The Turks, more specifically Reno and Rude.
Meet us in endless waves (You fight them in the Sunken Gelnika)
To send you to your graves (You battle them, someone's got to lose)
Find us by the madman's gun (You fight them again in Sector 8 just before you get to the Sister Ray, but this time Elena is with them)
No, we're not the hero, so run! (Not sure about this line)
Sidenote:
WEAPON killed by Sister Ray while stationed at Junon = Sapphire
WEAPON killed by Sister Ray while stationed at Midgar = Diamond
