|Picture of Karen Parker, from the Prototype Wiki|
Part I - Missed Opportunities
Part II - King of the Gods
Part III - My Dear Sister
Karen Parker is really a baffling and frustrating part of the story of Prototype. Just a quick rundown of what she does in the story should alert anyone that there's something really wrong here. She first appears in a photograph in Alex's apartment, triggering memories in Zeus. Zeus then goes and rescues her from Blackwatch. She implies she has unfinished business with Alex and that their relationship ended painfully. She is able to make significant progress on studying the virus working alone (!) She explains a little about how the infection works - the only character in the entirety of the game that ever does so.
Then, Karen betrays Alex to Blackwatch for no discernible reason, and disappears from the story.
What. The. Hell.