Hannah, I was thinking the very same thing the other day. For me, Dorcas is the main character. She is the most interesting to me. But, if you think about it, Laura is supposed to be telling the story through her eyes and who is the "main character" in Laura's life at this point? It's Dorcas. Dorcas is the main influence on Laura right now-- her teacher, her mentor.
Also, what interests me in a character is what lies just beneath the surface. The mystery of that character. There is no real mystery to Laura--we know who she is and what she's looking for. Dorcas is extremely enigmatic and whenever she's in a scene I sit up and pay close attention, trying glean clues from those eyes that are at once both revealing and trying so hard to hide something. Dorcas strives so hard to remain in control of her emotions, but it's those times when she slips up and reveals who she really is that I live for.