Demerzel is a gynoid, or female-presenting humanoid robot, who serves as the majordomo to the revolving trio of Emperor Cleon clones, Brothers Dawn, Day and Dusk.

[3] James Poniewozik of The New York Times wrote, "Birn gives an eccentric performance that is both disconcertingly mechanical and the most vulnerably human of the series.

Playing a robotic character is never an easy undertaking, but she makes it look that way, maintaining a real command of her physicality while we see small hints of her inner state cross over her features.

"[9] Michel Ghanem of TheWrap wrote, "[Demerzel's] backstory and motivations are explored in depth [in season two] and serve as a great performance for Birn, who has to straddle the nuances of showing emotion as an android.

[5][13][14] Demerzel's loyalty to Imperial Genetic Dynasty is "unwavering and steadfast",[5] as her programming seemingly makes it impossible for her to disobey, betray or harm the Emperor in any way.

[17][18] As the genetically corrupted Cleons increasingly veer from the path she has set for them, she begins to act more overtly autonomous, thwarting their choices and actions, and even killing and replacing them with new copies as necessary to preserve her grand scheme for humanity.

[19] Understanding her true role, in season two a Seldon avatar gifts her with his Prime Radiant, a device which stores the entirety of his psychohistorical equations, so that she may use it to guarantee humanity's survival.

[14] As depicted via flashback in the season two episode "Long Ago, Not Far Away", after the Robot Wars, Demerzel is kept as the prisoner and plaything of Emperor Aburanis, whose abuse teaches her the extent of human cruelty.

[22] Rafael Motamayor of Vulture wrote, "Demerzel is Foundation's biggest secret weapon—a character the show is mostly keeping a mystery while slowly unraveling just how central to every aspect of the story she really is.

[11][24][25] Demerzel is shown to be a devout Luminist in "Death and the Maiden", and goes so far as to kneel before the upstart Zephyr, Halima Ifa, who has spoken out against Imperial cloning on religious grounds.

[26] In "Mysteries and Martyrs", Day is furious at Demerzel for committing this heresy, but she counters that if her action was truly in opposition to the Empire, her programming would not have allowed her to physically do it.

[27] In the season one finale "The Leap", the revelation that rebels have tampered with the genetics of the Cleons, and effectively manipulated Dawn in their plot, puts Day and Dusk at odds with each other over whether the youngest clone should be destroyed.

[16][28] Faced with the reality that her true loyalty is to the millennia-spanning bloodline and not the Cleons themselves, Demerzel screams in frustration in her chambers, tearing the skin from her mechanical skull.

[28] In the season two premiere "In Seldon's Shadow", a subsequent Day survives an assassination attempt while he is having sex with Demerzel, and suspects Dawn and Dusk's involvement.

Day has decided to halt the degradation of the bloodline by marrying the newly crowned Queen Sareth of Cloud Dominion, and fathering an heir with her.

[33] In "The Last Empress", Day decides to deal with Hari Seldon's troublesome Foundation on the planet Terminus in person, despite the protestations of Dusk and Demerzel.

As Dusk and Corintha discover a secret chamber guarded by a projection of Cleon I, Dawn and Sareth also realize that Demerzel has been controlling the Empire all along.

[23][34] In "Long Ago, Not Far Away", Dusk and Corintha learn Demerzel's backstory, and her function as the true guardian of the Empire, after which Cleon I traps them in the chamber.

Demerzel has feigned acceptance of Day's plan to end the Genetic Dynasty, but orchestrated the attempted assassination to be rid of Sareth by blaming it on her.

[37][38] He next appears in The Naked Sun, serialized in Astounding Science Fiction between October and December 1956, and published in book form by Doubleday in 1957.

[44] Olivaw reappears in the 1986 novel Foundation and Earth, which takes place over 20,000 years later,[43] and reveals that he has been manipulating the progress of humanity for millennia.

[44]: 213–215 In Prelude to Foundation, reporter Chetter Hummin warns young mathematician Hari Seldon that his nascent theory of psychohistory has attracted the dangerous attention of Eto Demerzel, the First Minister and chief advisor to Emperor Cleon I. Pairing Seldon with Streeling University historian Dors Venabili, Hummin assists him in his danger-fraught tour of several of the capital planet Trantor's 800 varied sectors, evading capture by Demerzel while gathering information he hopes will inform if and how psychohistory can be developed into a predictive science.

[44]: 213–215 James E. Gunn compared Seldon's revelation that Demerzel is a robot to the sequence in the 1933 novel Lost Horizon in which Hugh Conway guesses correctly that the High Lama is the 250-year-old monk Perrault.

"[46] In Forward the Foundation, ambitious politician Jo-Jo Joranum schemes to replace Demerzel as First Minister, with the goal of ultimately deposing Cleon I.