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Koshiro is a tall man with black hair tied back in a long ponytail. He has a wrinkled face, and wears glasses with circular frames. He always wears a gray yukata and sometimes he has a beige haori over it with two crossed swords on the left breast.
When Kuina was born, Koshiro looked younger, and less wrinkled.
He was a believer that females are naturally weaker than their male counterparts and are therefore unsuitable to be warriors. Koshiro seems to appreciate talent as shown when he noted that as long as Zoro valued his swordsmanship he did not care if he had became a pirate. Although he is always seen smiling, he gives the impression of being a mostly emotionless person. Koshiro's reaction to his daughter's death was just a comment that "humans are fragile" when telling Zoro that she died.
Abilities and PowersEdit
He is skilled enough with a sword that he can teach others to use it. He showed Zoro how to "cut without cutting" by cutting a piece of paper without slicing it in half - however, this was done by cutting with the blunt edge of the sword, and is therefore not proof of his swordsmanship.
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.
It is revealed that, at least once, Isshin Dojo helped the Revolutionaries by providing them food and supplies. A man with a large face (probably Emporio Ivankov) was seen and Dragon said that the army received food from the dojo.
One night, Koshiro discussed about Zoro's progress with a fellow teacher and was confident that Zoro would one day surpass Kuina. Kuina, having overheard this conversation, became outraged saying that she will be the world's greatest swordsman. Koshiro stated that as a woman, Kuina cannot be the world's greatest swordsman causing her to be distraught.
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 Wado Ichimonji.
Several years later, Zoro leaves the village to become the world's best swordsman.
Water 7 SagaEdit
Post-Enies Lobby ArcEdit
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.
From the Decks of the WorldEdit
From the Decks of the World: The 500,000,000 Man ArcEdit
Anime and Manga DifferencesEdit
At the end of the anime's flashback of Zoro's past, Koshiro was shown with bits of gray hair when Zoro left the village. Yet, his hair was fully black when Zoro received his second bounty. This is most likely due to him not yet having been seen outside of Zoro's flashback in the manga at the time.
In the manga, his actions are limited to Zoro's flashback while in the anime his role is expanded. Kuina's increased fighting potential is noted by him because of her rivalry with Zoro, while in the manga he makes no statement. His traditional values are also touched upon as he contemplated making Zoro his heir rather than his own daughter Kuina simply because of her gender.
- Koshiro made a very small cameo appearance in one of the "Boss Luffy Historical Arc" filler episodes.