First, you must set the required mistress as your “first mistress”. To do this head to your bedchambers and down the long thin corridor to the left. This room will contain your mistresses. You can select which you want to be your first mistress by walking up to them and pressing . Next, head over to the tower upgrade station (also in your chambers) and buy every item there, which only takes 3100 gold.
See “Ghost Bringer” to find out how to have both Dark and Ghost Fay available in a single playthrough.