Kuroo Hazama, known as Black Jack, is a supremely skilled unlicensed surgeon. His distinctive appearance stems from a childhood landmine explosion that killed his mother and nearly destroyed his body. Reconstructed through extensive surgery by Dr. Jotaro Honma, he bears patchwork scars, mismatched facial skin tones from a graft donated by his half-African friend Takashi, and partially white hair due to shock. He retains these traits, refusing corrective procedures out of respect for his friend. The trauma of his mother's death and his father's abandonment fueled his lifelong resentment towards his father and those responsible for the explosion.

Operating outside the established medical system after his license was revoked, Black Jack defied superiors to perform unauthorized, life-saving surgery on Megumi Kisaragi, his romantic interest suffering from advanced cervical cancer. This act led to his expulsion. He established an independent clinic on a coastal cliff, adopting the alias "Black Jack" from his name "Kuroo" (black) and "Jack" (a common male name). He charges exorbitant fees, often millions of yen, primarily from wealthy or corrupt patients, yet frequently waives fees for those facing genuine hardship or injustice. A significant portion of his earnings funds charitable medical procedures or acquires land for environmental conservation.

His surgical abilities are legendary, handling complex, seemingly impossible cases rejected by others, including intricate organ transplants, limb reattachments, and treatments for rare or incurable conditions. He exhibits extraordinary precision, stamina—performing marathon surgeries solo—and adaptability, operating even in complete darkness. He wields surgical tools, particularly scalpels, for self-defense with remarkable accuracy. His practice extends beyond humans to animals and encounters with supernatural entities like spirits or an alien. His lack of a license stems from principled opposition to medical bureaucracy and corruption, not qualification.

Pinoko, a sentient being created by Black Jack from a teratoid cystoma and encased in an artificial body resembling a young girl, is his constant companion. She acts as his assistant and self-proclaimed wife, though he views her as a daughter. Despite occasional outward gruffness, he demonstrates deep care for her well-being. Pinoko provides essential emotional connection and domestic support, mitigating his isolation. Beyond Pinoko, he maintains distance from past patients and colleagues, valuing independence and anonymity, though possessing a network of contacts from his travels.

His personality presents a complex duality. Externally, he cultivates an image of coldness, formality, and greed through his imposing appearance, aloof demeanor, and insistence on high fees. Internally, he possesses a strong sense of justice, compassion for the suffering, and dedication to healing above all else. He despises medical corruption, environmental destruction, and abuse of power, often using his fees or influence to punish the corrupt or protect nature. Perceptive, he quickly diagnoses conditions and discerns patients' true characters. Generally stoic, he can display intense emotion regarding his past trauma, his mentor Dr. Honma, or threats to Pinoko. His quest for vengeance against those responsible for his mother's death represents a darker, relentless aspect.