Kyūtarō Ōba is initially introduced as a minor figure within the Kifūken school, a swordsman who handles its administrative tasks and appears unassuming and unremarkable. As the series progresses, he is unveiled as the primary antagonist and is himself a shokujinki, the very type of man-eating monster the Kifūken is sworn to hunt. In his younger years, Ōba was depicted as an innocent and kind-hearted individual. However, witnessing the brutal carnage wrought by another character in pursuit of power deeply traumatized him and fundamentally reshaped his personality, leading him down a path of cruelty and manipulation.

Ōba's precise motivations remain somewhat ambiguous throughout the story. He claims that his actions are driven by a desire to protect his illegitimate son, Bon Jōji Ōba, a gentle giant who is an outcast due to his size. Ōba states he wants his son to be strong and avoid being mocked by others. Another driving force appears to be a ruthless ambition for power and control. Additionally, Ōba harbors a long-standing, unexpressed attraction to Harumi, the elder sister of Yuka Kamitsuki. This attraction festers over time and warps into a sick and obsessive fixation; after Harumi's death, he preserves her severed arms and creates multiple clones of her body in a grotesque attempt to possess her.

In the story, Ōba initially positions himself as a helpful advisor to Kazuma Momota, the young and ambitious new leader of the Kifūken. He encourages Kazuma's aggressive modernization of the organization while secretly orchestrating a larger scheme. His role is that of a hidden puppeteer who manipulates events from the shadows to further his own ends. He deliberately exposes Kazuma to substances that trigger the latent shokujinki instincts within him, effectively corrupting and destroying the young leader from within. His ultimate plan involves seizing control and reshaping the world according to his twisted vision.

Ōba's key relationships are central to his character. His relationship with his son, Bon, is complex; Ōba claims his cruelty is for Bon's benefit, but Bon himself rejects his father's methods and ultimately aids the protagonists in stopping him. His obsession with Harumi defines his descent into depravity. He also interacts antagonistically with the protagonist, Toshihiko Momota, whom he seeks to destroy as an obstacle to his plans, and with Yuka, whom he subjects to horrific experiments.

Throughout the series, Ōba undergoes a transformation from a quiet administrator into a monstrous and calculating villain. His past innocence is completely eroded by trauma and obsession. As a shokujinki, he possesses the abilities common to his kind, including immense strength, the capacity to transform into a bestial form with devastating claws, and a need to consume human flesh. He is also highly intelligent and deceitful, using his administrative position and unthreatening demeanor to conceal his true nature and long-term strategy. His most notable abilities are not just physical, but also his skill in manipulation and his deep understanding of both the Kifūken and the shokujinki, which he uses to mastermind his schemes.