Haganemaru

Haganemaru is the other main character in the manga acting as Jintetsus 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 fianc 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 Jintetsus weapon from then on out his single eye being embedded into the hilt of the sword.