Sayuri Kanzaki is the adoptive younger sister of Ken Kanzaki, the series' protagonist, with no biological relation. Ken was adopted by police officer Genichirō Kanzaki after being discovered alongside his dying biological mother. Genichirō and his wife Kazumi raised both children as their own.

She shares an intensely close bond with Ken, exhibiting fierce protective instincts toward him. This bond endures despite Ken concealing his secret identity as the cyborg Red Jack and his perilous missions with the Metal Jack unit from his family.

A defining trait is her developing romantic feelings for Jun Zaizen, the young heir to the powerful Zaizen Group conglomerate. These emotions remain a consistent thread throughout her character arc.

Her infancy included a traumatic event: Criminal Makieda, harboring a grudge against her father Genichirō for a past arrest, broke into the Kanzaki home shortly after her birth. Makieda took infant Sayuri hostage and mortally wounded Genichirō during the confrontation, though a young Ken ultimately intervened to stop him.

Her anxiety for Ken's safety occasionally drives her to interfere with his police duties, particularly when he handles firearms. This protective impulse sparks conflict, including an incident where Ken physically restrained her to proceed with his obligations.