Specifically, I am trying to describe Agent Smith from the film The Matrix and his relationship to Neo. Agent Smith seems to be the anti-christ to the christ-like figure Neo. They are both very powerful; neither can entirely defeat the other. Their missions and beliefs are completely opposite. Neo cares for humanity; Smith hates it. Neo is selfless; Smith is completely selfish. etc..
I have some ideas: antithesis, the polar opposite, dark shadow..