Lieutenant Marla McGivers was a female Human Starfleet officer in the 23rd century who served in the operations division aboard the USS Enterprise in 2267 under the command of Captain James T. Kirk, during the historic five-year mission.
She was ship's historian; however, she rarely had active duties to perform. On the few occasions when she was called to service, McGivers regarded it almost as an inconvenience, and on one occasion she even kept a landing party waiting for her arrival. Judging from McGivers' cluttered quarters, crowded with paintings and sculpted busts, her primary pastime was art. Her favorite subjects were all strong, forceful leaders from Earth's history. McGivers' works of art depicted men such as Alexander the Great, Napoléon, Richard the Lionheart, Leif Ericson, and Flavius, a gladiator. McGivers was of the opinion that modern men did not live up to her heroes of the past.
As a historian with a fascination for bold, colorful characters, McGivers was particularly intrigued when, in 2267, the Enterprise discovered the SS Botany Bay and Khan Noonien Singh, a dictator who had ruled much of Earth in the 1990s. Though she protested that her interest in Khan was professional, she invited him to her quarters, where she began painting his portrait. She later arranged a full-dress welcome dinner in his honor, a function normally reserved for a fleet admiral. When Khan pursued her, McGivers was surprised and shocked by his bullying manner, but also deeply flattered. Khan demanded absolute loyalty from her; desperately afraid of losing him, she promised to do anything he asked - including helping him in his attempts to seize the Enterprise.
Fortunately, McGivers was uncomfortable with Khan's murderous plans; she turned against him for long enough to prevent Captain Kirk's death and to help retake the ship, but it was too late to save her career. When Kirk sent Khan and his followers on Ceti Alpha V, McGivers was given a choice between facing court-martial or accompanying Khan. She chose to go with Khan, and he called her "a superior woman." (TOS: "Space Seed")
At the time that Khan's party settled on the planet, it was perfectly fit for Human habitation. However, a mere six months after their arrival, the neighboring planet, Ceti Alpha VI, exploded, causing an ecological disaster on Ceti Alpha V which left the planet a barren wasteland. Khan's followers struggled to survive, but Marla did not survive to be freed from the planet. (Star Trek II: The Wrath of Khan)
Marla McGivers was played by Madlyn Rhue.
Although she was referred to as "lieutenant" in dialogue, she wore no rank stripe, denoting an ensign. Presumably, McGivers wore a red uniform because as ship's historian, her duties would fall under the classification of engineering and support services.
In the script for The Wrath of Khan, McGivers is stated specifically to be Khan's "beloved wife" who was killed by the ceti eel. Khan's line directly referencing McGivers did not make it into the final cut of the film, however, and thus McGivers' status as Khan's late wife remains unconfirmed, though probable. 
According to Harve Bennett, McGivers was actually to appear in Star Trek II. After he discovered Rhue was then using a wheelchair due to multiple sclerosis, however, the character was written out as it was felt it would be unfair to recast the role. (Set Phasers to Stun: 50 Years of Star Trek; )
"Madlyn" was established as Lt. Marla McGivers' middle name in the novel To Reign in Hell: The Exile of Khan Noonien Singh by Greg Cox. In the early part of the novel, Kirk, Spock and McCoy, returning to Ceti Alpha V in 2287, discover McGivers' tomb, hand-carved by Khan himself, showing a birth year of 2242 and a death year of 2273.
To Reign in Hell indicated that Marla married Khan shortly before the destruction of Ceti Alpha VI, and, because she and Khan appeared incapable of producing children by dint of her non-Augment genetics, she tended to the children of Khan's people in their cavern home after the cataclysm. She was captured by Harulf Ericsson, one of Khan's former operatives on Earth who sought to overthrow him, and infected with a Ceti eel with the intention of using her to kill Khan. However, as Clark Terrell did when ordered to kill Kirk, Marla committed suicide instead. This scenario played out similarly in issue 3 of the comic book series Star Trek: Khan - Ruling in Hell.