor looking down on regular humans.

Despite his apparent loyalty to Ōnoki, his first and most important concern is the success of the goal for which he was created. While appreciating what Ōnoki strives for in lasting peace and wishing to see it through, Kū has ultimately adopted an "ends justify the means" mentality, willing to go against Ōnoki's orders if he feels it will only impede the efforts towards "peace", including a willingness to sacrifice lives, though he had no ill-will towards his creator and wished him to remain safe. At the same time, in learning the potential evolution of his being, Kū became more arrogant, feeling superior to humans and the true future of the world.

Appearance
Like important Iwa figures, Kū dresses in a green and red robe. Underneath, he wears a green flack jacket. Like his template, he is a short man with shaggy brown hair reaching below his chin and covers his left eye while the rest of his hair reaches his mid-back. Also like the Third Tsuchikage, he has a broad face with a large nose and thick eyebrows.

Abilities
Created and trained by the Third Tsuchikage, Kū is shown to be a very capable ninja, enough to face the Fourth Tsuchikage one-on-one. He also a skilled leader, proving to be very methodical and perceptive, able to effectively read the situation and decide how to act.

Nature Transformation
Like his genetic source, Kū is highly proficient in the advanced nature transformation of Dust Release, which is a simultaneous usage of Earth, Fire, and Wind Release. With it, he can use the Dust Release: Detachment of the Primitive World Technique to disintegrate his targets. Like many Iwa-nin, Kū seems to specialise in Earth Release, able to use the Earth Release: Light-Weight Rock Technique to grant himself flight to move about as he pleases and also use the earth around him to attack his foes. He is also able to use an Iwagakure kinjutsu, Akuta, to create golems made from stone.

Physical Prowess
While mostly a schemer who operates from behind the scene, Kū is more than proficient in direct combat. Against Kurotsuchi, he showed noticeable taijutsu prowess, able to calmly defended himself from her attacks and likewise proficient in take-downs and body holds. He also is skilled in kenjutsu wielding a large cleaver-like blade with which he can strike down multiple targets with a single strike. He has considerable raw strength, able to strike a target as large as Akatsuchi through the air with enough force to shatter a massive stone column. He has blazing speed, able to quickly catch Boruto off-guard before subduing him.

Body Modifications
After receiving a transplanted human heart, Kū obtained an undecaying body, which allows him to fight at full strength without the need to worry about his body starting to decay and fall apart due to its incomplete state.

Mitsuki's Disappearance Arc
Kū met with Kirara and Kakō in what appears to be some type of research facility for the production of Akuta. Later, feeling that his preparations were complete, Kū, Kakō, and some of his artificial soldiers successfully attacked and abducted Kurotsuchi. Later, when Akatsuchi and five Konoha genin came to Ōnoki for help in finding their lost ally Mitsuki, Kū surrounded them with his Akuta. He then revealed to the captured foes that he was, in fact, Ōnoki's son and working in league with the former Tsuchikage.

When Boruto confronted the Third Tsuchikage about this, Kū quickly subdued him. The old man then explain the nature behind his goal and voiced his desire not to hurt anyone, the genin made their move to escape. Kū quickly engaged in the fight, facing off against Akatsuchi. The fight quickly escalating, knocking over a massive column towards Ōnoki, who was narrowly saved by Boruto, who escaped with the elder. An enraged Kū then ordered his Akuta to retrieve Ōnoki.

Growing irritating at Ōnoki's abduction, Kū had his soldiers send out more Akuta while insisting to keep their actions secret. He later finally met with Mitsuki. The boy asked about the essence of free will. Kū however noted that he can't truly answer that himself as an artificial human like Mitsuki and that Orochimaru, as Mitsuki's creator, was the best chance of him getting a straight answer. He did however note that Mitsuki betraying Konohagakure, an act of rebellion, is common among humans as a sign of maturing. He went on to insist that the unique nature of Mitsuki's being would be instrumental in completing the Third Tsuchikage's plans to ensure lasting peace through the Akuta.

Later, as his body began breaking down like the other artificial humans, Kū talked to his scientist about the progress on studying Mitsuki's more advanced body. While noting that Orochimaru's design was very difficult to decipher and the Cursed Seal made research even more difficult, he concluded that transplanting a normal human's heart into an artificial human would likely stabilise the artificial humans. While hating the idea of killing the innocent, Kū decided to look at the bigger picture on continue his mission of protecting all, conceded to make the sacrifice. Not wanting to sacrifice anyone from Iwagakure, he told the scientist to test his theory with Shikadai. He then learned that with Kirara weakening, her genjutsu failed on controlling an Iwa official. Realising that stealth was no long and option, Kū decided to forcefully take control of the village.

Soon afterwards, the village was put on lockdown by Kū and his army. As they continued to search for Ōnoki, Kū's fellow artificial humans began growing irritable, knowing of the high chance of full stabilising themselves by receiving a human heart transplant. Kū however refused to do something so drastic without Ōnoki's specific approval. Later, to his fury, he learned that Konohagakure was contacted about the recent coup d'état.

When Kirara returned with the Konoha-nin and Ōnoki, the elder was furious to see Kū's excessive measures to keep the world from learning of their plans. While Kū explained the reason for his actions, he also revealed that new research showed the Fabrications could be saved by transplanting human hearts into them. Ōnoki refused on the grounds that it went against his goal of saving all lives. Deciding that his creator was too sentimental to perform the greater good, Kū incapacitated the elder, taking over full control. As he decided to go forward with the transplant, Shikadai appeared, who helped his fellow genin escape. Kū sent Kirara and Kokuyō to capture the enemies again.

Later, Mitsuki arrived at the Iwa main building with a transplant heart for Kū. Kū met the boy with Kirara and Sekiei. They were interrupted by Sarada and Boruto. As they tried to plead with Mitsuki, Kū trapped the two genin with his Earth Release. Once Kū had the vial with his new heart in his hand, Kū began celebrating the next stage of his plans. Mitsuki suddenly ran his hand through Kū's chest. He revealed that the transplant heart was merely a cluster of his snakes and his betrayal of Konoha was a ruse to let Mitsuki destroy the enemy's plans from inside. While seriously hurt and weakened, an enraged Kū refused to let Mitsuki get away with this.

Kū retreated to the Old City in order to treat his wounds and decaying body, leaving Sekiei and Kirara to deal with the Konoha genin. Once he arrived, the doctor informed Kū the donors had escaped and there were no raw materials for the artificial heart. Deciding there was no other choice, Kū ended up killing the doctor, taking his heart and implanting it. Once fully recovered, he confronted Boruto, Sarada, and Mitsuki.

As Kū began pressuring the genin with his renewed power, Ōnoki appeared to try and reason with Kū again on how far he strayed from his original path, giving into such anger. Kū however argued that his anger was inherited from Ōnoki's own grief and attacked the group with his Dust Release. Sarada saved everyone by breaking through the ground. As Kū caught up to finish the battle, the combined effort of Team 7 managed to seriously damage Kū. Kū attempted a final Dust Release on the group, only for Ōnoki to counter it with a much stronger one, determined to not face his late grandson again without atoning. Ōnoki ultimately destroyed Kū.

Trivia

 * "Kū" means "sky" or "emptiness".