Gyousou Saku is a central figure in the kingdom of Tai, first introduced as its most famous and respected general. He is a tall man with long white hair typically tied back and striking crimson eyes, presenting a powerful and immediate physical presence that reflects his formidable reputation. His true name is Sou, though he is widely known by his title, Gyousou, and he hails from Kouki Province. Prior to his ascension, he was a Sen-nin, an immortal, having earned a place on the divine registry, and was already a celebrated warrior of significant renown across the twelve kingdoms.

His personality is complex, defined by a blend of martial hardness, deep-seated pride, and a genuine capacity for compassion. Gyousou is the epitome of a warrior, possessing a powerful, almost overwhelming aura that others find intimidating. Those who encounter him often describe this presence as an ambitious fire or a fearsome, predatory force, dangerous yet compelling. He is not arrogant in a boastful manner but possesses a profound self-assurance rooted in his own competence and achievements. This self-respect manifests as pride, his primary character flaw, which is so integral to him that he was prepared to resign his immortality and his rank as a general if he was not chosen to be king, viewing it as a necessary lesson in humility. Despite his fearsome demeanor, he is capable of great warmth and politeness, treating friends and strangers with courtesy. However, he is easily offended by dishonorable behavior and does not hesitate to use decisive action against those who cause disturbances.

Gyousou's core motivation is the prosperity of Tai, a kingdom left bankrupt and fractured by civil war due to his predecessor's extravagance. His driving ambition is to restore the kingdom to a state of peace and strength, and he believes himself to be the one capable of doing so. This ambition leads him to ascend Mount Hou as a candidate to become the next king. The selection process is complicated by his relationship with Taiki, the young kirin of Tai, who is tasked with choosing the new monarch. Taiki is initially terrified of Gyousou, halting the ceremony upon their first meeting. This fear stems not from cruelty but from Gyousou's sheer intensity. Through conversations and a youma hunt together, Gyousou directly asks Taiki why he is feared, learning that Taiki sees him as fire: mesmerizing and necessary, but also undeniably dangerous. Satisfied with this answer, Gyousou accepts it, showing his willingness to understand his own perceived failings.

Ultimately, Taiki recognizes that his fear was precisely how he knew Gyousou was the right choice, and he selects him as the King of Tai. Upon his ascension, Gyousou demonstrates immediate and decisive leadership, drastically reducing royal expenditures to manage the kingdom's dire finances. He also shows a softer, more thoughtful side in his new role. As a mark of deep affection and hope for a new era, he gives Taiki the nickname Kouri, the name of a high peak where the dead are believed to walk. He balances his own martial nature with compassion, such as when he summons Keiki, the kirin of another kingdom, because he is worried about Taiki's depression and works to ease the young kirin's conscience. In his rule, Gyousou learns to temper his instinctive, pride-driven decisions, coming to see Taiki as a necessary restraint on his own ambitions and relying on his counsel.

His development traces a path from a prideful, albeit honorable, general to a more measured and wise king. While he retains his powerful presence, his ascension tempers his arrogance. He learns the value of partnership and restraint, moving from a man ready to abandon everything if not chosen to a leader who actively seeks to balance power with compassion for his people and his kirin.

In terms of abilities, Gyousou is one of the twelve immortal kings. As such, he heals from nearly all wounds rapidly and cannot die from starvation, thirst, or injury; he can only be killed by decapitation with a specific enchanted weapon known as a sword of the underword. His health is also mysteriously linked to that of his kirin, Taiki, and he would die within a year if Taiki perished. Aside from his immortality, his only supernatural trait is a divine aura that is perceptible primarily to his own kirin. His true power lies in his martial skill, as he is widely considered one of the finest swordsmen in the world, second only to the ancient King of En. Despite these abilities, Gyousou eventually disappears and is believed by many to be dead, though the circumstances of his fate remain unknown.