This is Pinoko, Black Jack’s assistant. Like many characters from this show, her background is complicated. At first I was kind of turned off by the contrast between her apparent age and her behavior. After a little research, the situation makes more sense. Her story is related in the television series (episode 44) and not in the OVAs.

Several years ago Black Jack had an eighteen-year-old girl referred to him as a patient. It turns out the girl had a huge tumor in her abdomen, called a teratoma. In this case, the teratoma was a remnant of a Siamese twin. This twin was self-aware, had a neurologic system and internal organs. Unfortunately, she had no skin and was stunted in growth. Black Jack surgically removes the teratoma but then gives her a manufactured body of her own. I am assuming that since the remnant was so small he had no choice but to put it into a small plastic casing of a little girl. Sadly, she is completely rejected by the other girl, who demands that he “throw it away”. Instead, he adopts Pinoko as his daughter. For the most part, she role-plays her apparent age in public, but in private or in emergencies her maturity comes out. For example, she has no problems acting as a surgical assistant for Black Jack or in keeping up their home while he is gone.