Chika Naruse works as a nurse alongside her sister Hitomi in the same hospital department. She possesses vibrant orange hair styled in neck-length twintails, yellow eyes, and presents a teenage appearance, with recorded physical measurements of 79-51-80. Her standard professional attire is a nurse uniform complete with hat and apron.

Chika harbors significant envy toward her colleague Kaname, specifically desiring Kaname's curvier physique. This envy persists despite Chika witnessing Kaname's visible distress stemming from frequent workplace harassment, revealing Chika's occasional insensitivity in social dynamics. Her communication is marked by unfiltered bluntness; she speaks without restraint. When challenged about her lack of tact, Chika typically responds with childish sulkiness rather than introspection, projecting an overall demeanor of immaturity and naivety.

This inherent naivety proves pivotal when Dr. Makimura exploits it to manipulate her. Using a simple deceptive ploy, the doctor enrolls Chika as the fourth test subject in a clinical experiment without her full understanding or consent. The experiment tests specialized drugs—a blue version for males and a red version for females—aimed at treating reproductive impotence. Consequently, Chika's status shifts from medical staff to experimental subject within this framework.