The reason that Reva wants Obi-Wan is likely because she was a Jedi present during Order 66. In fact, she may have been one of the young children present in the opening scene during the massacre at the Jedi temple. She likely lost her family and friends, then turned to the dark side herself as an Inquisitor.
The Third Sister soon confronted her quarry at the shipyard, revealing to him that she intended to keep him alive so she could deliver him to Vader, revealing to Kenobi that he was still alive, and admitting that she was privy to her master's true identity as Anakin Skywalker.
The decoy worked, and Reva lived to become the Third Sister. During her confrontation with Kenobi, she blamed the Jedi Master that he didn't do enough to prevent Skywalker from executing what she considered her family.
It may have also revealed a surprising detail about the Inquisitors. Obi-Wan Kenobi shocked audiences in Episode 2 when the Third Sister, Reva, turned against her boss and stabbed the Grand Inquisitor in the stomach.
As the massacre began, Vader stabbed Reva and left her for dead. According to Obi-Wan Kenobi, Reva used the dark side to keep herself alive.
Reva knows that Anakin Skywalker is Darth Vader because she was one of his targets—a child—during the Order 66 killings that took place during the events of Star Wars Episode III: Revenge of the Sith.
Reva spent her adult life pretending to believe in the values of the Sith, climbing the career ladder of the Inquisitors in the hope of getting close enough to Vader to assassinate him. Her attempt was, obviously, a complete and utter failure - the series ends with Reva wounded, humiliated and directionless.
We learn she is indeed one of the Padawans seen in the opening flashback, and she faked her death after watching her classmates be murdered by Anakin Skywalker. While she hates Obi-Wan for failing to stop his apprentice from destroying the Jedi Order, Vader himself is the true target of Reva's wrath.
The Third Sister is the leading villain of the new series which heavily featured The Inquisitors in its marketing.
This is not because the Third Sister knows that Leia is Anakin Skywalker's child; it is because the Third Sister found a link between Obi-Wan and Bail Organa and she believed Obi-Wan would do anything to save Bail's daughter.
As the Grand Inquisitor made clear, neither he nor Vader had any further use for Reva, and therefore they didn't consider her even worth killing. Instead, they left her crawling in the dirt because, as he had told her before, she was “the least” of them.
Duchess Satine Kryze | StarWars.com.
Reva, like all other Force-sensitive children, had been inducted into the Jedi Order at a young age; she had no other attachments or family aside from those younglings. Reva explains to Obi-Wan how she watched her “family” be killed and had to pretend she was dead to survive the attack.
Reva was herself skewered by Anakin's lightsaber. But the blow was not lethal—it is said that she drew upon the dark side to keep herself from dying. She also successfully feigned death to avoid further brutality. This enabled Reva to survive the Great Jedi Purge, but the course of her life had unalterably changed.
Throughout her encounters with Kenobi in the series, he refers to himself as Ben. The most straightforward explanation would be that Leia knows the person who rescued her is Ben, and never connected the dots that Ben is actually Obi-Wan.
The clear implication is that all the Inquisitors knew Darth Vader's true origins, which is indeed something of a surprise. Still, it does make sense from an in-universe perspective. Inquisitors were not Sith, but they served the Sith's purposes, trained in many dark side arts to hunt Jedi across the galaxy.
Reva was never a true Jedi, as her training ended while she was a youngling. As an Inquisitor, Reva isn't a Sith either, as the Rule of Two prevents the title from being held by anyone other than Darth Vader and Emperor Palpatine, who also limit the training and strength of their Inquisitors in canon.
Reva Sevander, better known as the Third Sister, is the central antagonist of the 2022 Disney+ Star Wars miniseries Obi-Wan Kenobi. She was a Jedi youngling who survived Order 66 and fell to the dark side of the Force, joining the Galactic Empire by becoming part of the Inquisitorius.
For years, Vader hated Obi-Wan, having been struck down by his mentor and left for dead while also believing the Jedi had worked to turn his true love Padmé against him.
The tragic past of Reva, a.k.a. the Third Sister, is revealed in the fifth episode of the Obi-Wan Kenobi series, which Darth Vader is somehow already aware of.
Warning!
Kenobi managed to convince Riva to betray her master, Darth Vader, by encouraging her to backstab him. Of course, such a paltry attempt to kill the fearsome villain proved futile – Vader disarmed Reva and stabbed her in the chest with her own lightsaber.
Reva's redemption then was to come after Kenobi told her the truth, that it was actually Darth Vader (i.e. the fallen Anakin Skywalker) and not the Jedi Council who were to blame. Realizing she had been wrong, Reva then lied to Vader, telling him that she killed Kenobi, only to be killed by Vader.
She also knows Obi-Wan, and she knows him well. Her holographic plea for help, already iconic, will never be the same. She knows Obi-Wan's real name as well as his alias of “Ben.” When Luke comes barging into her cell and says, “I'm here with Ben Kenobi,” Leia jumps off the bunk in an instant.
In the original Star Wars: Episode VI – Return of the Jedi screenplay, Obi-Wan did have a brother, and it was none other than Owen Lars (Joel Edgerton)—Luke Skywalker's step-uncle and Anakin's step-brother.
Aftermath. Although Order 66 greatly depleted the Jedi Order's ranks, with an estimated less than 100 Jedi surviving it, it was only the beginning of the Great Jedi Purge, which stretched on for years and claimed the lives of many of the survivors of the initial onslaught.