Siona and Leto II
Siona Atreides (also known as Siona Ibn Fuad al-Seyefa Atreides was a distant descendant of Ghanima Atreides and Farad'n Corrino, and one of the first carriers of prescience-cloaking genes.
Early Life
Siona was born the daughter of Moneo Atreides, Leto's trusted attendant and advisor during his last years. Like her father, Siona harbored a strong rebellious streak, and by her teenage years became violently opposed to Leto's domineering rule of his Empire.
Actions against the God-Emperor
Siona's rough charisma and intelligence helped her emerge as the leader of a group of like-minded dissidents, who were determined to end the God-Emperor's rule through any means necessary. During one incident, Siona barely escaped alive after stealing some of Leto's personal journals and plans.
Her rebellious actions continued even after Leto had forced her into joining his Fish Speakers. Leto's ambition of breeding Siona with his Duncan Idaho ghola of that time also riled the young Atreides descendant. As a result of Leto's tacit breeding expectation, Siona and the Duncan Idaho ghola also did not get on initially.
However, Leto's persistence with the defiant Siona continued, in no small part because of her formidable potential, which he later tested in what remained of the deep desert of Arrakis. Here Leto's hopes were realized. After forcing the unhappy and distracted young woman to drink liquefied Spice Melange, she fell into a spice trance during which she experienced prescient visions showing the need for Leto's Golden Path. But her experience did not bring her closer to Leto, and her determination to destroy him continued.
The plot to kill Leto
Siona's intentions finally found some substance when she met Idaho again, and the two realized their common ground. Knowing Leto's vulnerability to water - thanks to his sandworm-hybrid nature - the pair hatched a plan to drown the God-Emperor in the Idaho River as he travelled to marry Hwi Noree.
That plan succeeded in no small part due to the involvement of Nayla, one of Leto's most fanatical Fish Speakers. Having been ordered by Leto to follow any command Siona gave, Nayla severely damaged the main bridge over the Idaho river with a lasgun. The action sent Leto falling into the river, where he devolved back into sandtrout, before dying.
Leto, seeing his death and devolution as a necessary step on the Golden Path, had used Siona, Nayla, and Idaho to fulfill his own fate. His final gift to Siona and Idaho was the stewardship of his Empire.
Later Years
After Leto's death, his plan for Siona and Idaho was realized, and the pair married and had several children together. Their children, and indeed all their descendants shared Siona's invisibility to the prescient vision of the Spacing Guild, another necessary element on the Golden Path.
Behind the Scenes
Siona's full name, Siona Ibn Fuad al-Seyefa Atreides, translates as "Siona son of Fuad the Seyefa" Atreides.