Knives Millions

He is one a sentient Plants and Vash's twin brother. He and his brother are both born during a mission to further the human race; they are taken care of by Rem until he disrupts the course of the ships, causing it to crash. He is the complete opposite of Vash; he believes in the survival of the fittest and doesn’t care if he has to kill hundreds of people, on the other hand, Vash believes all beings have potential for good and deserve life fittest. He discovers humans were conducting experiments on other plants, he decides that humans are evil and grotesque, and that they need to be eliminated. Vash refuses with his plans and seeks persuade him to stop, after not being able to stop him he realizes the only way to do it is by killing him.

In the manga many years after the ship crashed, the two brothers meet again this time in the the city of July. The battle leaves Vash without his left arm and Knives in a very critical state and the city of July totally destroyed. Knives slowly regenerates over a period of time. To prevent another battle that may leave him wounded and to kill Vash he forms the Gung-Ho-Guns an organization of highly trained assassins to act for him.

After reviving he learns that he is slowly dying due to his abilities, he begins absorbing other plants, which causes his powers to grow exponentially. He fuses with thousands of plants and is able to launch his "Ark", a floating ship designed to suck all of Gunsmoke resources therefore killing all humans. Knives will eventually fight Vash again, this time it will be their final battle, which results in the destruction of the “Ark”. After the battle, Knives is able to save Vash from the exploding Ark. Finally he turns into a tree.

In the anime, Vash locates Knives and the two have an impressive battle. Vash manages to gain the upper hand and defeats Knives by disabling him using Nicholas D. Wolfwood’s gun.