Doctor Kiriko is a recurring character who serves as a powerful and ideological rival to the protagonist, Black Jack. He is often referred to as a doctor of death, as his medical specialty is the polar opposite of Black Jack's life-saving surgeries. Where Black Jack fights tirelessly to preserve life against any odds, Doctor Kiriko acts as an agent of euthanasia, helping terminally ill or suffering patients end their lives on their own terms to relieve them of their pain.

Kiriko's background is essential to understanding his grim philosophy. Prior to becoming a traveling euthanasia doctor, he served as a military surgeon, an experience that exposed him to immense human suffering and the horrors of war. During this time, he witnessed countless severely wounded soldiers with no chance of survival, many of whom begged for a release from their agony. In response, he began administering lethal injections as an act of mercy, an experience that profoundly shaped his future career. This past is often cited as the origin of his cynical and pessimistic worldview, transforming him from a doctor who once shared Black Jack's ambition to save lives into one who sees death as a welcome relief from a painful existence.

In terms of personality, Kiriko is typically portrayed as a morbid, brooding, and calm individual who carries himself with a grim seriousness. He is the complete antithesis of Black Jack, embodying a philosophy that in nature, creatures die when their time comes, and that a painless death is often preferable to a painful life. This moral opposition forms the core of their rivalry. Despite their conflicting ideologies, there is a mutual recognition of the other's skills, and on rare occasions, they have been known to work together when their goals unexpectedly align, though their interactions are usually marked by intense ideological clashes. For instance, Kiriko once sought to euthanize his own father, Dr. Jorujyu, who was suffering from an incurable disease, a plan that Black Jack directly opposed by intervening to perform a life-extending surgery.

Doctor Kiriko's role in the story is primarily that of a foil to Black Jack, constantly challenging the notion that life must be preserved at all costs. He forces both the protagonist and the audience to confront difficult ethical questions regarding the quality of life versus the sanctity of life. His motivations are rooted in a twisted sense of mercy, believing he is providing a final, compassionate service to those who are beyond the help of conventional medicine. He charges a fee for his services that is comparable to Black Jack's famously high prices, further cementing their mirrored relationship.

Kiriko has a distinctive and striking appearance. He is gaunt and pale, with long, stark white hair and a face that typically carries a somber expression. One of his most notable features is a black eyepatch, which he wears over an eye he is implied to have lost during his time as a military doctor in the Vietnam War. He is almost always seen dressed in elegant, dark clothing such as black suits and long coats, which adds to his ominous and funereal presence. He is also frequently seen carrying a large suitcase, which contains a specialized machine designed to painlessly and peacefully stop a patient's heart, providing a pleasurable death on demand. His primary ability is not surgery, but his expertise in the medical and ethical practice of euthanasia, a skill he performs with cold, professional precision.