Master is the consciousness of a man who died in a traffic accident and was reincarnated in a fantasy world as a sentient, magical sword. His original name, appearance, and personal details from his previous life remain unknown. In his past life, he was an ordinary Japanese salaryman in his thirties who enjoyed anime, manga, VRMMOs, and light novels. He had a friendly personality, liked eating curry, had some cooking proficiency, and had never dated. After reincarnating, he awoke on a pedestal in hidden ruins and gradually learned to move and fight using telekinesis, discovering he could absorb monster cores to gain new skills and increase his power. After building up strength, he left the ruins but became trapped in a mana-depleting zone for weeks. He was eventually found by Fran, a young black cat beastkin girl fleeing from slavers, and helped her escape. She named him Shishou, meaning Master or Teacher, and they formed a contract that shares his abilities with her.

In terms of personality, Master acts as a father figure and guardian toward Fran. He is protective, caring, and dedicated to her well-being, often prioritizing her safety and growth over his own needs. His motivations are closely tied to Fran's goals: he promises to remain her sword until she achieves her dream of evolving into a mightier beast, a feat never accomplished by her tribe. He supports her journey to become an adventurer and reclaim the honor of her people.

Master's role in the story is that of a narrator and co-protagonist. He serves as Fran's primary weapon, mentor, and companion, providing combat support, strategic advice, and emotional guidance. Their partnership is central to the narrative, with Master handling tactical analysis and ability management while Fran executes their plans in battle.

Key relationships include his bond with Fran, which is the emotional core of the series. He also summons a darkness wolf familiar named Urushi, who becomes Fran's companion and mount, forming a close-knit trio. Master develops a respectful working relationship with other characters such as the blacksmith Garrus, who identifies him as an intelligent weapon and maintains his blade, and the adventurer Amanda, who serves as a mentor figure to Fran.

Master undergoes significant development over the course of the story. He evolves from a confused, power-seeking entity into a selfless protector who prioritizes Fran's dreams and safety. His journey involves mastering his growing skill set while learning to navigate the moral complexities of the world. It is hinted that he was forged using the remains of the wolf god Fenrir, making him kindred to the Goddess of Chaos. At a key point, the Goddess turns Master into a cursed sword that can only be wielded by Fran until her death, binding their fates together.

Master possesses a wide array of notable abilities. He has telekinesis, which allows him to move and fight independently. He can absorb magic stones from defeated monsters to gain their skills and enhance his own power. He can create doppelgangers that take the form of his former human self. He has a lie-detection ability and a charm skill. Eventually, he gains the capacity to summon familiars such as Urushi. His blade is described as a beautiful white metal with a blue line, featuring a golden guard carved with a brave wolf and a red oriental ribbon.