Mikhail Sergeyevich Cossack, better known simply as Dr. Cossack is is the secondary antagonist in Mega Man 4. He is a scientist who was blackmailed by Dr. Wily into attempting to kill Mega Man. He is the creator of the Cossack Numbers and becomes an ally to Mega Man onwards from Mega Man 4.
Mega Man 4
Dr. Cossack sent a letter to Dr. Light challenging Mega Man to defeat his Robot Masters, claiming that he would prove to the world that he was the greatest robotic scientist in the world. After defeating Cossack's robots, Mega Man rushes to his citadel in Siberia, where he meets the doctor and fights him. However, before Mega Man could defeat him, Proto Man interrupts the battle, along with Cossack's daughter, Kalinka, who was kidnapped by Dr. Wily. Wily was using Cossack for his mad schemes.
Mega Man 5
After Wily is defeated, Cossack befriends Dr. Light and gives Mega Man a pet bird robot named Beat as a gift for defeating Wily again.
Mega Man 11
While he doesn't appear physically Dr. Cossack is responsible for the creation of Tundra Man, one of the Robot Masters brainwashed by Wily in the game.
Dr. Cossack is an affable scientist who cares about his daughter. Despite having good intentions, he doesn't hesitate to do Dr. Wily's bidding against his will for the sake of protecting his daughter. After he breaks from Wily's influence, he shows remorse and gratitude by assisting Dr. Light and Mega Man on their endeavors to stop Dr. Wily.
Dr. Cossack on the Mega Man Wiki