Nancy Crater was the primary personification for the last living M-113 creature to inhabit planet M-113. Her appearance was the form of the former female Human civilian Nancy Crater, the late wife of archaeologist Robert Crater.
In 2264 or 2265, the last of the M-113 creatures murdered Nancy Crater, an act that almost drove her widower, Robert, to destroy it. In the end, the fact that it was the last of its kind, or perhaps its ability to assume any form, stayed his hand, and he lived with it for a year or more.
In 2266, the creature was given an opportunity to secure more salt – which was especially vital as the Crater expedition was dangerously low on salt by that time – when the USS Enterprise visited M-113 for routine medical checks of the two scientists working there, unaware Nancy Crater had been killed.
Captain Kirk led a landing party to the surface of M-113 to perform the routine medical examinations required of all scientific expeditions. On the planet, the landing party encountered who they thought was Nancy Crater. What the landing party saw was actually the M-113 creature impersonating Nancy Crater.
Each member of the landing party saw a different woman, although they weren't aware of that.The first image McCoy saw was Nancy as he remembered her, like a girl of twenty-five. The first image Kirk saw was Nancy as he expected her to be, an older woman with gray in her hair. The closest the landing party came to realizing they were seeing different women was when Crewman Darnell remarked that Nancy reminded him of a woman he'd met on Wrigley's pleasure planet – a comment that drew a sharp rebuke from McCoy. Later, McCoy saw the same woman Kirk saw; he chalked his earlier view up to affectionate memory and dismissed it.
The creature's hunger drove it to murder crewmen Darnell, Sturgeon and Green on the surface of M-113 (which was strewn with archaeological remains). As Nancy Crater, it blamed Darnell's death on ingestion of a Borgia plant.
Impersonating Crewman Green, it returned to the Enterprise, where chance saved Yeoman Janice Rand from becoming its next victim. It followed her when she brought Sulu his dinner, and might have murdered both officers except that Beauregard, a curious plant in the botany section of the life sciences department, scared it off.
The creature returned to the form of Nancy and sought out McCoy. She convinced the doctor that he was tired and convinced him to lie down. Around this time, Captain Kirk and Lieutenant Commander Spock found Professor Crater on the surface of M-113, and returned to the ship with him. The creature, then impersonating Dr. McCoy, sat in on a staff meeting at which Crater admitted he knew how to identify it. Before Crater could reveal or be made to reveal how this might be done, the creature murdered him, attempted to feed off Spock, who survived, presumably due to the differing composition of Vulcans' blood salts, and fled to McCoy's quarters.
Nancy Crater again
There was some evidence that McCoy's strong love for Nancy Crater affected the creature. On several occasions, it could have easily killed him, and did not. Additionally, as Nancy, it sought him out for protection when its nature had been at least partially discovered, and it was being hunted. In said case, Kirk found the creature in McCoy's quarters, and attempted to lure it to him with salt.
In a resulting scuffle, it overpowered Kirk and began to feed on him. It wasn't until the creature dropped its hypnotic projection, and Kirk began to scream from the pain of salt extraction, that McCoy shot and killed the creature. (TOS: "The Man Trap")
In its true form, the creature was portrayed by Sandra Gimpel. When the creature posed as other characters, it was played by Jeanne Bal, Francine Pyne, Bruce Watson, DeForest Kelley, and Vince Howard. As such, the character was played by more performers than any other single character in Star Trek: The Original Series. (The Star Trek Compendium 4th ed., p. 36)