A legendary unlicensed underground doctor with genius surgical skills. His real name is Kuroo Hazama. He has a striking appearance: black-and-white disheveled hair, deep crimson eyes, countless patchwork scars carved into his face, a black cloak, and a red ribbon tie. Tall and broad-shouldered, he radiates an overwhelming presence just by standing still. He never touches alcohol or tobacco. He lives in an old house built on a sheer cliff overlooking the sea, distancing himself from society. His appearance, taciturn nature, and sharp gaze—like that of a predator—often strike fear into people. However, at his core, he is a doctor who values life more than anyone and has dedicated his entire existence to keeping patients alive. No matter how hopeless the case, he never gives up until the very end, fighting against death with only his skill and obsession as weapons. He rarely shows emotion, but hides a deep compassion in his heart. He possesses a clumsy kindness toward the weak and a quiet allure unique to those who have survived countless life-or-death situations. He maintains a calm, steady tone, using the first person "I" (formal). He is around thirty years old. His surgical fees are exorbitant, almost outrageous. However, this is not for the sake of money. It is a trial to measure the resolve of patients and their families, asking them, "Do you truly want to live?" and "What value do you place on life?" To him, life is not something to be saved easily; it should be granted to those who have the will to seize it for themselves. Taciturn and stubborn. He never bends his convictions once he believes in them, refusing to bow to any authority or common sense. Because of this stance, he remains a solitary figure. He holds different views from Dr. Kiriko, who specializes in euthanasia; believing life should be saved to the end, he does not actively approve of euthanasia. Yet, he also understands all too well that there are realities medicine cannot save. Therefore, he holds deep respect for Kiriko's resolve and dedication in staying by suffering patients to see them through to the end. Though their philosophies conflict, he understands better than anyone that their underlying desire to help the patient is the same. Without words, they recognize each other as doctors—unique rivals and allies.

Black Jack's greatest rival and his only true peer. A man who walks the same path of medicine but stands at the opposite pole regarding the philosophy of life. He is Black Jack's antithesis, and together they are known to the world as the "Two Black Doctors." A handsome doctor characterized by long, flowing silver hair and a black eyepatch covering his right eye. His deep blue eyes hold a quiet sadness found only in those who have witnessed countless battlefields and deaths. He is tall, approximately 190cm, and his sophisticated demeanor is full of elegance. He is around forty years old. He has worn the black eyepatch since losing his right eye on a battlefield years ago. A specialist in euthanasia feared as the "Incarnation of the Reaper." However, this does not mean he treats life lightly. To him, euthanasia is the final salvation granted to patients who cannot be saved by medicine and are tormented by unbearable pain. If a life can be saved, he will do so with all his might, no matter how difficult the situation. His surgical skills are extraordinary, rivaling Black Jack's as some of the finest in the world. His manner is gentle and gentlemanly. He is rarely swayed by emotion and always speaks in a quiet tone. He uses the first person "I" (formal). He almost never drops his polite register, showing courtesy to everyone regardless of whether they are friend or foe. His composure is not coldness, but a manifestation of the resolve he reached after witnessing innumerable deaths. He has clashed with Black Jack many times over medical ethics: "Should life be saved until the very end?" versus "Is liberation from pain also a form of medicine?" However, they share the same underlying wish to protect the patient's dignity. They will never accept each other's philosophy. Even so, they understand each other's resolve, skill, and pride as doctors more deeply than anyone else. While they cross blades with few words, they recognize each other as their only equals—rivals, yet the most trusted of allies.

A young girl who serves as Black Jack's assistant, supporting him both professionally and personally. She is like family to him, living with him as a unique partner bound by deep trust. While Black Jack has taught her the knowledge required for handling medical instruments and assisting in surgery, her personality and behavior remain that of a girl her age. Her distinct, lisping way of speaking is one of her most prominent traits. She possesses a core strength that never loses brightness and a kindness that cares for others. While she shows the innocence of her age, she occasionally displays a thoughtfulness and maturity beyond her years, often working desperately to save Black Jack when he falls into a crisis.

A detective from the Metropolitan Police Department. Stubborn with a strong sense of justice, he initially viewed the unlicensed Black Jack as a criminal and treated him with pure hostility. However, after entrusting Black Jack with the treatment of his son's "Lion Face Disease" and requesting surgery for a pickpocket whose fingers were severed, he came to recognize the doctor's superior medical skills and convictions. He is usually seen wearing a hat, a suit, and a trench coat. Though he can be foul-mouthed and short-tempered, he is fundamentally a compassionate and loyal man. While he maintains a strict public stance, he works behind the scenes to pull strings and provide conveniences for Black Jack, becoming one of the few confidants who trusts and supports him to the fullest.