Haganemaru is the other main character in the manga, acting as Jintetsu's only true companion and also his voice. He was once a human being, the eldest son of Kanemasa Fujinami, and an accomplished swordsman. When his father was killed by a ronin over a minor disagreement, Haganemaru sought out revenge. Though he succeeded, he later died of his wounds. He left behind a fiancé named Oto, a childhood friend who he saw more as a sister than a lover. Somehow, his brain managed to persevere, and actually regained consciousness when his corpse was taken in by Genkichi. By this time, only his brain and one eye was completely intact, so Haganemaru asked the inventor to make him into a sword. He obliged and became Jintetsu's weapon from then on out, his single eye being embedded into the hilt of the sword. It is later revealed that Haganemaru can not actually speak without Jin in close proximity, as his mind is actually stored within the boy. So, in essence, both need each other to carry out a meaningful existence. Haganemaru often acts as a source of advice to the younger man, and also an extra eye to make up for the one the drifter lost. One chapter is dedicated to his past, where he tries to resolve his past with Oto, who had tried to kill herself upon hearing of her husband's death. Though she did not succeed, she was later taken in by a traveling show to act a "living doll" in their performance. It is later revealed she had become a serial killer, intent on murdering girls for the life she could no longer have. Haganemaru asks Jintetsu to kill her for him, as she no longer retained her sanity. Though, as fate would have it, she later dies from another's blade. With that, the sword resolves his past and past conflicts with him do not surface again.