Millions Knives, originally named Nai or Kni, is the twin brother of Vash the Stampede. An Independent Plant born aboard SEEDs Ship Five during humanity's interstellar colonization, he and Vash were discovered and adopted by navigational officer Rem Saverem. The twins matured rapidly, appearing as young children within their first year. Knives demonstrated early proficiency in generating matter and energy, unlike Vash, who initially showed no apparent abilities. Despite their shared origins, Knives exhibited emotional reserve and skepticism toward human customs, viewing practices like eating as wasteful compared to Vash's enthusiastic embrace of human connection. His childhood interests included classical piano, played with Vash, and cowboy films, though he increasingly resented Rem's efforts to encourage human-like behavior, perceiving this as deceptive conditioning.

A pivotal shift occurred when the twins accessed restricted ship archives and discovered Tesla, an earlier Independent Plant subjected to decades of brutal experimentation and dismemberment by SEEDs researchers. Witnessing Tesla's suffering—and learning Rem had concealed this truth—shattered Knives' trust in humanity. He concluded humans were inherently exploitative and incapable of coexistence with Plants, cementing his belief that their extermination was necessary for Plant survival. This trauma contrasted with Vash's enduring faith in Rem's teachings, creating the brothers' ideological rift. Knives later used a password inadvertently revealed by Vash to sabotage the fleet's navigation, causing the catastrophic "Great Fall" crash onto the desert planet No Man's Land. He urged Rem to join their escape pod, but she chose to remain and save others. After planetfall, Knives laughed amid the wreckage, declaring Vash complicit in the genocide, which drove Vash to flee in horror.

In the years following the crash, Knives adopted the name "Millions Knives" as a tribute to the countless Plants subjugated by humanity. He located Dr. William Conrad, a surviving SEEDs scientist, and coerced him into researching Plant physiology. Knives later founded the Eye of Michael, a cult dedicated to his worship, and appointed Conrad as its high priest. His powers evolved from rudimentary blade generation to large-scale destruction, enabling him to annihilate settlements effortlessly. He systematically stole Plants from human cities, dooming inhabitants to resource scarcity, and framed Vash for these attacks to isolate him from humanity. Knives believed this would force Vash to acknowledge human cruelty and return to his side.

Knives' philosophy centers on humanity as a parasitic "virus" consuming Planets and their own world. He frames his genocidal mission as righteous liberation, invoking biblical rhetoric to portray himself as an avenging angel. His actions are driven by profound fear of human exploitation, protective devotion to Vash, and a pathological need to control his brother's choices. He dismisses Vash's pacifism as weakness induced by human manipulation, employing gaslighting, physical coercion, and strategic isolation to undermine Vash's relationships. This dynamic culminates in a confrontation aboard a SEEDs ship where Knives witnesses humans conducting a "Last Run"—forcing a dying Plant to expend its final energy reserves. After slaughtering the crew, he gives Vash a revolver, symbolizing humanity's detached cruelty. When Vash defends Luida, a human ally, Knives severs Vash's arm to halt his uncontrolled Angel Arm activation, deepening their enmity.

Knives' abilities stem from his Plant physiology. He generates monomolecular blades and chains from his body, often manifesting as feathered wings or prehensile appendages for offense and defense. He manipulates matter, creating barriers or reshaping his cloak into weapons, and exhibits heightened regeneration. His attire—a white biomechanical suit and gray cloak—mimics dependent Plants' forms, rejecting human aesthetics. A subtle mole beneath his right eye distinguishes him from Vash, who bears one under his left.

The July Incident marks Knives' most destructive act in this continuity. He crashes an orbital Plant station into the city of July, intending to demonstrate humanity's helplessness. After Vash redirects the resulting energy blast to minimize casualties, Knives attempts to fuse with him, seeking to harness Vash's unique ability to absorb matter. The fusion fails, and Knives is seemingly disintegrated by energy feedback, though his final fate remains unresolved.