This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
Siranna was a female Kelpien who lived during the mid-23rd century. She was the sister of Saru and the daughter of Aradar. In 2239, when Saru had left their homeworld of Kaminar aboard the USS Archimedes, Siranna had witnessed a light in the sky (the Archimedes going to warp speed), and had chosen to become a priest to seek out that light. After her father had undergone vahar'ai and been taken by the Ba'ul, the predator species on Kaminar, Siranna became the priest of their village. (ST: "The Brightest Star", "The Sound of Thunder")
Siranna was later mentioned by Saru in 2257, during a conversation that he had with Michael Burnham; when it seemed that he was going to die as a result of vahar'ai, Saru admitted that Burnham had taken Siranna's place in his life, as a sort of surrogate sister. (DIS: "Brother", "An Obol for Charon")
That year, during the USS Discovery's investigation into the "Red Angel", one of the red bursts that typically heralded its presence appeared in orbit over Kaminar; Siranna had seen the burst from the surface, interpreting it as a "fiery sign". Shortly afterward, she was shocked to see Saru alive and well, but her joy turned to anger when she demanded to know why he had never come back before then, combined by her fear of retribution by the Ba'ul. Siranna was later taken prisoner by the Ba'ul along with her brother at a Ba'ul stronghold. There, she discovered that Saru had undergone (and survived) vahar'ai some time before, proving that the "Great Balance", the Kelpien belief cycle that Siranna had upheld, was a lie; the Ba'ul, who had previously been the Kelpiens' prey, sought to prevent the extinction of their race by culling the Kelpiens before they could evolve past vahar'ai. She agreed to help Saru trigger vahar'ai in their entire race, allowing them to grow beyond their ingrained culture of fear. While Saru returned to the Discovery, Siranna chose to remain with her people, knowing that they would need help to find a "new balance" with the Ba'ul. (DIS: "The Sound of Thunder")