"Number Thirteen" was a previously unknown model of the humanoid Cylons. Thirteen's tend to be willing to do whatever it takes to survive, along with being braver then the other models.
Little else is known about her except that the model line initially rebelled against Cavil's take over of The Cylons. Up until the Fall of The Colonies, they were the only ones, aside from Cavil himself that knew the truth behind the fate of The Final Five. Instead of allowing him to reprogram their memories involving the significance of The Five. They fled from The Colony and lived in exile on Colonial frontier worlds as miners, security officers, or mercenaries.