Once you unlock the ability to enter the Nexus go visit the General Store in town and buy the matches than go to her Demanse, there should be a candle at the entrance you can light. Once you light the candle follow where it says the flame flickers towards. Eventually you will reach her main chambers, after which you will enter her Demanse from there every time you go there. From there you should be able to meet her and get her card after a short cutscene.