If you escape slavery with her in the prologue, you can find her in the refugee camp in the Valos farmer's country.
Then, once you advance enough of the main quest and get housing for the refugees, you can talk to her in her house, then in the stable in the back of her house in order to get a scene of her with her dog. After which you can recruit her.
not sure what happens if you dont escape with her, either being sold off yourself or taking the smuggler's route and never meeting her. I'm guessing you either have to additionally rescue her from somewhere (slavers in rathpike, maybe?) Or maybe she's replaced with someone else... or maybe she's just unavailable! the game sure likes to boast how your choices have significant outcomes.