Keigo Oonishi is the leader of the Organo, the primary criminal organization that controls much of the city of Lux. He is a man in his early thirties who rose to his position of power through a combination of shrewd political maneuvering and personal sacrifice. In his youth, he was manipulated into selling his own legs to the enigmatic ruling class known as the Class, using the money as capital to climb the ranks of the Organo. These legs were replaced with texhnolyzed prosthetics, a fact that defines both his physical appearance and his unique connection to the city.

His personality is defined by a calm, collected, and often stoic demeanor. He is a man who prizes order and control above all else, believing that a fragile peace is preferable to the chaos of all-out war. Oonishi is a reluctant leader who prefers dialogue and negotiation to violence, a stance that is frequently mistaken for passivity or weakness by his more aggressive subordinates. Despite this preference for peace, he is not incapable of ruthless action; he will authorize brutal retaliations, known as "spectacles," when he deems it necessary to maintain the Organo's authority and the city's fragile stability. He is considered by some to be the most moral character in the series, a strong and capable man who commits his life to bringing order and liberty to a dying city, even if his methods are often unscrupulous. He maintains a respectful duty to the comatose head of the Organo, Hirohisa Gotoh, whom he visits regularly.

Oonishi's primary motivation is the preservation of Lux. He is driven by a sense of responsibility for the city and its inhabitants, believing he is the only one capable of holding the factions together and preventing the city's total collapse into anarchy. This commitment is tied to his ability to hear the "voice of the city," which manifests as visions or prophecies that guide his decisions. The source of this voice is later revealed to be a young seer named Ran, who is telepathically in contact with him. This connection grants him information no one else can access, making him a uniquely perceptive leader.

As the head of the Organo, Oonishi is the central figure of authority in Lux's underworld. His role in the story is to act as a stabilizing force against the various threats that emerge, including the surface world outsider Kazuho Yoshii, his own treacherous council members, and ultimately the Shapes of the Class. He serves as a mentor and benefactor to the protagonist, Ichise, after the young fighter is texhnolyzed. Recognizing Ichise's potential and will to live, Oonishi takes him under his wing, trying to channel the fighter's blind rage into a more focused purpose and later considering his debt to him repaid in full. His key relationships are largely political and fraught with betrayal. He is repeatedly undermined by ambitious subordinates like Mizuno and Kohagura, who see him as an obstacle to their own power and form alliances with his enemies to oust him. His personal life is isolated and cold; he is married to a catatonic woman, Mana, who shows him no affection, and his sole comfort is an affair with his secretary, Michiko Hirota. The death of his wife at the hands of Yoshii is one of the few events that breaks through his stoic exterior.

Throughout the series, Oonishi undergoes a significant development from a calculating political leader to a tragic, almost messianic figure. He loses his position, his family, and his city as the forces of the Class and the Shapes invade and destroy Lux. In the face of total annihilation, he abandons his political maneuvering and takes a direct, heroic stand. He personally leads what remains of Lux's men in a bloody, desperate battle against the Shapes, securing a costly victory. In his final act, he confronts the obelisk that serves as the city's heart. Following the voice of Ran, he fulfills her request to kill her by driving his katana into the obelisk, an act that brings an end to the city's suffering but leaves him vulnerable. He is then gunned down by an angry mob of Lux's citizens, who have turned against texhnolyzation. His body is destroyed, but his texhnolyzed legs remain standing upright, a final, haunting symbol of his unyielding will. His notable abilities include his expert skill with a katana, making him a formidable fighter, and the precognitive information he receives through his texhnolyzed legs, which allows him to hear the voice of the city.