We’ll start with the most obvious. Seriously, just look at these two side by side. You’ve got a robot boy wearing a helmet, gloves, metal underwear, and knee-high boots. Mighty No. 9’s outfit is considerably more detailed, but Mega Man was designed in the 8-bit era, so the gulf is pretty easy to dismiss. They even have little ear thingies on their helmets, though No. 9’s look to be headphones to go with his goggles. Perhaps those and a hat come together to appear exactly the same as a helmet, which would then make it totally unique from Mega Man’s headwear.
The minor detail that separates them: Mighty has green eyes to Mega Man’s blue peepers.