Koushirou is Zoro's sensei that taught him swordmanship. He was also the father of Zoro's rival who died before he could beat her, who is the one that drives his goal to become the worlds best swordsman. He does have a smaller role, because he is mainly shown in flashbacks, but he does have an important role as he is a big part of who made Zoro who he is. He is shown in present day a few times keeping track of Zoro's path.