Pinoko is Black Jack's sidekick and she usually claims to be his wife (which she is not). Despite her appearance she is actually eighteen years old even, though her body size looks like a toddler, making people disbelieve her when she says she is eighteen. She is known for her famous catchphrase, "acchonburike", which has been translated as "oh my goodness" or "ohmigewdness" in some English adaptions, but is devoid of any real meaning.
Pinoko is described to be loyal, handy and neat. She helps with house chores and sometimes assist Black Jack in surgeries whenever she can. Despite this, she is shown to have hatred to anyone who calls her a kid and will angrily claim she is eighteen years old. She also develop a romantic feeling for Black Jack which is shown throughout the manga and anime and even claims that she is his wife. Whenever Black Jack is with other women, she becomes very alert and will start asking questions, which sometimes annoy either Black Jack or the women. She is often also shown to be good emotional contrast to the more edgy Black Jack. She is sometimes emotional support in times of distress and has given the doctor something to care about.
Pinoko began life in her twin sister's stomach as a Teratogenous Cystoma. She was supposed to be born as one half of a pair of twins but was trapped inside her twin at some point when they are still in the womb. Since then, Pinoko starts to develop as a tumour inside her twin sister's stomach and lived there for eighteen years before being separated by Black Jack through operation. Before the doctors consulted Black Jack to proceed with the removal of the tumour, previous surgeons find it hard to operate on her sister due to Pinoko's curse by making the surgeons lose their sanity while saying: "Don't cut me!" The surgery has to be cancelled due to the problem and the doctors have no choice but to seek Black Jack's help in order to remove the tumour.
During the surgery, Pinoko starts doing the exact thing she does to the previous surgeons until Black Jack consoles her by saying that he will promise to keep her alive once she was removed from her twin sister's body. After trusting and agreeing with Black Jack, she lets him proceed with the surgery without disturbance and was kept inside the culture medium for some time after the removal. While her twin sister is taken back to the hospital, one of the doctors are horrified when they saw the tumour being kept inside the culture medium and asks Black Jack to throw it away but he refuses to do so saying that it was up to him to do what he wants. After they left, Black Jack proceeds another surgery by giving the tumour a body to make it look like a real human being and was named Pinoko afterwards.
While it's not shown in the manga, the anime shows Pinoko's attempts at getting used to having her own body, Black Jack refusing to help as he himself has to recover from his own operation as a child on his own. After an incident late at night, Pinoko manages to train herself to become a real human without Black Jack's help and quickly adapted to it. A year after their separation, the doctors brought Pinoko's sister back for the final checkup. Before leaving, Black Jack tries to introduce Pinoko to her twin sister, where, in a comical fashion Pinoko begins to jump on her sister, angry at the multiple attempts to get rid of her, causing her sister to disown her. However, in the anime this is changed to her she rejecting Pinoko right on the spot by saying that she has no twin sister. Pinoko, being sad and angry by this, starts to hit her sister and they quickly left the house, leaving Black Jack and Pinoko behind. Since that day, Pinoko became Black Jack's assistant and adoptive daughter.
Pinoko has orange short hair, a pair of orange eyes and a height of a toddler since she couldn't grow normally due to her plastic exoskeleton. She wears four pink bows on the back of her head, a red overalls with a yellow shirt inside, a pair of white socks and red shoes.
- Her twin sister's name is Yurie as mentioned in Black Jack Final OVA but was usually nameless in the manga and anime series. Her face was never shown in both the manga and the anime series as she wears the mask all the time.
- In the manga series, Pinoko develops leukemia from working too much and her twin sister came back to donate her blood to Pinoko since her blood is more compatible as they are twins.
- Pinoko's exoskeleton was inspired by a young girl who used to be a model. Black Jack mentioned he chose her because he thinks she looks cute. Also her having someone make a more human looking body for her is similar to the main protagonist of Dororo, who had to have heavy prosthetic body parts after having 48 pieces of his body taken from him before he was born.
- Due to her plastic exoskeleton, this limits some of her abilities like swimming and growing. Moreover, she speaks with a strong lisp due to her exo-skeletal skull.
- Pinoko is technically only a year old as stated in her birth certificate but she is eighteen since she grew together with her twin sister in her stomach for eighteen years, due to the time she spent as a parasitic twin.