Jōizumi Masamune is a Mitsurugi maiden, the physical embodiment of an ancient sword. She possesses crimson waist-length hair, pink eyes, and appears to be a teenager. Her personality radiates positivity and optimism; she speaks directly and honestly. Masamune possesses an overwhelming drive to help others, especially those wrestling with internal conflicts or dishonesty with themselves. This well-intentioned Chronic Hero Syndrome frequently plunges her into troublesome situations due to her compulsion to aid anyone in need.

She existed during Japan's warring era but retired after the Battle of Sekigahara, embracing the peaceful intentions of the "Oath of Hyakken" in Tsugaru. However, three centuries later, during an alternate Meiji Era, she was summoned back as a Mitsurugi to combat emerging threats endangering the peace, marking a departure from her peaceful existence.

Within group dynamics, she forms a Power Trio with Kuwana Gou and Goou Yoshimitsu, serving as one of the franchise's recognizable figures. Designated No. 057, she is a Fire-element sword of the Balanced type, embodying the virtue of Gratitude. She likes apples and dislikes insects. Functionally classified as a Starter Sword, she is typically among the first Mitsurugi acquired or encountered.

Her narrative emphasizes the transition from peaceful retirement back to active duty out of necessity. She operates under an unseen Master or Commanding Officer, carrying out missions summoned and empowered by this leader. Her actions consistently reflect her core optimism and drive to aid others within this renewed conflict.