I've used this concept from the very start of cRPG, I use the term "nemesis". It's more ominous, and really anyone not on your team is an "opponent". It needs a word with a more specific target. My first nemesis that I remember was Ptah, a fucking my old friendcher of course. Currently, I dunno, VanDamne probably. I really, really want to kill him whenever I see him on servers, and he is a worthy opponent, suitably rage inducing.