Ken Kanzaki is a detective within the Tokyo Metropolitan Police Department. Adopted by police officer Genichirō Kanzaki after being found as an infant beside his dying biological mother, Ken witnessed Genichirō's death during convict Makieda's prison escape when he was ten. In that moment, Ken used his adoptive father's magnum to shoot Makieda, saving his family; this firearm remains his weapon of choice.

Ken, alongside F1 racer Ryo Aguri and professional wrestler Go Goda, suffered mortal wounds protecting Jun Zaizen during a terrorist attack on a Zaizen Group event. All three underwent cybernetic reconstruction, forming the core of an armored police unit. Ken assumed the identity of Red Jack, initially keeping this double life secret from colleagues, though he later revealed his status to his superior in episode 30.

Personality-wise, Ken is cheerful yet quick to irritability, driven by a strong sense of justice. He possesses exceptional marksmanship skills. He shares a close bond with his adoptive younger sister Sayuri, whom he protects vigilantly, and partners with fellow officer Eriko Yoshizawa. He shows particular affection for his cybernetic police dog, Lander. Ken's rare RH minus T blood type becomes critical after he sustains secondary mortal injuries, requiring "Hyperization." This process enhances his capabilities but causes cellular necrosis due to biological incompatibilities, resulting in his upgraded form, Hyper Red.

A notable humanizing trait is his intense fear of snakes. His development involves confronting the criminal organization Id and its bioroid leaders, with personal connections emerging through Jun Zaizen—creator of Id's central figures. Family dynamics intensify when Sayuri develops feelings for Jun. Ken's evolution encompasses reconciling his human identity with his cybernetic enhancements, navigating the demands of secrecy, and managing the physical consequences of repeated transformations.