Sonic Man is an aquatic Robot Master whose main attack consists of using a Sonic Wave that is hard to see. He also attacks with three small mines that he drops directly onto Mega Man. Sonic Man originated from the 1990 DOS computer game Mega Man (not to be confused with the more famous 1987 NES game of the same title), where he is one of the three Robot Masters fought.
In MKF, Sonic Man 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 Sonic Man, 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 Sonic Man 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.