Knives Millions is the main antagonist of the anime Trigun and the older twin brother of the protagonist Vash the Stampede. Both are rare examples of independent Plants, a race of beings born from dependent Plant generators that provide energy and matter for human colonies. Knives and Vash were born on a Project SEEDS colony ship and raised primarily by a human crew member named Rem Saverem. As a child, Knives was analytical and reserved, though he initially appeared normal and shared a bond with Rem and Vash. However, he grew distant as he began to question his place among humans, and his worldview was shattered when he discovered that another independent Plant, Tesla, had been subjected to horrific experiments by the human crew. This revelation convinced Knives that humans were inherently cruel and exploitative, and he decided that humanity could not coexist with Plants. In response, he manipulated the fleet into crashing onto the desert planet known as No Man's Land, killing most of the human settlers and causing the death of Rem.

Knives is cold, calculating, and utterly convinced of the superiority of Plants over humans. He views humanity as a parasitic disease that must be eradicated to create a pure and peaceful world for his Plant brethren. He is prideful and austere, believing in the survival of the fittest and rejecting what he sees as Rem's idealistic nonsense. Despite his outward logic and emotional detachment, he becomes intensely emotional when it comes to Vash, experiencing a possessive and obsessive love that drives much of his actions. He seeks to persuade or force Vash to join his crusade, viewing Vash's pacifism as weakness and betrayal of their kind. When Vash refuses, Knives becomes determined to either bring him back by any means or, in some interpretations, absorb his power to prevent their conflict.

As the primary antagonist, Knives is the driving force behind much of the series' conflict. He orchestrates the crash of the colony ships, later forming the Gung-Ho Guns, a group of assassins and mercenaries, to hunt Vash and torment him. For much of the story, he remains in the background, regenerating after a catastrophic confrontation with Vash in the city of July, where both brothers used their Angel Arm abilities, resulting in the destruction of the city and Vash gaining his infamous bounty. His return in the final episodes leads to a climactic battle with Vash that ends with Knives being shot and immobilized, though still alive.

Knives's key relationships center on his twin brother Vash, who represents everything he despises: hope in humanity and a commitment to non-violence. Their ideological opposition forms the core tragedy of the series. Knives also has a complex relationship with Rem, whom he respects but ultimately rejects for teaching Vash what he considers dangerous ideals. He holds the loyalty of Legato Bluesummers, who leads the Gung-Ho Guns in his absence and reveres him as a godlike figure.

In terms of development, Knives starts as a curious and protective brother but hardens into a ruthless misanthrope after the trauma of Tesla's fate and his disillusionment with humanity. His hatred deepens over time, though in the original manga his character is more nuanced, showing apathy toward humans but genuine care for other Plants and a desire to protect Vash even at the cost of absorbing him. The anime portrays him as a more straightforward sociopathic villain, though his final defeat hints at the possibility of rehabilitation.

Knives possesses immense Plant abilities, including the power to generate blades from his body, regenerate from severe injuries, manipulate energy, and control other Plants. He can also fly and, at full power, unleash devastating energy blasts known as Angel Arms. His skills make him a formidable combatant, able to destroy entire settlements in moments. Despite his power, he can be emotionally vulnerable when his beliefs are challenged by Vash's unwavering compassion.