Mercury is an extraterrestrial Robot Master found, reactivated, and used by Dr. Wily. He was first seen in Mega Man V as one of the games 8 Robot Master Stardroids. Like the Bio-Devil series of robots, Mercury is capable of shifting between a solid and liquid state. His main weapon is the Grab Buster, a fist-like attack that snatches powerups like E-Tanks and P-chips (currency) from Mega Man. Mercury is the only Robot Master hitherto to have this ability.
Mercury attacks by transforming his body into a liquid state and throwing himself drop by drop across the screen. If is on the left side, he will spread the drops out as he goes, and they will fly back towards the main blob one at a time. If Mega Man does not defeat him quickly, he will steal currency (called P-chips), E-Tanks, W-Tanks, and other powerups and abilities and the player must buy them again from the shop. Mercury's weakness is the Black Hole, but the Mega Arm is more effective as it does the same damage when fully charged and can be fired faster than Black Hole. Also, Mercury will use the Grab Buster immediately after being hit with the Black Hole.