Mr. X

Mister X is basically a fake name of Dr. Wily, which he used to mask his true identity in Mega Man 6. Mr. X is a billionaire that sponsored the 1st Annual Robot Tournament that he created to take control of eight of the strongest Robot Masters in the final event of the tournament and conquer the world.

X Crusher
Mr. X fights with a machine called X Crusher that swings in the screen and shoots a purple ball that splits in two when it touches the floor, each part sliding a different direction. When the machine is destroyed, he flies out from it and reveals that he is really Dr. Wily before escaping to his fortress.

Strategy
Stay to one side and attack it by using any weapon (Flame Blast is it's weakness). Dodge the energy balls by jumping. A good strategy is to use a charge shot.

Trivia

 * Just before unmasking himself, Mr. X cries out: "My plan for world domination has faild". This typo was fixed in the Mega Man: Anniversary Collection.
 * Mister X is commonly used as a pseudonym for someone whose name is secret or unknown.