Kiyomitsu Kashū began as an uchigatana forged in 1684 by a swordsmith sharing his name. He later belonged to the historical Shinsengumi figure Okita Sōji, an association that fundamentally influenced his identity and relationships. As the first sword awakened to human form by the citadel’s saniwa, he gained intimate knowledge of the saniwa’s preferences and daily operations. His origins in an impoverished forging environment instilled a conviction that maintaining an attractive appearance is vital to being cherished by his owner, driving his emphasis on aesthetics and personal presentation.

His tie to Okita Sōji forged a foundational bond with Yamatonokami Yasusada, another of Okita’s former swords. Together, they are known as "Okitagumi," characterized by frequent quarrels yet profound mutual reliance and friendship. When Yasusada arrived at the citadel in January, Kiyomitsu guided his adjustment to their new life. During a confrontation with the History Retrograde Army at Ikedaya Inn in 1864, Kiyomitsu shielded the injured Yasusada, ensuring their retreat. Afterward, he provided Yasusada a hairpin to repair battle-damaged bangs, cautioned against arrogance, and affirmed their shared reverence for Okita—blending practical aid with emotional support.

In daily citadel life, Kiyomitsu displays a lively, occasionally vengeful demeanor, exemplified when chasing those who hit him with snowballs. He conducts routine activities like guiding newcomers such as Tsurumaru Kuninaga and Dōdanuki Masakuni on tours, sometimes including Yasusada. His design features sleek black hair and a distinct aesthetic, reflected in merchandise with traditional motifs like lacquerware. Across seasons, films, and spin-offs, his narrative remains consistent without documented evolution or expanded background.