@Getz: The player probably ends up fighting her.
Oh, come now folks she is the classic 'honey pot' character meant to trick you. They set up Songbird who this big, powerful and menacing looking thing, and Elizabeth as an innocent, beautiful yet powerful thing. Yet, who do you think the BIG BAD of the game is really going to be? At the end of the game you (the player) will be given the choice as to if you kill Elizabeth or let her live. To let her live will mean knowing you are releasing some great powerful evil into the world. Elizabeth is cute, doe-eyed, and innocent looking because they want you to be invested in helping her throughout the game until the "twist" when it is revealed Songbird is protecting the world from the great evil -- Elizabeth.
Just read this and it's exactly what I'm thinking.