Pinoko began as a teratoid cystoma, a parasitic twin developing inside her sister's abdomen for eighteen years. During this period, she formed a complete brain, nervous system, and organs but remained an unstructured tissue mass. Dr. Black Jack surgically removed her during his operation on her twin sister, reconstructing her viable tissues into a functional humanoid body using synthetic materials. He named her Pinoko after Pinocchio. After creation, she adapted independently to her new form, learning to walk, talk, and perform tasks without Black Jack's aid, including escaping a fire that spurred her mobility.

Her appearance resembles a child aged five to eight, though she insists she is eighteen based on her developmental period. Her synthetic body prevents physical growth, causing frustration due to her wish for an adult form to marry Black Jack. Her facial features were modeled after a girl with a lung ailment from Black Jack's medical journal. She lives as his assistant and self-proclaimed wife, handling domestic duties like cooking and cleaning while aiding in medical procedures. Her personality blends loyalty, practicality, and orderliness with impulsiveness, jealousy, and emotional volatility. She reacts angrily to being called a child or mistaken for his daughter, asserting her age and marital status. She exhibits fierce protectiveness toward Black Jack, lamenting his absences and attempting rescues during perceived dangers. Her jealousy surfaces around other women, leading to confrontations or accusations of infidelity. Her emotional nature contrasts with his stoicism, sometimes evoking his compassion but also becoming a target for his frustrations, prompting tears, arguments, or attempts to solve his problems.

Her behaviors include childlike interests: overeating (often causing stomach aches), craving sweets (ordering massive sundaes like "Pinoko's Special"), watching television, playing with friends like Sharaku, diary writing, taking self-portraits in modeling poses, experimenting with makeup and fashion, and writing love letters to Black Jack (sometimes requesting spelling help). She uses unique exclamations like "acchonburike" (meaning "Oh my goodness") and "Aramanchu" ("okey dokey"). Despite her size, she displays resilience and combat ability. She enjoys accompanying Black Jack on trips, treating them as vacations for sightseeing, hot springs, and hotels, and decorates spaces with ornate styles.

In the short film *Dr. Pinoko no Mori no Bōken*, she attempts to deliver Black Jack's forgotten medical bag but gets lost in a forest. Guided by a butterfly, she encounters sick animals and independently treats them using medical knowledge gained from observing Black Jack, demonstrating competence and initiative. Across other media, she endures kidnapping, gunshots, poison, car crashes, and leukemia. Her origins cause psychological conflict; she harbors deep resentment toward her twin sister for attempted rejections, leading to physical attacks upon meeting. One episode shows her mistakenly believing a fictional letter claiming Black Jack stole her, prompting an intoxicated search for her "real" family and subsequent punishment. She exhibits occasional telepathic abilities, initially controlling surgeons during her extraction and later communicating with another teratoid cystoma. Her interests extend to karaoke and boxing in OVAs, and she bonds closely with Sharaku, teaching him to confront bullies.