Knives, through most of the anime series, is not shown. You don't meet him until episode seventeen, and then only in Vash's memory. He's Vash's twin, but unlike Vash, he doesn't really see pacifism as the way to go. He wants to kill humans because he sees them as a bacteria that is infesting the Universe, and he wants to create his own little Eden for him and Vash on the planet using the Plants.
His whole reasoning is shown in episode seventeen when he kills a spider so that the butterfly in the spiders web can live. Vash doesn't take this too well, so they fight about it. Meanwhile, one of the SEED crew members is brutally abusing Vash, and Vash tells Knives. Knives goes on to make sure that he's frozen, but then somehow convinces Rowan, the science officer, to kill him. Rowan proposes to Mary, but when Mary finds out, she obviously does NOT want to marry him. So Rowan pulls out his gun and threatens Knives.
Knives is a powerful pyschic, even more powerful than Legato. Behind the scenes he controls the Gung-Ho-Guns and commands enormous respect from Legato. In fact, Legato is totally and completely BLIND in his adoration of Knives. Knives is a very good fighter, and he has the benefit of TWO Angel Arms, while Vash only has one. He was also the one who blew off Vash's other arm trying to get Vash to side with him and destroy humankind. Originally Knives had possesion of both guns, then he gave one to Vash, then Vash took BOTH of them, then Knives blew off Vash's arm and got the black gun, while Vash has the silver one.
In the manga, Knives shows up just before Maximum, reborn from the Plant(The big light bulb thing) When he first appears, he's buck-naked, and he literally crushes Legato in two. If you look, there's one half of Legato on top, and the other half kinda wedged underneath. He's taunts Vash and forces him to grow his Angel Arm.
