Hard Demon is one of the five demons, From Easy Demon, Medium Demon, Insane Demon, and Extreme Demon, being basically the middle ground of them. They're aggressive, usually picking a fight with anyone who tries to provoke them.

He is aggressive, irritated, and a demon. Basically challenging.

from Geometry Dash.

Easy Demon is one of the five demons, being the weakest of the five. They don't try to get into fights often, but if threatened they'll gladly rip someone to shreds.

Easy Demon difficulty from Geometry Dash.

EXTREME DEMON IS ONE OF THE FIVE DEMONS, BEING THE STRONGEST AND MOST DANGEROUS. THEY WILL ATTEMPT TO KILL ANYTHING NEARBY.