Mega Man ? is the result of a virus that Dr. Wily uploaded onto his wave 2 Robot Masters. When Mega Man defeated a Robot Master he downloaded it's power and the virus along with it, when he had defeated all of the robot masters the virus turned him into "Mega Man ?" and he was forced to attack his friends, although upon entering the battle, he seemed quite happy to decimate them all. Luckily, the original Robot Masters managed to turn him back to normal before he could do any permanent damage.
Since Dr.Wily assumed Mega Man would come and try to defeat his robot masters, he installed a virus that would be copied along with the vanquished robot's power when Mega Man would use his copy chip. So, each time Mega Man copied the power of the robot he defeated, more of said virus was downloaded into him. As time went on, Mega Man gained a more aggressive attitude, losing his normal sadness when blowing up a robot, and even going so far as to smiling malevolently as he shoots Heat Man in the head. Whenever Mega Man was annoyed or mad at Doctor Light, his eyes would turn a red color, which nobody addresses.
Since Mega Man ? is a result of a virus Dr. Wily created, Mega Man ? had absolute loyalty to him. He would do any given task for Dr. Wily, even if he himself did not want to. When told to attack, Mega Man ? entered the battlefield smiling in an evil and malicious manner, ready to attack his own friends. Mega Man ? is ruthless, since even the minor influence of the partial virus was enough to get Mega Man to take cruel pleasure in shooting Heat Man in the head, and that Mega Man ? said that he had to shoot Elec Man into decommission. He was about to before he was hit away. In summary, Mega Man ? is cruel, ruthless, merciless, brutal, evil, and completely loyal to Dr. Wily.
Mega Man ? is identical in appearance to Mega Man apart from his eye color being red instead of its usual blue, and him having a purple scarf around his neck
His appearance is based on the evil Mega Man that shows up in a Robot Masters own level in the video game Powered Up