Shimotsuki Koushirou

Koshiro is Roronoa Zoro's teacher and Kuina's father. He is the dojo Master of Isshin Dojo in Shimotsuki Village and he was the one who trained Zoro.

He is typically seen in flashbacks. However, in Chapter 440, he makes a brief appearance when another student of his asked if he was really Roronoa Zoro's teacher. He wished Zoro to go on ahead and fulfil his dream. It is implied he can also cut steel as he follows the method - Swordsman that can cut nothing can also cut steel and anything else they wish, he tries to teach this to Zoro as a child but it isn't until Zoro is 19 before he finally masters it.

Personality
He was a believer that females being naturally weaker then their male counterparts and therefore unsuitable to be warriors. Koshiro seems to appreciate talent as shown when he noted that as long as Zoro valued his Swordsmanship he didn't care if he had became a pirate. Although, he is always seen smiling but gives the impression of being a mostly emotionless person. Koshiro's reaction to his daughters death was just a comment that "humans are fragile" when telling Zoro that she died.

Abilities and Powers
Its been heavily implied that he is a swordsman capable of cutting steel as shown when he was able to slash at a piece of paper without harming it or at least he is aware there is a method of doing it.

Past
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, Koshiro helped the Revolutionaries granting them with food and supplies. He's seen talking with a man with a large face (probably Emporio Ivankov) and Dragon says the Army received food by 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 Wadō Ichimonji.

Several years later, Zoro leaves the village to become the world's best swordsman.

Post-Enies Lobby Arc
After the Enies Lobby incident, Koshiro was seen again in his dojo when his students were asking him about Zoro. Koshiro stated that as long as Zoro treasures his sword, it will not matter what person he becomes.

Two Years Later
After the news of the Straw Hats' spread across the world, Koshiro is seen next to Kuina's grave.

Anime and Manga Differences
In the manga, his actions are limited in 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.

In the anime after Zoro had a bounty, many students of Koshiro's school wanted to be just like Zoro. Koshiro is happy.