Yoriichi Type Zero

Yoriichi Type Zero is a mechanical battle doll constructed by Kotetsu's ancestors during the Sengoku period. According to Kotetsu's father, it was based on an actual swordsman, that being Yoriichi Tsugikuni.

Appearance
Yoriichi Type Zero's appearance largely resembles Yoriichi Tsugikuni, the person it was modeled after. It has six arms and hands that each hold a katana, due to the fact that two arms weren't enough to come even close to replicating Yoriichi's movements. Over the 300 years it has existed for, the doll has suffered extensive damage, with the head half cracked and the arms having some amount of wear on them.

History
During the Sengoku period, members of the Swordsmith Village created a specialized battle doll to train Demon Slayers. It was modeled after Yoriichi, the progenitor of Breathing Styles and the strongest Demon Slayer to have ever lived. During its construction, they realized that just two arms was inadequate to replicate Yoriichi's own movement speed, and they added four more to make it as close as possible to him.

The doll was passed down to its creator's descendants, with its latest owner being an orphan ten year old swordsmith named Kotetsu. According to Kotetsu, the technology used to create the battle doll was so advanced that the present day members of the Swordsmith Village have yet not been able to replicate it.

Swordsmith Village Arc
After hearing the "secret weapon" mentioned by Mitsuri, Tanjiro decides to look for it in the forests of the Swordsmith Village. Eventually, he find a young swordsmith, Kotetsu, having a squabble against the Mist Hashira Muichiro to prevent him from damaging Yoriichi Type Zero further. Despite Tanjiro's intervention, Muichiro obtains the key to activate the doll by force due to Kotetsu's inability to physically fight back against him.

During his training session, Muichiro is able to evade its fast flurry of attacks and breaks the doll's left shoulder armor and one of its right arms. Due to his own blade snapping, he decides to take a sword from the doll as his own, showcasing the broken arm still attached, dismaying Kotetsu and Tanjiro. The two find the doll now soaked with rainwater and even more damaged, but they use the key to test its functionality and see that the doll is still capable of movement. Wanting to beat the arrogant Muichiro, Kotetsu gives Tanjiro a long and grueling training session with Yoriichi Type Zero.

Over several days, Tanjiro engages the doll whilst it is armed with wooden clubs instead of katanas, but he nonetheless is unable to land a hit. His task is made more difficult when Kotetsu denies him food and water in his failure. On the seventh day, in his exhaustion, Tanjiro faints mid-battle but he suddenly gains the ability to smell a scent that can predict an attack's incoming movements. Now armed with his new ability, Tanjiro lands his first blow on Yoriichi Type Zero, though it was nothing more than a nick on its legs.

After regaining his stamina, Tanjiro engages the doll more efficiently as he is now able to evade its slashes, even whilst the doll is armed with katanas. Eventually, with Kotetsu's blessing, Tanjiro lands a powerful strike on its neck, causing the head to break off and the body to crumble, revealing inside the doll was a katana over 300 years old.

Following the battle in the Swordsmith Village, it was decided that the village would be relocated within four days to safeguard against future demon attacks. When Muichiro returned two days after the battle, he discovered Kotetsu diligently working on repairing Yoriichi Type Zero. Kotetsu explained that if the repairs weren't completed before the relocation, the village chief, Tecchin, had ordered the item to be discarded due to the limitations on what they could transport through their secret means. Motivated by this revelation, Muichiro joined in assisting Kotetsu by replicating the movements he could recall. However, after they successfully reconstructed one style, Kozo revealed that the time limit was merely a fabrication by Tecchin to motivate Kotetsu.

Abilities
Yoriichi Type Zero has 108 different movements based off of the person it was modelled after. Its power surpasses most humans, hence why it is used as a training dummy for Demon Slayers. However, as centuries passed on, it has vastly withered and eroded, making it likely far weaker than what it was capable of during the Sengoku period.

The doll can be adjusted and fine tuned to exploit an opponent's weakness by rotating its fingers a certain amount of times and configuring the settings in its head. These functions were purposefully left out by Kotetsu when Muichiro used the doll to make his training session less fruitful than an optimized session.

Trivia

 * The quantity of arms that Yoriichi Type Zero possesses (six) along with the position of its joint connection to the torso is a representation of the "Asuras", a race of supernatural demonic deities in Buddhism. The katanas that the doll wields may also represent the weaponry that they possess for warfare against the "Devas".