Koushirou ran the dojo where Zoro grew up and trained under him as a kid. He's Kuina's father—the swordmaster whose daughter became Zoro's rival before her tragic death. Most of what we see of him comes through flashbacks, though he did pop up briefly in Chapter 440 when one of his current students asked if the famous pirate was really his former pupil.
Koushirou's teaching philosophy centered on a specific principle: a swordsman who can cut nothing can eventually cut steel and anything else. He drilled this into young Zoro, but it took years—Zoro didn't actually pull it off until he was 19. The implication is that Koushirou mastered the technique himself and understood its deeper mechanics in a way most swordsmen don't.
By the time Zoro left to chase his dreams, Koushirou had already given him everything he could. When they crossed paths again, he simply wished him well.
First Appearance: Chapter 5; Episode 19
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