Dr. Cain is the scientist and archaeologist who found Mega Man X on April 14th, 21XX. X was in a capsule buried deep underground in the abandoned warehouse of Dr. Light's ruined lab. He based the Reploids on X's design, creating the first on November 22nd. As Cain couldn't fully understand X's design, and the Reploids didn't have a diagnostic like X, the Reploids could have problems and started to become "Mavericks" (Irregulars). The Maverick Hunters were founded to counter the Mavericks. Sigma, Dr. Cain's most advanced creation, became the leader of the Maverick Hunters, and subsequently leader of the Mavericks. Cain Labs also created Dr. Doppler and the Repliforce. Dr. Cain is never mentioned again after Mega Man X4, it can most likely be assumed he died of old age or by other reasons.

In The Day of Σ, an unlockable movie in Maverick Hunter X, Cain has a life support to extend his life, with four tubes connected in his back. He was caught in an explosion of missiles that Sigma launched, although it is not made clear if he was killed in it. It is possible that he was meant to be dead in the new continuity that the remake would have established, but as this never came to fruition there is no official comment.

(Source: Mega Man Knowledge Base)