Rexford "Rex" Lewis, also known as Cobra Commander and formerly as the Doctor, is the main antagonist of the G.I. Joe film series, serving as one of the two main antagonists (along with Destro) of G.I. Joe: The Rise of Cobra and the main antagonist of G.I. Joe: Retaliation. He was a former military scientist who became influenced by the nanomite research from M.A.R.S. Industries, and thus, his thirst for knowledge gave him little regard for human life. He eventually took over M.A.R.S. and became the founder and supreme leader of the terrorist organization COBRA. He was a close friend of Conrad Hauser/Duke and brother to Ana Lewis, who later became the Baroness.