User blog comment:JediSpectre117/Citadel Trailer is out REPEAT OUT !!!!!/@comment-124.168.113.204-20130304184841/@comment-24174486-20130304190742

To quote our mysterious mastermind: "I'm going to take everything you have, and everything you are."

The only character I can think of that falls along those lines is Kai Leng, and while it would be possible for Leng to play the role (given that Shepard first clashes with him during Priority: The Citadel II and doesn't dispose of him until Priority: Cerberus Headquarters), I doubt it will be him given that he already has an antagonistic role in the story.