Mercury is a Stardroid that appeared in the 1994 Game Boy game Mega Man V as one the game's bosses. His body is made of liquid metal, allowing him to take any form and dismantle himself to attack and is quick at running away after causing mischief. Mercury's main weapon is the Grab Buster, an attack that can steal energy and items from enemies and absorb them to recover his health.
In Mushroom Kingdom Fusion, Mercury is one of the eight Portal Masters found in the Wily Portal Chamber featured in Wily Citadel, which is the final level of World 3. The player must defeat all eight "Portal Masters", including Mercury, without dying, or be forced to fight all of them again (although an extra life is provided beforehand to prevent the player from running out of lives). After Mercury and the other Portal Masters have been defeated, a path to the final checkpoint will open up. Inside the player will enter one last teleporter, which will transport them to the boss room and the final battle against Dr. Wily.