"Number One" was a female Human Starfleet officer in the 23rd century. She was a lieutenant who served in the command division aboard the USS Enterprise in 2254 under the command of Captain Christopher Pike. As well as being one of the ship's helmsman, she was its first officer. She was one of the most experienced officers on the ship. Her name was not Number One; this was the naval slang term given by Pike to his first officer on the USS Enterprise.
According to the Talosian magistrate, known as The Keeper, she had exceptional intelligence and rationality, and that her seeming lack of emotion was largely a pretense and she often had fantasies that involved Captain Pike.
In 2254, when Pike was kidnapped by the Talosians, Number One led the effort to rescue him, first by unsuccessfully attempting to use a laser cannon to blast open an entry to a Talosian underground lair where Pike was being held, and then by using the transporter in an attempt to infiltrate this layer. Number One was subsequently kidnapped, along with Yeoman Colt, for the purpose of providing Pike with a mate with whom he could procreate. In response to this, Number One set her laser pistol to overload, telling the Talosian Keeper that it was wrong to keep a colony of Humans as slaves, and that they would rather die. This, combined with an examination of the Enterprise's historical records, convinced the Talosians that Humans' unique hatred of captivity made them nonviable subjects, and they were subsequently allowed to return to the Enterprise. (TOS: "The Cage", "The Menagerie, Part I", "The Menagerie, Part II")
"It's wrong to create a race of Humans to keep as slaves."
"Who would have been Eve?"
Background information Edit
This character was portrayed by Majel Barrett-Roddenberry.
It was rare that an officer with the rank of lieutenant served as the first officer of a starship. The likely reason for this was that, when Gene Roddenberry first created Star Trek, the first rank structure he employed was based on the system used in the 18th and 19th century British navy, in which a ship's second in command was generally a First Lieutenant in the sense of the most senior lieutenant, rather than 20th century naval ranks we have come to associate with Starfleet.
In the original version of the series outline Star Trek is... (as reprinted in The Making of Star Trek, pp. 22-30), Number One was initially described as "a glacierlike, efficient female who serves as ship's Executive Officer." (The Making of Star Trek, p. 24) A more detailed description of the character from exactly the same document stated;
- The Executive Officer
- Never referred to as anything but "Number One", this officer is female. Almost mysteriously female, in fact – slim and dark in a Nile Valley way, age uncertain, one of those women who will always look the same between years twenty and fifty. An extraordinarily efficient officer, "Number One" enjoys playing it expressionless, cool – is probably Robert April's superior in detailed knowledge of the equipment, departments, and personnel aboard the vessel. When Captain April leaves the craft, "Number One" moves up to Acting Captain. (The Making of Star Trek, p. 29)
In the scripts of "The Cage", Number One was described as "Female, slim and dark in a Nile Valley way, age uncertain, one of those women who will always look the same between the ages of twenty and fifty [....] Almost glacier-like in her imperturbability and precision. From time to time we'll wonder just how much female exists under that icy facade." She was also scripted to be slightly more wary of J.M. Colt than she is in the final version of the episode, such as being initially reluctant about allowing her to join a landing party assigned to rescue the captain. 
Number One was dropped from the series as NBC executives did not respond well to her character. Reactions to her in the screen test of "The Cage" were also poor (by the women more negatively than the men, according to Gene Roddenberry). Her highly logical, steel-trap mind was given to the character of Spock in TOS: "Where No Man Has Gone Before" and maintained there onward.
Although Majel Barrett-Roddenberry was credited as Majel Barrett in "The Cage", she was credited as "M. Leigh Hudec" in "The Menagerie, Part I" and "The Menagerie, Part II" (probably because the production team were endeavoring to hide from NBC the fact that the actress whose departure from the series had been demanded by the network was actually still in the series, recurring as Nurse Christine Chapel).
In the Marvel Star Trek: Early Voyages comic book series, this character was named Lieutenant Commander Robbins. Her first name was interrupted by other dialogue, but starts with "Eure-."
The novel Vulcan's Glory, by TOS writer D.C. Fontana, suggested this character's moniker was not simply a nickname or title – she was an Illyrian who was called "Number One," as the best intellect among her generation. The novels The Children of Kings and Child of Two Worlds explain that Number One does have a given name, but it's difficult to pronounce for non-Illyrians; the former suggests that she liked to be called by this name rather than her own as it enabled her to maintain a professional relationship with her captain. In the novel Captain to Captain she is referred to as "Una", however this is not her real name either.
Star Trek II: Biographies gives her real name as Leigh Hudec.
Peter David's New Frontier series of novels had a character named "Morgan Primus" who was an immortal like Flint (from TOS: "Requiem for Methuselah") and was the mother of Robin Lefler. It is hinted that she and Number One are the same person.