That would be Sarah Lyons, actually. Now that I think about it, getting to know Sarah better over the course of the game, including "Broken Steel" so your interactions could go further (assuming you were a saint, of course), was some of the best material in the entire game. The shady details surrounding why you couldn't become an initiate no matter WHAT you did early on in the game seemed less absurd when it was Sarah telling you in her stern fashion that your help was appreciated but that it was another matter entirely from joining the Brotherhood. Being able to continue the game made the sacrifice interaction with Sarah much more meaningful, and her parting words with you at the end of Broken Steel, wishing that the 2 of you had met under different, better circumstances, so that you could have been closer, was a really nice touch. Amidst all the crap in this game, there was at least one relationship and character arc that was done decently.