The Keeper was charged with the upkeep of the Talosian menagerie, an elaborate test with the aim to find a servant species that was supposed to repopulate the planet later. Under his command, the Talosians rescued Vina, the only survivor of the crashed Earth vessel SS Columbia, and later lured the USS Enterprise to the planet in 2254. (TOS: "The Cage", "The Menagerie, Part II") The Keeper then sent two Talosian underlings to kidnap Captain Christopher Pike, then captain of the Enterprise, from the surface of the planet. (TOS: "The Cage", "The Menagerie, Part I") Using telepathic illusions and a supply of a blue liquid that he claimed was a "nourishing protein complex", The Keeper intended to trap Pike into accepting a life on Talos IV as Vina's mate and breeding stock for a new, stronger race. The Keeper also detailed alternative mating selections from the Enterprise, Number One and Yeoman J.M. Colt, once they were also captured. (TOS: "The Cage", "The Menagerie, Part II")
Upon being caught and strangled by Pike, The Keeper became threatening, momentarily assuming the appearance of an illusory anthropoid ape. After the captain managed to free himself and the female officers by overcoming The Keepers' mental powers, however, the magistrate had to accept that Humanity was unsuitable for his plans, due to their strong will and refusal to submit to captivity. While Pike and the Enterprise were let go, the magistrate agreed to take care of Vina, even providing her with an illusory Pike to keep her company. Before the real Captain Pike was beamed back aboard the Enterprise, The Keeper wished him, "May you find your way as pleasant." (TOS: "The Cage", "The Menagerie, Part II")
When Spock attempted to return Pike to Talos IV against orders in 2267, The Keeper remotely took control of a screen in the Enterprise's briefing room, transmitting images of Pike's earlier encounter with the Talosians to a disciplinary hearing aboard the ship. The Keeper later telepathically contacted the ship directly, relaying footage of himself to the screen, and revealed to Captain Kirk – the current captain of the Enterprise – that a witness of the hearing, Commodore José I. Mendez, had actually been yet another illusion, nullifying the hearing. Finally, The Keeper welcomed Pike back to the planet, ensuring it was what the former captain wanted to do, and wished Kirk, "May you find your way as pleasant [as Pike's]." (TOS: "The Menagerie, Part II")
The fact that, in the story outline of "The Cage" (as reprinted in The Making of Star Trek, pp. 47-65), the Talosians were written as crab-like aliens effected how The Keeper was first conceived. For instance, the outline introduces him by stating, "The crab-creature at the televisor controls turns from the screen, using claw-snap and clatter for speech." He is referred to, in the last scene of the outline's first act, as "the 'Keeper'" and the outline goes on to frequently refer to him as that (though without the use of quotation marks). The story outline also twice refers to him as "the crab-creature Keeper". (The Making of Star Trek, pp. 48 & 58) In one scene included in the story outline but not in the episode's final version, The Keeper was apparently killed by an axe-wielding Captain Robert April, though this scenario was yet another of the illusions concocted by the Talosians, and The Keeper was actually still alive, unharmed. (The Making of Star Trek, pp. 59 & 60)
The character is commonly referred to with the articulation "The Keeper" in the second revised final draft script of "The Cage", whose introduction of the character is not quite as elaborate as his initial description from the episode's story outline. After a written direction instructing the trio of Talosians to be shown, the script goes on to say, "Emphasizing one who wears an authoritative-looking jeweled pendant on a short chain around his neck. We will come to know this Talosian as THE KEEPER. He watches the screen intently, then leans in as if seeing something of more than average interest." When he first uses telepathic communication in the episode, the script specifies that he has an "unemotional and almost pedantic tone which will become familiar to us." When he first uses vocal language in the episode, the script reads, "At first the words will come a bit carefully, as if out of practice in communicating this way. As our scene progresses, speech will follow a bit more smoothly."
Despite being written consistently in both the outline and the script as a male (and established in the episode as having that gender), The Keeper was played by actress Meg Wyllie, a casting choice that director Robert Butler took credit for. "When the femininity idea came up for the Talosians," he recalled, "I thought of Meg because I thought it would have been challenging to her, and amusing to her to do that, and I just thought she'd do it really well." (The Star Trek Interview Book, p. 98) Indeed, Wyllie found the part to be one of her most challenging, later remarking, "I had never played such a role nor had such a makeup job applied to me [....] The makeup was not comfortable [....] I was never given a full script, just the sections in which I appeared." Wyllie also related that, due to her unfamiliarity with such a part, she "was most intrigued" and revealed that she was given no unusual advice to prepare her for the performance. "No special instructions," she said, "merely to play the part with dignity and control. A mental, rather than physical, approach was needed to concentrate on the words I was saying. The pulsings of the veins in my skull–and very little facial expressions–were to be the only visible effects of my thought transfers." (Starlog issue #117, pp. 52 & 53)
The Keeper's vocals were performed by Malachi Throne. Meg Wyllie was at first entirely unaware of Throne's involvement. "I wasn't even aware my voice would be dubbed," she explained. "I was quite surprised to hear a man's voice issuing from my head." (Starlog issue #117, p. 53)
The "new" footage of The Keeper in "The Menagerie, Part II" evidently reused footage from "The Cage".
The Keeper's mirror universe counterpart appeared in the short story "The Greater Good" by Margaret Wander Bonanno contained in the anthology Shards and Shadows. As in the primary universe, the Talosians had used a distress call to lure the ISS Enterprise to Talos IV with the intention of having Christopher Pike mate with Vina so as to create a race of Terran slaves. However, Pike rejected her, refusing to mate with an "insipid human female." Upon learning of the Terran Empire's fear of telepaths, the Keeper decided to release Captain Pike and use him as their eyes and ears throughout the empire. The captain would give the Talosians an early warning should the empire ever decide to attack and obliterate their planet. After assassinating Pike and assuming the captaincy of the Enterprise in 2264, James T. Kirk returned to Talos IV and ordered that its surface be leveled, annihilating the Talosians for the good of the Empire.