(coexisting on another dimensional plane)
Normally, this woman would have other duties aboard ship, but her main role would be to provide companionship and sexual favors to the captain. However, captain's women often proved to be deceptive and manipulative, using their position to build their own power base on the ship. They would often react very quickly to a change in the command situation on the ship, and would offer their services to a new commander almost immediately, but only if it served their purposes to do so.
Hoshi Sato served in this capacity first to Captain Maximilian Forrest and then to Commander Jonathan Archer, before murdering Archer and attempting to seize the Emperor's throne for herself. (ENT: "In a Mirror, Darkly", "In a Mirror, Darkly, Part II")
Marlena Moreau was Captain James T. Kirk's woman. She occasionally assisted him in committing assassinations (using the Tantalus field) that helped preserve his command. When she believed that she had lost her position as Kirk's woman, Moreau threatened to leave him for Commander Kenner, although Kirk dissuaded her from this action. Having been introduced to her in this role while briefly in the mirror universe, the prime reality Kirk was flustered when he encountered her prime reality counterpart shortly after returning to his own reality. (TOS: "Mirror, Mirror")