Referencing a character from Mega Man 4 on the NES of the same name, a Dr. Cossack is any character in a story who at first seems to be the main antagonist, but then you find out that he/she is really just the slave or middle man of the true, often more threatening antagonist.
In Mega Man 4, it seemed as though Dr. Cossack was the new evil scientist on the block. He built 8 powerful Robot Masters and had them conquer different areas around the world and challenged Mega Man to try and stop him. After penetrating Dr. Cossack's fortress however, Proto Man teleports in with Dr. Cossack's daughter, Kalinka, who explains that he was only following orders from Dr. Wily (The main series villain) because the mad scientist had her captured and threatened to harm her if Dr. Cossack didn't obey him. After Dr. Wily was defeated, Dr. Cossack actually demonstrated himself to be a nice guy. He even helped Dr. Light (good guy scientist) build Beat (Robot attacking bird) to assist Mega Man in his future ventures.
by FatherofGray September 18, 2013