Koshiro is a Dojo Master of the Isshin Dojo in the Shimotsuki village and Zoro’s teacher of the sword.He taught Zoro how to cut steel. Cut without cutting. Listening to the rhythm. That shows me he is a very skilled swordsman at a higher level than Zoro up to at least the Water 7 arc. I bet One Piece fans really want to find out his true identity. Fortunately, it was revealed in SBS One Piece Volume 96.
Koshiro is originally the son of Shimotsuki Kozaburo, the sword maker who left Wano more than 50 years ago. Thus, the name of Zoro’s true teacher is Shimotsuki Koshiro and his father Shimotsuki Kozaburo who made Sword Enma and Wado Ichimonji. According to Hitetsu, Kozaburo is a legendary master swordsmith whose craftsmanship was known far and wide.
Kozaburo’s journey started 55 years ago specifically when he went out to sea and a great adventure took place for Samurai Wano. He arrived at an area in Eastern Blue, then decided to save people from bandits, he began to love a girl while he was training the villagers to use the sword, he decided to stay in the area.
He found a village that he named Shimotsuki after that Koshiro was born, then their granddaughter Kuina, and continues, stating that the elderly man in his village is actually Shimotsuki Kozaburo from Wano country.
Author Oda “Huh? Could it be …. connected to the Zoro dynasty?!” This will reveal the samurai that established Shimotsuki Village.