Lieutenant Marla McGivers was the historian aboard the USS Enterprise, 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, Napoleon, 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 banished Khan and his followers to Ceti Alpha V, McGivers opted to accompany them rather than face a court-martial. Khan was pleased by her choice and dubbed her "a superior woman." (TOS: "Space Seed")
At the time that Khan's party was exiled to 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, and the ordeal proved too much for Marla. (Star Trek II: The Wrath of Khan)
Marla McGivers was played by Madlyn Rhue. Although she was referred to as "lieutenant" in dialog, 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 Star Trek II: 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 non-canon but probable.
According to Harve Bennett, McGivers was actually to appear in Star Trek II. After he discovered Rhue to have been confined to a wheelchair due to multiple sclerosis, however, the character was written out as it was felt it would be unfair to recast the role. (citation needed • edit)
In the comic book series Khan: Ruling in Hell, she is kidnapped by Khan's enemies in issue 3 who deliberate plant a ceti eel in her to brainwash her into murdering him. The assassination attempt fails, and she dies from exposure to the creature soon after.