K.I.A

K.I.A is Alex's arch enemy and is almost impossible to defeat - especially if you're Alex. K.I.A was one of the MVP clones of Alex, but went rogue and was on constant killing sprees throughout America. He came back to England to kill Alex at StarGate base 2, but failed and was destroyed. Alex almost showed him mercy, but ultimately realised that however amazing this artificial human was, he was dangerous.
A few months after his destruction, he was brought back to life by Sonake, and made more powerful than ever before.
A future version of him once came back in time to stop Ender from being killed. So he's not all bad.