Kazuho Yoshii is introduced as a mysterious traveler who descends from the legendary surface world to the decaying underground city of Lux. He is a 28-year-old man with short brown hair, a mustache, and a gentle smile that contrasts sharply with his actions. He typically wears a blue hat, a blue golf shirt, an orange jacket, and orange pants, and he almost always carries a large backpack which holds supplies including a firearm. Physically, a significant portion of his body has been texhnolyzed, replacing his limbs with cybernetic prostheses.

Yoshii's background is defined by his origin on the surface, a world that has become stagnant and lifeless where people exist as empty shells, going through motions without genuine emotion or ambition. He found this existence unbearable and was determined to reach Lux, the underground city where humans still possessed raw sensation, passion, and conflict. Even if not officially selected to go, he was resolute in finding his own way down, which he did by traversing a massive staircase and airshaft.

His personality is a study in chilling contradictions. On the surface, Yoshii is unfailingly polite, soft-spoken, and rational, often wearing a calm, pleasant smile. However, this demeanor masks the nature of a sociopath. He is capable of cold-blooded murder without a change in his gentle tone, and he displays no scruples in manipulating or eliminating anyone who serves his purpose. He is a provocateur who finds genuine fascination in observing human emotions and reactions, particularly when they are heightened by violence and chaos. He presents himself as a calm, inquisitive observer, but he is an agent of anarchy who takes pleasure in watching the city tear itself apart.

Yoshii's primary motivation is to prevent Lux from suffering the same fate as the surface world. He believes that the people of the surface have become ghosts because they lost their passionate will to strive, change, and even fight. To combat this, he seeks to ignite a war among the factions of Lux, believing that by forcing the citizens to act on their most violent and desperate impulses, he can stir their passions and reaffirm what it means to be truly alive. For him, a society driven by intense conflict, however destructive, is preferable to one rendered catatonic by stagnation and apathy. While this goal is framed by some as a twisted form of well-intentioned extremism, a substantial part of his actions also seems driven by personal boredom and a desire for amusement, as he finds the volatile, emotional residents of Lux far more interesting than the passive ghosts above.

In the story, Yoshii serves as the primary catalyst for the initial collapse of Lux's fragile peace. After arriving and being guided to the village of Gabe by the seer Ran, he makes his way into the city proper. Using his charm and intellect, he gathers information from the city's outcasts, including the homeless and prostitutes. He then systematically sows chaos by assassinating key figures, planting bombs, and pitting the three major powers of Lux against each other: the mafia-like Organo, the militant Salvation Union, and the rebellious Racan. His actions are a deliberate attempt to create a city-wide spectacle of violence.

Several key relationships define his time in Lux. He meets Ran, who guides him to the city, and makes a specific request that she never reveal his future to anyone, including himself, indicating his embrace of a fate he cannot foresee. His primary antagonist is Keigo Onishi, the executive director of the Organo. Yoshii personally targets Onishi, murdering his wife Mana in a cold and brutal fashion to provoke him and force his participation in the escalating conflict. He also has a significant, though adversarial, encounter with the protagonist Ichise, whose raw, misdirected anger intrigues Yoshii before Ichise ultimately becomes his killer.

Yoshii's development across the series is not one of redemption but of revelation. His calm, mysterious exterior gradually peels away to reveal the ruthless, calculating mastermind beneath. His true nature as a sociopathic terrorist is unveiled as he moves from being a helpful stranger in Gabe to the secretive force destabilizing all of Lux. His character arc culminates not in the success of his plan, but in his death. He is punched by Ichise and sent falling from a great height. In keeping with his philosophy, he dies with a smile on his face, accepting his unexpected fate without regret. This death, treated with a surprising sense of somber tragedy, solidifies his role as a chaotic, messianic-like figure whose destructive influence reshaped the city forever.

Yoshii's notable abilities are rooted in his intellect, physical augmentation, and psychological manipulation. As a texhnolyzed individual, his cybernetic limbs grant him superhuman strength, allowing him to deliver kicks forceful enough to be fatal. He is a highly skilled marksman, proficient with both handguns and sniper rifles. His most dangerous ability, however, is his charismatic and strategic mind. He is a master manipulator who can seamlessly blend in, gather intelligence from the lowest members of society, and orchestrate a multi-sided war without any of the factions realizing he is the common enemy. He is a man who wages psychological warfare with a calm smile, proving that in the world of Texhnolyze, a placid demeanor can conceal the most monstrous of intentions.