Sai Shou serves as Deputy Provincial Head of the Merchant's Guild in Sa Province and Prefectural Head in Kinka. He is the son of the Kinka Thaishu and has a twin sister, Sai Rin, the Provincial Head of the Merchant's Guild in Koren.

He first encounters Shuurei Hong during her travels, facilitating her introduction to the Merchant's Guild leadership. After Shuurei secures the guild's support, Sai Shou informs her of the recent Rin Clan murders, casting doubt on her pink-haired companion's identity. Shuurei departs without learning his true role. Later, during her recovery, Sai Shou formally reveals himself as Head of the Merchant's Guild, surprising her with his youth and capability. Rou Ensei then discloses Sai Shou's familial connection to Sai Rin.

Sai Shou negotiates a partnership with Shuurei and Ensei, pledging eighty percent of the Merchant's Guild's resources to assist newly appointed governors. This support is contingent on restoring peace to Sa Province and dismantling the corrupt Sa Clan's influence. He mobilizes the guild's mercenaries to stabilize Koren after the Sa Clan's selection ceremony.

Ordered by Sa Chuushou to forge a replica of the Sa Clan ring, Sai Shou betrays him, refusing on moral grounds despite his merchant background. He also acts as a discreet intermediary, delivering poison requested by Seiran.

Following the Sa Clan conflict's resolution, Sai Shou resigns from the Merchant's Guild. Recognizing his inherent inclination toward public service, he pursues studies alongside Rou Ensei to become a government official.

His personality is defined by calculation and pragmatism. He adheres to a principle of never fully committing his resources or abilities, always retaining a portion in reserve as a precaution—a merchant-oriented mindset. Yet he demonstrates integrity through actions like his defiance of Sa Chuushou.