Koushirou is a master swordsman and was the teacher of Roronoa Zoro. He is also the father of the late Kuina and son of Shimotsuki Kozaburo. He is the master of Isshin Dojo in Shimotsuki Village.
Koushirou has black hair tied back in a long ponytail, his face is wrinkled 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.
He was a believer that females are naturally weaker than their male counterparts and are therefore unsuitable to be warriors. Koushirou deeply 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. However, he was rather dismayed that the students of his dojo hoped to become great pirates like Zoro and started practicing Three Sword Style to better emulate him.
Although he is always seen smiling, he gives the impression of being a mostly emotionless person. Koushirou's reaction to his daughter's death was just a comment that "humans are fragile" when telling Zoro that she died, though it is possible that he was masking his pain.
Abilities and Powers
He is skilled enough with a sword that he can teach others to use it. Having learnt from his father a samurai, Koushirou has knowledge of swordsmanship techniques known only in Wano which he would pass on to Zoro.
One day, a young Zoro came to his dojo for a challenge. His terms were to fight with Koushirou, and that if he won, he would own the dojo. However, if he lost, he would become a student there. Koushirou, deciding not to fight Zoro himself, pitted Zoro against his daughter Kuina. Zoro accepted the change in the duel. Zoro lost to Kuina, and became Koushirou's student as agreed.
One night, Koushirou 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 would be the world's greatest swordsman. Koushirou stated that as a woman, Kuina cannot be the best sword's greatest swordsman, causing her to be distraught. 
Concludes non-canon section.
Water 7 Saga
Post-Enies Lobby Arc
After the Enies Lobby incident, Koushirou 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. Koushirou also stated that as long as Zoro treasures his sword, it will not matter what person he becomes.
From the Decks of the World
From the Decks of the World: The 500,000,000 Man Arc
Anime and Manga Differences
At the end of the anime's flashback of Zoro's past, Koushirou was shown with bits of gray hair when Zoro left Shimotsuki 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.
- Koushirou made a very small cameo appearance in one of the "Boss Luffy Historical Arc" filler episodes.
- Koushirou is possibly the last surviving member of the Shimotsuki bloodline.
- His favorite food is watermelon.