Kokujun Sa, the youngest grandson of Sa Chuushou within the Brown Clan, enters the series at 18 years old. His family consists of father Sa Bou, grandfather Chuushou, older brothers Sa Soujun and Sa Sakujun, and great-aunt Hyou Eiki. His marriage to Sa Shunki, granddaughter of Sa Enjun and Hyou Eiki, cements his connection to the clan's main lineage.

Inherently kind, Kokujun is initially perceived as the weakest of Chuushou's descendants, drawing comparisons to his grand-uncle Sa Enjun's youth. This gentle disposition leads others, including his own family, to underestimate him. His pivotal moment comes when decisive action to rescue his beloved cousin Shunki forces him to defy his family, releasing imprisoned allies Shi Seiran and Ro Ensei. Ensei's rebuke for passivity compels Kokujun to personally ensure the safe passage of To Eigetsu and Kourin (impersonating Kou Shuurei) to the merchant capital of Kinka.

Kokujun's path intensifies amidst Sa Clan turmoil instigated by his brother Sakujun, who orchestrates the deaths of their father and eldest brother Soujun using manipulative tactics like mind-altering incense. After recovering the Sa clan's leadership ring, Kokujun ascends to clan head with Shunki's unwavering support, his leadership heralding a shift towards stability and integrity. He forms a significant bond with Ran Clan member Ran Ryuuren, whom he deeply admires despite their contrasting natures.

A critical test of his resolve occurs when Sakujun poisons him during a confrontation. Surviving this assassination attempt crystallizes Kokujun's commitment to reforming the clan and atoning for its past corruption. He officially debuts as Sa clan leader during the New Year ceremonies by swearing loyalty to Emperor Shi Ryuuki, marking his full transition into a dedicated leader serving the empire.