In Dune, Gaius Helen Mohiam is the Imperial Truthsayer, and the mentor of Lady Jessica, the Bene Gesserit concubine of Duke Leto Atreides.

Mohiam is interested in Jessica's young son Paul Atreides, who is a key figure in the Bene Gesserit breeding program but has also displayed unusual potential.

In the novel Dune, the Baron Vladimir Harkonnen has this impression of Gaius Helen Mohiam: An old woman in a black aba robe with hood drawn down over her forehead detached herself from the Emperor's suite, took up station behind the throne, one scrawny hand resting on the quartz back.

Her face peered out of the hood like a witch caricature—sunken cheeks and eyes, an overlong nose, skin mottled and with protruding veins.The Baron stilled his trembling at sight of her.

He passes the test, and Mohiam (though still furious over Jessica's choice to disobey the Sisterhood and bear a son instead of a daughter) is somewhat intrigued by the potential she sees in 15-year-old Paul, as she notes he has survived more pain than any other Bene Gesserit inductee.

[3] Later, Mohiam accompanies Emperor Shaddam IV to Arrakis in her role as Imperial Truthsayer; there she is faced with Paul's four-year-old sister, the pre-born Alia, whom she calls an Abomination.

[3] In Dune Messiah, the Reverend Mother joins the conspiracy to topple the rule of Paul Atreides after he ascends the Golden Lion Throne and unleashes his Jihad upon the Empire.

In exchange for the guaranteed safety of his concubine Chani, and the Sisterhood's acceptance of his decision to father no heirs with Irulan, Paul offers something of the utmost value: his sperm.

[4] In the Prelude to Dune series, Mohiam is instructed by the Bene Gesserit to collect the genetic material of Baron Vladimir Harkonnen (through conception) for their breeding program.

[16] The Los Angeles Times wrote that Rampling "is the only actor living who can project both world-weary wisdom and utter ruthlessness while wearing an absurdly high hat and a nearly impenetrable black veil ... her relationship with acolyte Lady Jessica plays a bit like a perpetually skeptical head of the CIA dealing with an exasperating but productive rogue agent.