Young Knives, also known as Millions Knives or Knives Millions, is the twin brother of Vash the Stampede. Both emerged as sentient Plants aboard a SEEDS colony ship, artificially created and raised by Rem Saverem. As a child, Knives displayed kindness and curiosity about humanity but possessed a more melancholic and naive disposition than his brother. A pivotal trauma occurred when he and Vash discovered the abused remains of Tesla, an earlier independent Plant. Witnessing this cruelty shattered his perception of humanity. This ignited his intense distrust and hatred toward humans, whom he viewed as inherently exploitative and lacking self-control. He internalized his grief, often inflicting pain on himself alone while masking his anguish and plotting against humans.

In *Trigun Stampede*, further exposure to human brutality solidified his ideology. He observed the "Last Run" procedure, a final harvest where humans callously extracted energy from dying Plants aboard another SEEDS ship. Dr. William Conrad, though privately empathetic, failed to intervene, reinforcing Knives' belief in human apathy. Security officers explicitly stated Plants existed solely to serve humans, deepening his conviction. This culminated in a defining moment symbolized by a butterfly trapped in a spider's web. Knives killed the spider to save the butterfly, declaring it "the easiest way" since "you can't save both," foreshadowing his later belief that humanity must be eradicated to liberate Plants. He adopted the name "Millions Knives" to honor the myriad sacrificed Plants.

Knives' resentment manifested in violent action. After causing the SEEDS fleet catastrophe to prevent human colonization, he and Vash crash-landed on Gunsmoke (Noman's Land). He spent years attempting to recruit Vash to his genocidal cause, viewing humans as a "resource-hoarding, planet-killing intelligent virus." Their ideological rift became irreparable when Vash refused, leading to escalating conflicts. In one confrontation aboard a human ship, Knives slaughtered the crew and gave Vash a revolver from a dead guard, condemning it as a symbol of human detachment from killing. When Vash used his innate Plant ability, the Angel Arm, to protect Luida (a human reminiscent of Rem), Knives severed Vash's left arm, demonstrating his willingness to inflict physical and emotional pain to sever his brother's human connections.

Across adaptations, Knives' methods evolved but his goals remained consistent. He formed the Gung-Ho Guns, a group of assassins, to act during his regeneration after July's destruction. He absorbed other Plants to augment his power, causing physical changes like a black streak in his hair that spread as his life force depleted. His ultimate plan involved launching "The Ark," a mobile weapon designed to strip humans of all Plant-dependent resources, believing this would trigger their self-destruction and prove his point about their inherent nature. Despite his hatred for humanity, he retained a twisted affection for Vash, persistently trying to convert him and viewing their bond as the only meaningful connection. His final fate varies across narratives: he vanishes after a climactic battle or briefly finds peace with Vash before disappearing.