The main thing we learn about Ruby in this episode is that she was abandoned by her birth mother, who left her at the door of a church. Throughout the special, we learn that Ruby wants to reconnect with her biological family, and is disappointed when she learns that there is no trace of them. Whoever Ruby’s parents are, they’ve never left any records of their DNA, which the episode establishes as being somewhat rare. It shouldn’t be this difficult to track down Ruby’s mother, but it somehow is.
What makes this more interesting is that a large part of the special’s climax involves the Doctor returning to the night Ruby was abandoned, to make sure those pesky goblins don’t go back in time and eat baby Ruby while she’s waiting at the church’s doorstep. The Doctor successfully saves her, and we end the sequence with a long, lingering shot at Ruby’s apparent mother, hidden under an ominous cloak, as she walks away in the dark. To anyone who knows anything about planting and payoff, the implications are clear: Whoever Ruby’s mom is, she’s definitely important, and she’ll probably end up being a huge dramatic reveal in the upcoming season’s finale.
It’s a slight departure from Russell T. Davies’ usual approach to his companions. Rose Tyler, Martha Jones, and Donna Noble were all regular women in London with fairly straightforward living situations. They may have lost loved ones before we’ve met them, but there was never any doubt over who those loved ones were. This is the first time Davies has given us a companion where a major part of her backstory is hidden from us.