Black is an orphaned street child living with another boy named White in the fictional metropolis of Treasure Town. These two homeless, parentless prepubescent boys inhabit an abandoned car and call themselves "the Cats," asserting dominance over their urban environment through random violence and territorial control. Black is physically marked by a prominent scar across one eye and serves as the primary protector and fighter of the pair.

Exhibiting a hardened, street-smart personality contrasted with White's innocence, Black views Treasure Town as his territory and responds ruthlessly to perceived threats, engaging in violent confrontations with yakuza enforcers and other adversaries. His pragmatic nature drives survival skills like mugging and stealing, yet he displays unwavering devotion to White, whom he dresses, protects, and guides through daily struggles. This protective dynamic defines his existence, with one observer noting Black "has already lost all faith in the living. His only purpose in life is to protect White."

Black's development centers on his psychological unraveling after assassins severely wound White and authorities take him into protective custody. Separated from White, Black descends into madness and violent impulses, manifesting a destructive alter ego known as the "minotaur" – a monstrous embodiment of his inner turmoil, trauma, and capacity for brutality. This transformation represents his struggle between inherent darkness and residual humanity.

Reconciliation occurs when White returns. After a climactic confrontation where the minotaur persona threatens to consume him, Black overcomes this internal darkness through White's unconditional love and their restored bond. The narrative concludes with the pair playing together on a beach, their equilibrium regained. Throughout the story, their relationship is described as symbiotic: both are "flawed beings who can only be made whole by each other," possessing the "extra screws" the other lacks.