I feel there's got to be more to her, but also that we'll never know. The thing is, she's more brightly lit than almost anyone else in the picture, and oddly so because she's in the second or third rank, whereas the light is falling naturally on the first rank. So she's been specially picked out.
I'd guess she's someone's daughter, someone who probably did a great kindness to Rembrandt's bank account.
The real question though is: When is Zirndorf coming out with a figure of her???