Pinoko is a central recurring character in the Black Jack story, introduced as a medical marvel rather than an ordinary girl. She first appears as a teratoma, a type of tumor, that developed inside the abdomen of her twin sister. Within that growth, her brain, limbs, and internal organs existed in a scattered, unassembled state, and she survived inside her sister's body for eighteen years. During that time, she displayed extraordinary psychic abilities, using telekinesis to destroy surgical instruments and telepathy to drive doctors mad, because she feared that any attempt to remove her would end in her being discarded or killed. When she was brought to Black Jack, he persuaded her to allow the operation by promising to keep her alive in a culture solution rather than dispose of her, and after removing her he assembled the scattered parts into the body of a small girl, giving her a face and torso covered in synthetic fiber skin. Her name is said to derive from Pinocchio, reflecting the idea of a constructed being brought to life.

Pinoko presents herself as an eighteen-year-old lady and insists that she is Black Jack's wife, referring to herself as his okutan, while Black Jack regards and treats her as a daughter. Her physical form remains that of a young child, and her behavior and mental outlook often match that appearance, with a childlike, willful, and emotionally direct manner of expressing herself. She is nevertheless described as precocious and spirited, loyal, practical, and orderly. She speaks in a distinctive babyish dialect with mispronounced syllables and catchphrases like atchonburike, and her emotions run close to the surface, whether she is overjoyed, angry, jealous, or upset. She is deeply possessive of Black Jack and dislikes seeing him involved with young women, though she also channels that devotion into caring for him. Because her face and parts of her body are made of synthetic materials, she cannot swim normally, as the material absorbs water, and Black Jack once built her a heavily salted pool so she could float and swim for short periods. Her hair does not grow, and her face was modeled on a photograph of a girl who had suffered from a pollution-related illness.

Pinoko's primary motivation is her bond with Black Jack. She wants to be useful to him, to protect him, and to be recognized as his partner, and this devotion drives much of her behavior. She takes on nearly all household duties, cooking, cleaning, laundry, and answering the door, even though her small stature forces her to use stools and step ladders. Her cooking initially reflects her lack of knowledge, but she gradually becomes genuinely skilled, and her curry rice becomes a signature dish. She also loves animals and shows great compassion for injured creatures, at one point persuading Black Jack to treat a stray dog that then joins the household. In the medical setting, she serves as his constant companion and assistant during operations, preparing equipment, handing him instruments, and helping care for patients, and although she has no formal medical education, her observations and instincts sometimes give Black Jack the insight needed to save a life.

Her relationships are defined largely by this partnership. With Black Jack, she shares the closest bond in his life, and he is the only family she truly has. He once attempted to place her with adoptive parents because he feared leaving her alone if he died, but she returned of her own accord and ended up saving his life during a dangerous self-operation, after which he accepted her as his permanent assistant. Their relationship is warm but complicated, since she sees herself as his wife while he sees himself as her guardian. She also has a difficult connection to her twin sister, who rejected her after learning of her origin because of family reputation and social standing. Despite this coldness, the two once met without recognizing each other and briefly enjoyed a genuine sisterly bond, and her sister eventually provided a blood transfusion when Pinoko fell dangerously ill with leukemia. Characters like Dr. Kiriko and other doctors appear in her orbit mainly through their interactions with Black Jack, and she often reacts to them with the same blunt, emotional honesty she shows everyone else.

Over the course of the story, Pinoko develops from a terrified, disembodied creature into a capable and resilient companion. Soon after being given her body, she had to undergo strict rehabilitation to learn how to move and function, a process Black Jack pushed her through with considerable difficulty. Her knowledge of the world grows over time, from early moments of social naivety to a more settled competence in running the household and assisting in surgery. She also matures emotionally, showing courage in crises, comforting frightened patients, and proving repeatedly that her loyalty to Black Jack will not waver even when he tries to push her away or when circumstances separate them. In the final chapter, a dream sequence shows her as an elegant, grown woman, and Black Jack tells her that she is his wife and the best wife he could have, acknowledging the depth of their bond.

Her most notable abilities combine her unusual origin with her everyday skills. Before being given her body, she could use telekinesis and telepathy, though these powers are not shown afterward. In her assembled form, she relies on practical competence rather than superhuman gifts, functioning as a devoted nurse, housekeeper, and assistant whose quick thinking occasionally turns the tide of an operation. Because much of her skin is synthetic, some physical sensations differ from those of an ordinary person, and she has shown surprising resilience in dangerous situations. Above all, her defining ability is her unwavering devotion, which gives the often grim medical world around her a source of warmth, humor, and human feeling.