Kojiro Sasaki, a renowned swordsman from feudal Japan, perished in his legendary duel against Miyamoto Musashi. In the afterlife, he trained ceaselessly for centuries, aging beyond death to achieve his ultimate form through mental and spiritual refinement. This earned him the epithet "History's Greatest Loser," reflecting his lifetime strategy of deliberate losses to absorb every opponent's techniques. His appearance manifests prolonged growth: an elderly man with silver hair, a scar-etched body, and dignified presence, clad in red-and-orange flower-patterned haori and hakama while wielding the exceptionally long katana Monohoshizao.

His personality merges surface nonchalance with profound dedication. At Seigen Toda's dojo, he seemed lazy and unreliable—arriving late and forfeiting fights preemptively to avoid injuries hindering his long-term growth. This masked relentless discipline; he vanished into wilderness for self-guided training, learning from nature and animals before returning transformed. He prioritizes personal evolution over wealth, fame, or victory, refusing rematches against mentally surpassed foes. Unfussy in habits, he consumes unprepared wild game or distasteful foods like Brunhilde’s salmiak pies without complaint. In combat, he deeply respects rivals as essential catalysts for his growth, openly thanking them for honing his art.

Kojiro’s combat prowess hinges on supernatural prediction: **Senju Muso** ("Unmatched Anticipation of a Thousand Moves") simulates thousands of battles by analyzing opponents' physiology, movements, and environment. Against Poseidon, it evolved into **Manju Muso** ("Ultimate Vision of Ten Thousand Moves"), perceiving air and ground vibrations for near-omniscient foresight. His swordsmanship synthesizes styles from every master he faced—Chujo-ryu, Ittō-ryu, Shinkage-ryu, Yoshioka-ryu, Hozoin-ryu, and Niten Ichi-ryu.

Signature techniques include **Tsubame Gaeshi**, a downward slash instantly reversed upward to strike the blind spot (hitting objects at 200 km/h); **Torakiri**, a reverse-grip sideways slash; and **Niten Ganryu Style**—activated after his Volund split—which dual-wields katana and kodachi to unleash combined techniques like the dismembering **Swallow Tiger Combo, Thousand Blades**.

For Ragnarok, he bonded with Valkyrie Hrist, who became **Monohoshizao**—a divine nodachi twice a standard katana's weight. Shattered by Poseidon, it underwent **Re Völundr**, splitting into dual blades to enable Niten Ganryu. His physical capabilities, scaling to Poseidon's divine power, include universe-level durability and speed, deflecting thousands of god-speed strikes.

In Ragnarok's third round, Kojiro volunteered against Poseidon. Initially overwhelmed, he adapted via Senju Muso, evolving it mid-fight to counter techniques like **Amphitrite** and **Medusa Alope Demeter**. Bolstered by history's swordsmen during a crisis of doubt, he beheaded Poseidon with the same cross-slash that killed him in life—achieving his first victory and fulfilling his wish to "win just once." He collapsed post-battle, sustained by Hrist.

Later, he supported allies like Buddha against Loki and the Seven Lucky Gods, analyzing matches alongside fellow Japanese swordsmen. He noted parallels between Poseidon and Hades when the latter whistled a similar tune and explained concepts like Qi during Qin Shi Huang’s fight. Relationships include reverence for past rivals like Musashi and camaraderie with Einherjar such as Soji Okita.

Trivia notes his catchphrase "Even just once, I want to win" and appreciation for unconventional foods. His sword, historically named **Bizen Nagamitsu**, exceeded three shaku in length.