Dune is more than just one novel. It's a whole franchise that includes six original novels by Frank Herbert, and several more books about the lore and legend of House Atreides and their rise to power. Part of this extended library was put together from Herbert's own notes after the author passed away in 1985, and many characters in the later novels don't appear in the earlier books or any of the live-action adaptations.
None of the following characters appear in either the Villeneuve or Lynch versions of Dune, or the original novel. They only make their debuts in the sequels or supplemental materials written by Frank Herbert or his son Brian, in collaboration with other authors. Their roles are pivotal to the events of the series even if they don't appear in the first installment, which is really just an introduction to a vast epic.
7 Queen Anirul
The saga of Dune starts during the reign of the 81st Padishah Emperor Shaddam IV. He had five daughters, two of which are featured prominently in the series. One of the most important people in this dynasty is mentioned in the book: the mother of those daughters, Queen Anirul.
Anirul was not only a Bene Gesserit, she was also a Kwisatz Mother, one of the participants and guides of their Kwisatz Haderach breeding program. Her marriage to Shaddam, and her bearing him only daughters, was a part of that plan. She appears as a main character in the prequel trilogy Prelude to Dune, as well as the peripheral books Dune: House Corrino and Dune: House Atreides.
6 Murbella
The last book that Frank Herbert finished in the original series was about a new desert planet named Chapterhouse, where the Bene Gesserit had intended to cultivate worms and therefore spice once again. This is the book in which the character Murbella appears, and even though she appears later in the series, it seems that Herbert intended Murbella to be a part of the story's ultimate resolution.
Murbella eventually became a Reverend Mother of the Bene Gesserit, but she begins this story as part of a different order called the Honored Matres, rivals to the Bene Gesserit. One of the major conflicts of the novel is resolved when she becomes the leader of both orders, uniting them.
5 Sheeana Brugh
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Sheeana Brugh appears in the last two books of the original six, Heretics of Dune and Chapterhouse: Dune. A sand worm attacks her Fremen settlement on the planet now known as Rakis, and she is the lone survivor. Rather than harming her, the worm takes her to the nearby town of Keen. The Rakian Priesthood who live there, impressed by her abilities, adopts and raises her.
Years later, the Bene Gesserit seek out Sheeana as part of their plan to adapt Chapterhouse as the new Arrakis. They use her abilities to lure a worm into captivity in order to transport it to another planet. Once her DNA is analyzed, it's discovered that she's a descendant of the Atreides bloodline, which is why the worms obey her.
4 Supreme Bashar Miles Teg
Miles Teg was both a Mentat and a Bene Gesserit, and was held up as an ideal example of the success of the Order's breeding program. He was a descendent of House Atreides and was said to have an eerie resemblance to his famous ancestor, Duke Leto Atreides I.
Miles' various powers made him instrumental in both Heretics of Dune and Chapterhouse: Dune. His military knowledge and loyalty to the Order made it possible to hold off the forces of the Honored Matres until Sheeana was able to escape with a worm. The planet Dune and Miles himself were lost, but Chapterhouse was a work in progress and Miles would be resurrected as a ghola, a type of clone or replicant, to return in the last book.
3 Siona Atreides
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One of the direct descendants of Paul Atreides through his daughter Ghanima, Siona was one of the architects of the plan to murder the God-Emperor Leto Atreides II after he had reigned for more than 3,500 years. She only appears in God Emperor of Dune, but she's mentioned in two more books, Heretics of Dune and Chapterhouse: Dune.
Siona isn't just important because of how she hatches a plot to kill Leto II. It's because of her that a crucial part of his plan for humanity, called the Golden Path, is able to continue. Part of this is that humanity is now immune to the prescient vision of the Spacing Guild, thanks to the Atredies genetics that she will pass on.
2 Reverend Mother Darwi Odrade
As mentioned above, near the end of the series, the two rival female orders of the Bene Gesserit and Honored Matres join to become one. The last Reverend Mother of the Bene Gesserit before this union was Reverend Mother Darwi Odrade.
Odrade wasn't from a prominent or rich family, but her genetics were important. She not only carried the bloodline of the ancient Atreides, from which her name was derived, but also that of Corrino and Harkonnen, and it is said she bore a resemblance to the Lady Jessica. Odrade was known among the order for having high emotional intelligence, and this is why Mother Superior Alma Mavis Taraza sent her to Rakis to assess Sheeana's abilities.
1 Reverend Mother Lucilla
Lucilla's contemporaries were Odrade and Alma Mavis Taraza, and she had a similar background and abilities. She was also a descendent of Siona Atreides, with the same mysterious powers typical of their lineage.
When Odrade became the Reverend Mother, she made Lucilla the Vice-Chancellor of the planet Lampadas, where the Sisterhood kept most of their records. Despite her devotion to the Order, she seemed to have ambitions outside the Bene Gesserit in the form of her own house and title.