Three of my players chose some sort of nemesis for themselves at character creation as part of their Aspect-choosing. One's an ex-Denariian host, so that was an easy one. Another chose his former employer, a CEO of a corporation who attempts to acquire magical items on the sly. Dude's just a human, but his power comes from all the junk he has access to and his hired goons. The third nemesis is a warlock one PC encounted during his military service. There isn't anything so thematic as "mirror image" or anything about them, but my players came up with them and that's good enough for me.
As far as power level goes, the Denarian I think I need to make tougher after their brief encounter 2 sessions ago. They've tangled with the CEO's goons before, but the man himself? Well, I suppose it depends on the Items of Power he's got, but take those away and he's just a dude. The warlock I'm working on now, actually, but I'm thinking his true power is going to come from hordes of brainwashed civilians.