If you keep saving butterflies, the spiders will die.

Knives is Vash's twin brother, and you could most definitely tell by the eyes. However, his hair is shorter and a bit lighter than his sibling's, and he has something of a crazed look in his eye at times.
Knives is still very cynical about the human race, believing that they are horrible creatures who claw anyone on their way to the top. Having been defeated by Vash in a sense has left him in a rather confused and devastated state, and his will to punish is severely diminished. Though he is in fact grateful that his brother spared his life, he taunts Vash constantly about still being weak, but his ways are beginning to change, if just by a little bit.
In contrast to Vash's playful and boisterous nature, Knives is quiet and observant. While Vash can be emotional and intense, Knives is contained and casual.
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Both Knives and Vash carry AgLArms .44 Colts, which Knives presumably made himself. These are twin guns, very heavy, very bad, and hold a special power core in the middle which reacts to the "Plant" Energy (Plants being, what Knives and Vash are made of). They form the arm holding the gun into the Angel Arm (above, right), which Knives used to destroy Julycity and Agusta. Knives has the ability to use both arms, while Vash does not, and even at 13% capacity produce devastating effects (such as the leveling of cities).
The angel arm, however, is hardly ever harnessed. It doesn't matter, though. The colts are lethal in themselves.
Like Vash, he is a sentient plant created by humans. He was abused by humans on the ship where they lived, and after seeing the disturbing experiments being done on other plants, decides that the human planet is grotesque and disgusting, and unfit for this world. He destroys most of the people in the SEED program and he and Vash set out on their own ways. Vash, completely in horror over Knives' killing (especially Rem, their surrogate mother), refuses to help him and instead says he sees the good in everyone. Eventually the two meet in the city of July, and the face off leaves Vash without his left arm and Knives in a very critical state, the city destroyed. He slowly regenerates over a period of time, and forms the Gung-Ho-Guns to act for him. After he revives and learns that he is slowly dying by using his abilities, he begins absorbing other plants into himself, which causes his powers to grow exponentially.
He and Vash have a kind of duel to the death, which he loses. His life is spared, however, and he is brought unconscious to the city with Vash, Milly, and Meryl. He joins them out of sheer loss for anything else to do.