Muramasa is a blacksmith and the maker of the famous Muramasa swords. He was a former Taishirou. Muramasa crafted Kyo's, Sasuke's and Benitora's legendary weapons. However, he carries a heavy burden of guilt for what the Mibu have done and, in Kyo, he saw the fall of the Mibu. He left the Mibu to teach Kyo, whom he loves as a son. (Kyo was then called the Demon Child due to the unknown manner of his birth and the fact that as soon as he was born, the Mibu stopped being able to have children.)

Muramasa married a woman named Mayumi and stayed with her and her younger sister, Mahiro, in the mountains. He finally finds happiness with Mayumi but it is short-lived, as she is assassinated. Mahiro at first believes it was Kyo and thus swears to kill him, but it is later revealed to be Chinmei.

Muramasa is several hundred years old, but appears young due to a technique called "Sei", used by Mibu elders to stay alive indefinitely. Unfortunately for Muramasa, even his Sei is being wasted away by a deadly illness. He has attained Satori, which allows him to read the thoughts of others. He is also a powerful swordsman, as strong as Kyo. The illness is later reveal to be called the Death Disease, a design flaw in all Mibu constructs. The disease disintegrates their bodies. After helping Kyo in his mission, he dies of the disease and crumbles away into the wind above his wife's grave.

Before catching the Death Disease, Muramasa was probably the most powerful Mibu. Even Kyo admits that Muramasa was above him.