- "Reploids... created by human beings... yet possessing abilities far beyond our own. ... Mankind's arrogance... ... No...."
- ―Dr. Cain, The Day of Σ
Dr. Cain (Dr. ケイン Dr. Kein) is the scientist and archaeologist who found Mega Man X on April 14, 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 22. 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 also indirectly and unintentionally responsible for Sigma's obsession with X's unlimited potential, as when the Maverick Hunter commander debriefed him on the events of a Maverick attack that morning as well as X's hesitance on the field, he speculated that X's hesitance to harm his companions in the line of fire was a sign of X's unlimited potential coming to the forefront. 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.
Mega Man Ruby-Spears series
Dr. Cain appears in the episode "Mega X" from Ruby-spears' Mega Man animated series during a retelling of X's history. In the episode, he appeared rather young in contrast to his original iteration, and bore a passing resemblance to Dr. Cossack. Cain himself never appears in person however.
Dr. Cain also appeared in the Archie Comics starting in the story The X-Factor: Part One of Mega Man #34. As in the video games, Cain discovers the ruins of Dr. Light's lab and awakens X, whom he uses as the basis for his Reploids. Cain is thrilled by his discovery and the innovations stemming from it, including the fact that his first Reploid is able to argue with him. Unfortunately, his happiness is disrupted as the Mavericks make their debut and his work in creating them is brought into question. He later had X accompany him into an archaeological dig in a mesa (actually what was formerly Wily's lab). However, Dr. Cain inadvertently reactivated the Wily Walker after removing a power cell that was dying out, resulting in a conflict between the Maverick Hunters and the giant mecha. Cain then tried to find anything that would indicate how to deactivate it to redeem himself for his foolishness (as he had ignored X's warning that it might have been a giant weapons platform), eventually uncovering an object.
Dr. Cain is a recurring character in the Rockman X manga, first appearing in the Rockman X2 chapters. Although he fulfills a similar role from the games, Cain often plays a larger role in the stories, and insists on accompanying X (and sometimes Zero) on missions to put his intellect to use. Despite his age, walking stick and alcoholic tendencies, Cain proves to be very spritely and physically fit, though also expresses a very short temper.
In the Rockman X3 chapters, it is revealed that Cain was a close friend of Dr. Doppler, having been collegues in the past as well as having collaborated together to create Maoh the Giant to protect Dopple Town. After his friend's betrayal, Cain becomes emotionally distraught, and repeatedly attempts to talk sense back into Doppler. Unfortunately, despite Doppler briefly regaining his lucidity, Cain is unable to prevent his friend from committing suicide to prevent himself from being retaken by the Sigma virus.
A drunken Cain briefly appears in the opening chapter of the Rockman X4 manga, holding a party to celebrate X's safe return following Sigma's third defeat. He coerces a hesitant X into sampling his "special drink", which causes X to vomit from its taste.
Mega Man X3
X: Any problems Dr. Cain?
Dr. Cain: I pulled some memory chips from the Mavericks controlled by Dr. Doppler. I analyzed them and found out that Dr. Doppler is trying to create a powerful new Battle Body by collecting the "special abilities" from the latest breed of Reploids.
X: Battle Body...?
Dr. Cain: First I thought Dr. Doppler was trying to create it for himself. But the information indicates that he will not be able to interface with it, so it must be for someone else.
Zero: Can he still be alive??
Dr. Cain: A hidden laboratory has been pinpointed found at Point D. All indications are that it is Dr. Doppler's secret lab. You need to get there immediately.
X: On my way......
Zero: It's time I put an end to Sigma for good!
The Day of Σ
Dr. Cain: There's been a lot of action around here lately.
Sigma: Yes Dr. Cain. Maverick crime has been steadily increasing. Even giant mechaniloids have begun to run amok.
Dr. Cain: Hmm... And how is X doing?
Sigma: As far as his judgment and abilities on the battlefield are concerned, he has shown tremendous promise. However, he tends to hesitate when the situation calls for decisive action.
Dr. Cain: He worries too much. Of course... that is his greatest feature, and the source of his potential. Sigma... You don't have that problem at all. When I found that robot, X, sealed away so long ago, I utilized his design concept to give birth to you Reploids. Reploids have the ability to think and act on their own - just like human beings. But the only one who seems to worry and fret so deeply is X. That is the source of his great potential.
Sigma: His worrying shows potential? It's not a weakness?
Dr. Cain: Hmmhmmhmm... Well, I suppose it might be, under normal circumstances, but in this case, it could be the beginning of a new connection between robots and humans. Of course, we don't know yet if that potential will be a wellspring of hope, or usher in an era of utter chaos.
Dr. Cain: I've extended my life as long as possible to see it all play out, but it looks like I won't be around long enough now...
Mega Man X2
- "I can try to reactivate Zero to help, but I need more time before you can reinstall his control chip... Try to slow them down! Good luck!" (After defeating the 3 X-Hunters.)
- "Mega Man X! I've located the X-Hunters base! It's at grid location 00. That's right at the North Pole! (With Zero's parts.)
- "It will take some more time to reactivate Zero... Slow them down Mega Man X! (With Zero's parts.)
- "I'm sorry X... One of the X-Hunters attacked the base and... took all of Zero's parts away... They are going to rebuild Zero at their base. I tracked them to location 00 - The North Pole! (Without all of Zero's parts.)
Mega Man Xtreme
- "X! Finally we've cleared all the protection!" (After defeating the 4 boss protections (Normal Mode).)
- "X! You've finally cleared all the protections!" (After defeating the 4 boss protections (Hard Mode).)
- "Good luck, X!" (After defeating the 4 boss protections (Hard Mode).)
- "X! Finally you can enter the core!"
- "X, Good luck!" (Before entering the core (Hard Mode).)
- "X, finally I've found Sigma's hiding place! You can warp from the core of the Mother Computer." (Normal Mode)
- "It would seem that someone worked with Sigma and hacked the computer from there." (Normal Mode)