Kai Opaka was the spiritual leader of Bajor in the latter days of the Cardassian Occupation and the early days of Bajor's independence from Cardassian rule. One of the most respected figures in the Bajoran culture, Opaka was left in the Gamma Quadrant in 2369.
Constantly working to protect her people and serve the will of the Prophets, Opaka became a revered figure among her people. During the last decade of the Occupation, however, she secretly collaborated with the Cardassians. Opaka sent Prylar Bek to inform them of the location of a Bajoran Resistance cell in the Kendra Valley. This saved the lives of 1,200 other Bajorans, but Opaka sacrificed her son in the process – as a member of the cell, he was killed in the resulting massacre. Opaka's involvement was never discovered, even after Prylar Bek made a full confession. Vedek Bareil lost the kaiship to keep secret the fact she had sacrificed her son and his group in order to save hundreds of civilians from retribution. (DS9: "The Collaborator")
The Coming of the Emissary
When the Cardassians retreated from Bajor in 2369, the Bajoran people began forming factions, vying for control of their new found independence. Each side looked to Opaka for support. She went into hiding on Bajor, refusing to meet with anyone.
When the Bajoran Provisional Government invited Starfleet to take command of the former Cardassian space station Terok Nor in orbit of Bajor, Commander Benjamin Sisko was placed in command of the renamed station, Deep Space 9. Hoping to unite the various Bajoran factions, Sisko met with Opaka. She surprised many by identifying Sisko as the prophesied Emissary of the Prophets. Opaka believed that Sisko's arrival had a deep spiritual purpose. She believed that Sisko's arrival was fulfillment of prophecy and that he would be the savior of Bajor. She entrusted Sisko with a powerful and mystical orb, telling him his destiny was to reclaim eight other orbs stolen by the Cardassians. Kai Opaka used the orb to transport Sisko to the beach where he first met his wife. Sisko later discovered the home of the Prophets, the Bajoran wormhole, which the Bajorans identified as the Celestial Temple. Later, he spoke with Kai Opaka, who informed Sisko that he was the Emissary of the Prophets and this was not last time he would work with the Bajoran prophets to secure the future of Bajor. (DS9: "Emissary")
After several months, Opaka left Bajor for Deep Space 9 to visit Sisko. Compelled by the Prophets to travel into the wormhole, Opaka somehow knew that she would not be returning. During a trip to the Gamma Quadrant, Opaka died in a runabout crash on a moon.
The moon was discovered to be a prison on which artificial microbes kept the prisoners forever alive to wage war with one another. These microbes resurrected Opaka, but forced her to remain on the moon. The artificial microbes restored a person's body after death, but that body then became permanently dependent on those microbes for all cellular functions. Anyone with the microbes would die if taken away from the moon. Accepting her new situation as the will of the Prophets, Opaka remained behind. She hoped to teach the warring prisoners peace. (DS9: "Battle Lines")
Vedek Winn Adami told Benjamin Sisko that she and Opaka once spoke about Sisko being the Emissary of the Prophets and she asked Opaka why an unbeliever was chosen to fulfill this role. Opaka told her that, "one should never look into the eyes of one's own gods". (DS9: "In the Hands of the Prophets")
Opaka appeared in an orb shadow that Sisko experienced in 2372. He had stepped down as the Emissary when Akorem Laan, who had been in the Celestial Temple for 200 years, returned and said that the prophets had chosen him to be the Emissary. Opaka convinced Sisko that he should accept his destiny and that he was the rightful emissary. (DS9: "Accession")
The non-canon novels Rising Son and Unity states that Opaka's given name is Sulan and have her return from the Gamma Quadrant accompanied by Jake Sisko in 2376. She did not return to the position of Kai, and during a hostage situation, served as a midwife for Kasidy Yates-Sisko, when Kasidy gave birth to her daughter.
Her mirror universe counterpart appeared in the Star Trek: Mirror Universe novel Rise Like Lions. The novel stated that she was elected as Kai in 2377 after the mirror Bajorans converted en masse to the long abandoned faith of their ancestors shortly after the discovery of the Bajoran wormhole by the mirror universe counterpart of Iliana Ghemor, who was hailed as the Emissary of the Prophets.
In Star Trek Online, a ship was named after Kai Opaka, the USS Opaka. When the 2800 Dominion ships that were lost in the Bajoran wormhole after the Battle of Bajor reappeared and attacked Deep Space 9, the Opaka was at the station, and aided in the station's evacuation.