The mechanical doll otherwise known as Yoriichi Type Zero appears in the Swordsmith village arc, season 3, episode 2 and was no ordinary mechanical doll, but a fine mark of mechanical craftsmanship that gave Tanjiro the necessary training he needed to level up his skills and become an even stronger Demon Slayer, the enabled him to refine his abilities and challenge himself to become stronger.
The origin of the doll traces back to over 300 years ago during the Sengoku era, and was believed to be crafted by none other than the ancestors of the young and ambitious swordsmith, Kotetsu. Yoriichi Type Zero was instrumental in Tanjiro's training with its lifelike movements and reflexes, but what was this mechanical doll capable of? And what made it so special?
Yoriichi Tsugikuni
Crafted by highly skilled swordsmiths many years ago, Yoriichi Type Zero was way ahead of its time, said to have been modeled after an extremely powerful swordsman, Yoriichi Tsugikuni. Yoriichi was a legendary Demon Slayer and lived 500 years ago during the Sengoku era and was known to be the creator of the breathing style techniques that Demon Slayers use today, he was also known to have incredible abilities and was also responsible for creating the sun breathing technique, which the Kamado family harnessed. The doll was meant to be used for Demon Slayers to practice and train with, so they could become stronger while visiting the Swordsmith Village. Since the mechanical doll was based off Yoriichi Tsugikuni, it had to be given six arms to recreate his swordsmanship style, the doll was also able to execute 108 various moves and is said to be stronger than an average human. In episode 1, Yoriichi can be seen in a shared memory (dream) Tanjiro has when he is still in bed recovering from his fight against Daki and Gyutaro, a closer look at this Yoriichi also reveals that he bears a strong resemblance to another character in the series, Upper Rank demon 1, Kokushibo, as the duo were brothers in a past life.
A 300-Year-Old Sword?
Kotetsu then takes it on himself to train Tanjiro and puts him through extremely tough training sessions, and also replacing the swords on the mechanical doll with wooden clubs. Kotetsu turns out to be a ruthless trainer and goes to the extent of starving Tanjiro if he wasn't able to land a hit on the mechanical doll. After many failed attempts and hunger pangs, Tanjiro gives up because of exhaustion and faints, but this is when he suddenly feels a new ability taking over his body, and he is able to see the doll's movements clearly, giving him the opportunity to finally get a hit in on the doll.
Once fed and after regaining his strength, Tanjiro comes back with his new-found ability and uses it to land a spectacular hit on the doll destroying the head of it. What was inside the body of the mechanical doll is what surprised Tanjiro and Kotetsu the most, a katana which was more than 300 years old. Later on a swordsmith named Haganezuka arrives along with Kanamori and attempts to take the katana away with him, after a much heated yet funny confusion as to why Haganezuka wanted to take the katana away from them, Kanamori explains that Haganezuka has been training in the mountains so that he could build a sword for Tanjiro to use that wouldn't break so easily. Haganezuka then hands Tanjiro another sword to use for the time being and disappears into the woods taking the 300-year-old katana with him as Tanjiro and rest watched on.