Cai Ze, also known as Sai Taku, is a significant political figure from the state of Yan who spends the majority of his long career in the service of Qin. He is introduced as a tiny, elderly man with white hair, a long beard, and a perpetually unreadable expression, always seen in the formal robes of a high-ranking official. His advanced age belies a remarkably sharp and calculating mind, honed over decades of service to four successive kings of Qin, from the era of King Sho to the reign of Ei Sei. This longevity is a testament to his adaptability and deep understanding of the currents of political power.

As a person, Cai Ze is highly intelligent and a master of layered communication, often cloaking his insights in wit and irony to prompt introspection or reveal uncomfortable truths without direct confrontation. He is a pragmatist who holds the philosophy of serving only the strong, yet beneath this practical exterior lies a more complex inner world. In his youth, he had hoped to forge a path to unify China that would reduce the immense bloodshed he foresaw. Over time, however, he became disillusioned, convinced that such an idealistic path did not exist. This changed when he witnessed Ei Sei's tenacity and vision, which convinced him that the young king possessed the qualities of a ruler that appears only once in a thousand years. This rekindled hope and became the driving force behind his final, most consequential actions.

Within the Qin court, Cai Ze holds the distinguished position of the highest-ranked and most senior diplomat. His role is that of a strategic linchpin, responsible for stabilizing ties with foreign states, particularly his native Yan, where his assignments frequently anchor him. He is also recognized as one of the Four Pillars within Ryo Fui’s powerful faction, a testament to his influence during that era of volatile power struggles. However, his ultimate loyalty shifts to Ei Sei, whose grand ambition for unification he comes to support fully. His most pivotal contribution occurs during the Coalition Invasion arc, where he is dispatched to the state of Qi. Leveraging his old friendship with the King of Qi, Ou Ken, he successfully negotiates Qi’s withdrawal from the coalition army by promising a greater reward than what the invaders had offered. This single diplomatic coup is a major turning point in the war.

Cai Ze's key relationships are deeply intertwined with his political maneuvers. He shares a long-standing friendship with Ou Ken, the King of Qi, a connection that proves invaluable for Qin. He interacts frequently with other powerful figures in the court, such as the strategist Ri Shi, whom he advises to be patient in his ambitions. His interactions with the military leader Shouheikun can be provocative, using humor to subtly highlight political fissures within the state's leadership. His relationship with Ryo Fui is complex; while a trusted pillar of his faction, he is not above maneuvering to ensure Ei Sei’s vision ultimately prevails.

The character's most significant development comes at the end of his life. Acting entirely on his own initiative, Cai Ze organizes a clandestine meeting between Ei Sei and the King of Qi. To arrange this, he takes the immense risk of paying a bribe to the state of Zhao and stopping all military actions on the Qin-Zhao border, actions which some officials in Qin decry as treason. Ei Sei, however, agrees to the meeting without hesitation. During this summit, Cai Ze witnesses Ei Sei convince Ou Ken of his path to unify China, securing a verbal promise from the Qi king that he will surrender when Qin's armies finally reach his border. Immediately after this monumental achievement, the elderly diplomat passes away, his life ending with one of the greatest successes of his long and illustrious career.

Cai Ze's notable abilities lie almost entirely in the realm of intellect and statecraft. His statistics reflect a master diplomat, with the highest possible rating in experience and exceptional scores in intelligence and leadership, while his physical strength is negligible. He possesses a shrewd and calculating nature, allowing him to navigate the most volatile political scenarios with precision. His true genius is in his diplomatic mastery, specifically his capacity to negotiate under pressure, exploit personal connections, and see the grand strategic picture that others miss. He is, ultimately, a perennial architect of Qin's ascendancy, whose legacy is securing a crucial alliance and setting the stage for the eventual unification of China under one ruler.