Break Man is Proto Man's alias in Mega Man 3. Like the other Proto Man battles as the battles never play the normal boss music, the battle with Break Man will have the normal boss music.
After Mega Man beats all of the Doc Robots, he has to fight Break Man before entering the castle of Dr. Wily.
In a stage that looks like Hard Man's, Break Man will hop around the room, like Toad Man and will fire a pair of shots. Mega Man must use his arm cannon as the other weapons never hurt him. After shooting Break Man, Break Man will go up, along with Mega Man.
The reason why Break Man is immune to other weapons, because his armor protects him.
In the ending, Dr. Light recognizes his whistle, revealing that if Break Man is Proto Man.