Mistresses are the Overlord's recurring female companions that become available after specific events in the storyline. These maidens offer the ability to decorate the Dark Tower / Netherworld Tower in Overlord and Overlord II, respectively. Minion upgrades will also be made available later.
After winning your primary mistress' heart by buying her gifts (decorating the tower based on their tastes), you are invited to her quarters to engage in particularly raunchy affairs.
Should an Overlord decide to have more than one mistress, he can decide which one to select as his First Mistress (in Overlord II only). The primary mistress will get to implement her own unique decorations in the tower. This decision is not permanent and can be changed when desired.
There are two mistresses in Overlord. Rose is the default who joins you in your tower following her liberation during your investigation of a raid on Castle Spree. Her sister, Velvet, is introduced later in the game; you must choose to either remain faithful to Rose by leaving the room, or ditch Rose for her seductive sister by sending your minions to her bed. Predictably, Velvet contributes to a 100% Corruption end, and Rose to a lower Corruption end. The Third Overlord, regardless of your decision, had evidently chosen Rose as his mistress, as it is revealed in Overlord II that she is the mother of the Fourth Overlord. However, this is not made entirely clear due to plot inconsistency and incoherent oddities (such as Velvet's upgrades in the Wasteland Dark Tower ruins being present despite Rose's residency, and Rose's warning to the Overlad on the chaotic nature of his father). This aspect was likely left ambiguous to have the returning player's choice matter, albeit messily.
Overlord II Edit
There are four distinct mistresses in Overlord II, although only three are available in each playthrough. The two orientations of Fay are coined as two different entities. Your selection of the version of Fay you receive is directly influenced by the decision to either corrupt Queen Fay at the climax of the events in the Wasteland Sanctuary Depths, resulting in her becoming the mistress Dark Fay, or your impulse to kill her using the Domination spell after corrupting her, resulting in her other alias, Ghost Fay.