Haunted by the trauma from her past, the Third Sister sought to exact revenge on the Dark Lord of the Sith Darth Vader for his slaughter of the Jedi during Order 66, which had taken away the only family she had ever known.
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. She became an Inquisitor in the hope of getting close enough to Vader to kill him, though that doesn't seem to have worked out for her.
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.
“We were always faced with the fact that Obi-Wan could never kill Darth Vader. So he needed to defeat someone. And so Reva was my attempt to give him someone to defeat or someone to save, because he's not going to save Darth,” Beattie summed up.
After making an issue of it, Obi-Wan put two and two together and figured out that Reva was a Youngling who survived the massacre at the Jedi Temple perpetrated by Anakin during Star Wars: Episode III – Revenge of the Sith. Reva then confirmed Obi-Wan's assertions and admitted to her plans to betray Darth Vader.
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.
Turning to the dark
Sevander eventually escaped the Jedi Temple and turned to the dark side. Wanting revenge on Vader, currently acting as the commander-in-chief of the Galactic Empire's forces, she joined the regime's Inquisitorius as an Inquisitor, in order to get close to him.
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.
The Third Sister was at the Jedi Temple during Order 66. She either saw Anakin kill her fellow Jedi or saw the recording of his crimes. That's why she has such hatred for Obi-Wan.
Background. Reva was a Jedi youngling who was present in the Jedi Temple during Order 66. When the clones raided the temple, Reva and her fellow younglings found Anakin Skywalker and asked him for help, only for him to kill them all. Reva survived by playing dead, and she swore revenge on Anakin.
While Anakin notoriously killed younglings in Revenge of the Sith, perhaps his first steps in embracing the dark side saw him extending mercy where he couldn't bring himself to kill them all, choosing to spare Reva instead.
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.
The duel between the two dark siders also highlighted a few important facts. First off, the fight proved that none of the Inquisitors are very powerful. Reva gave it her all, but she was nowhere near a match for Darth Vader.
Reva's hatred of Obi-Wan comes from the fact that he wasn't there to save her and the rest of the younglings during Order 66 — in his defense, Obi-Wan was hanging on for dear life at that exact moment because his trusted Commander Cody (Temuera Morrison) had shot at him.
Reva and the other younglings were eventually cornered in the Jedi High Council chamber by the newly-christened Darth Vader and the clones. 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.
SPOILERS for Obi-Wan Kenobi episode 2. Obi-Wan Kenobi episode 2 revealed Reva knew Darth Vader's greatest secret: that he is really Anakin Skywalker.
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.
Duchess Satine Kryze | StarWars.com.
Reva is Mace Windu's daughter who blames Kenobi for his death.
Arguably, most Star Wars plot points revolve around characters simply not knowing something, very often about their family history. But, on top of all of that, the biggest reason Obi-Wan doesn't know Anakin is alive is that the last time he saw Anakin Skywalker in Revenge of the Sith, Ben literally left him for dead.
Star Wars reveals Emperor Palpatine learned Luke Skywalker is Darth Vader's son during The Empire Strikes Back. After Anakin Skywalker turned to the dark side and became Vader, his children were hidden to keep them save from the clutches of the Emperor.
If you are asking about Vader post-Mustafar, then no, she doesn't. Padmé doesn't seem to know Obi-wan cut off his limbs and left him to burn alive, Obi-wan didn't believe Anakin to be alive and Anakin's surgeries were still happening as Padmé died so there was no way for her to know.
The reason why the Third Sister hates Obi-Wan Kenobi so much is because she was a Jedi initiate when Order 66 happened. On that infamous day, Reva was training with her fellow Jedi younglings in the Jedi Temple of Coruscant.
Moses Ingram, the actor who played the Third Sister, received the lightsaber as a gift from Lucasfilm president Kathleen Kennedy. The weapon was customized to fit Ingram's hands. Ingram spent a lot of time icing her hands before she learned how to wield her lightsaber.
Yeah, killed by Vader at the end of it. Yeah, I wanted this story, I wanted her story to end.