I'm guessing the following:
The photo that Reddington took out of that book of victims was of Keen's mother. The photo was dated back in 1990.
After reviewing last week's episode again, the photo of the woman that Reddington took out of The Stewmaker's book of victims was dated 12/90. In the Pilot episode, Agent Ressler reviewed Reddington's history from his file and Reddington went missing after not returning home to visit his wife and child for Christmas in December, 1990. I definite connection there. Is that woman in the photo, Keen's mother? Maybe, maybe not, but one thing is for sure, it's all connected and Agent Keen is part of that connection. I still don't think she his biological daughter as that would be too easy, but he feels a responsibility towards her and her well being. I've read their connection will begin to be revealed over the coming weeks.