Keith Black is a central antagonist from the Project ARMS series, serving as a major figure within the secretive Egrigori organization. He is a clone, part of a series of genetically engineered beings all created from the genetic material of the original scientist Keith White. Originally, he was designated as Zero, one of the first clones brought into existence. In his early life, he was a timid and weak child who suffered frequent abuse from other clone batches in the organization's laboratories, finding solace only in his bond with his brother, Edward.

As he matured, Zero proved to be compatible with an Advanced ARMS, receiving the unit known as Humpty Dumpty in his right limb. Following this successful implantation, he took the name Keith Black. With his new power, he made a decisive play for control, murdering his genetic progenitor, Keith White, and seizing leadership of the Egrigori for himself. His personality transformed from a vulnerable youth into that of a calculating, erratic, and formidable leader. He is driven by what he claims to be the will of Alice, a powerful and hate-filled entity born from a dying girl's anguish, which seeks the end of the world. Black is utterly dedicated to bringing about this apocalyptic outcome, believing everything proceeds according to Alice's will.

In the story, Keith Black acts as the primary force opposing the protagonists, the young wielders of the original ARMS. He deploys his fellow Keith Series members, such as Keith Silver, Keith Violet, and Keith Red, to recover or eliminate the ARMS children. He is personally responsible for several pivotal events, including the murder of Hayato Shingu's father and the severing of Hayato's arm. His ultimate goal is to fully awaken the Jabberwock Program, an entity of mass destruction that would fulfill Alice's hatred for humanity. His most important relationships are fraught with tragedy; he was once close to his brother Edward, who later escaped the organization to become the renegade Keith Blue, setting him on a collision course with Black. He also holds a position of authority over the other Keith clones, who serve as his subordinates, though he is known to discard them when they are no longer useful.

Keith Black undergoes a significant, albeit complex, development. While he appears as an architect of destruction, it is later revealed that the consciousness of the original Keith White was absorbed by the Humpty Dumpty ARMS when Black murdered him. This remnant of the creator remained hidden within Black's psyche, subtly manipulating his actions toward its own apocalyptic ends. This revelation re-contextualizes Black's villainy, suggesting that his adult cruelty was not entirely his own. His final development involves a desperate internal struggle; aware of White's control, Black seeks to be killed by the very Jabberwock entity he helped awaken, hoping to destroy the program and White's influence. Though his body is eventually taken over by White, Black's lingering consciousness works from within, alongside his brother Blue, to ultimately weaken and defeat their creator from the inside.

Keith Black's notable abilities are derived almost entirely from his Advanced ARMS, Humpty Dumpty. This ARMS has the unique and fearsome power to absorb the qualities of anything it consumes, granting it the ability to replicate the special abilities of all the other ARMS units. This makes him an extraordinarily versatile and dangerous combatant, capable of countering nearly any power used against him. He is also shown wielding a bladed weapon called Fragarach. Despite his immense power, his abilities are inextricably linked to the parasitic consciousness of Keith White, which ultimately makes him as much a vessel as a wielder of his own strength.