Mutant Year Zero: Road to Eden

This is almost unmissable, but since I suppose it might be possible to sneak right by her (though I’ve never tried, it might be impossible, like when meeting Selma). The achievement makes it sound like you have to do something but really you just have to get close enough to trigger the cutscene and after that she’ll be a member of your party and this will pop. Click the spoiler for location.