Who is the lady Pip has never seen before?

The "lady" is Estella.

From the text:

The lady whom I had never seen before, lifted up her eyes and looked archly at me, and then I saw that the eyes were Estella’s eyes. But she was so much changed, was so much more beautiful, so much more womanly, in all things winning admiration, had made such wonderful advance, that I seemed to have made none. I fancied, as I looked at her, that I slipped hopelessly back into the coarse and common boy again.