Yoriichi Type Zero

Yoriichi Type Zero is a Battle Doll constructed by Kotetsu's ancestors during the Sengoku Era. According to Kotetsu's father, it was based off of an actual person, 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. It is also shown to be heavily damaged, with half of its face and one of its arms missing after being damaged by Muichiro Tokito in training.

Swordsmith Village Arc
After hearing the "secret weapon" mentioned by Mitsuri Kanroji, Tanjiro decides to look for it in the forest of the Swordsmith Village, only to find Kotetsu having a squabble against the Mist Hashira to prevent him from damaging Yoriichi Type Zero.

In the end, Muichiro obtained the key to activate the doll by force due to Kotetsu's inability to physically fight back against him. During the training, Muichiro broke the doll's left shoulder armor and one of its right arms. Afterwards, Kotetsu fixed it and began to train Tanjiro using the battle doll. Due to the intense training that Tanjiro has went through, he had awakened the ability to predict his enemy's movements by smelling them and can tell where his opponent would aim next.

After being cut by Tanjiro, the head from Yoriichi Type Zero tumbles down only to reveal a 300 year-old katana hidden within its body.

Abilities
Yoriichi Type Zero has 108 different movements based off of the person it was modelled after. Its power surpasses most humans, which is 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 Era.

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".