Koshiro is Roronoa Zoro’s teacher and Kuina’s father.
One day, a young Zoro came to his dojo for a challenge. Koshiro pitted Zoro against his daughter Kuina and stated that if Zoro loses, he would become a student of the dojo. Zoro accepted the challenge and lost and became Koshiro’s student as agreed. On the day after Zoro and Kuina made their vow, Kuina had fallen down the stairs while trying to get a sharpening block and died. On Zoro’s request, Koshiro handed him the family’s Meito Wadō Ichimonji.
After the Enies Lobby incident, Koshiro was seen again in his dojo when his students were asking him about Zoro. He wished Zoro to go on ahead and fulfill his dream. Koshiro also stated that as long as Zoro treasures his sword, it will not matter what person he becomes. Zoro is grateful to Koshiro for teaching him the sword and giving him the Meito Wadō Ichimonji. He states that he can not lose this sword no matter what. In addition, he remembers his words while he is fighting a tough opponent or while he is on the verge of death. Although he didn’t understand it at the time, Koshiro taught Zoro that there is much more to being a swordsman than the ability to cut through anything. A sword that can simply harm anything isn’t a real sword, and there is no sword mightier than the one that protects what it wants to protect. Zoro later remembers Koshiro’s teachings during his battle with Daz Bones, and overcomes his inability to cut steel.