i have this guitar that's playing its own melody comprised of mostly powerchords, and another guitar that's supposed to be the lead playing the main melody. what would i even describe the purpose of the first guitar as? harmony? backing? countermelody? (actually, the idea of what even classifies as a countermelody already confuses me)
sometimes i refer to melodic powerchord guitar parts as the 'riff', or even 'metal riff'. but without hearing what you're doing, i can't say exactly what i'd call it, in your case. but in the situation i'm thinking of, the bass most often has no choice but to follow along with the riff also, so in this case, the power-chord part might also be considered the bass. but this might all be untrue because i'm just guessing as to what you have there based on the way you described it