The following article contains spoilers for Obi-Wan Kenobi Part V, streaming on Disney+.
Obi-Wan Kenobi Part V included some serious developments and revelations in its storyline. After learning Reva's tragic Youngling past and witnessing how the Jedi Master Obi-Wan desperately pushed her to attack Vader without being fully prepared, Part V saw the character taking an unfortunate turn when Vader drove a lightsaber into her body. However, as shocking as it sounded, Reva seemingly escaped death at Vader's hands... again.
As Reva, the Third Sister, revealed earlier in the episode, she faked her death to cheat Anakin Skywalker the first time. However, it seemed that Darth Vader might have left her alive on purpose this time. Instead of giving Reva a fatal hit in the chest, Darth Vader only severely injured her. Given his history of being rescued after the duel on Mustafar, and how Obi-Wan escaped a similar fate, it's not natural that Darth Vader didn't finish Reva off himself or order the Fortress Inquisitors to do so.
Reva and Darth Vader are similar in many ways. They both experienced death and the unimaginable pain of loss in the most personal ways. Anakin Skywalker gave in to the dark side partially due to his desire to reverse and control death. Reva's revenge is also rooted in the death of her fellow Younglings. Rage and revenge also played a huge part in Anakin's story before he became Darth Vader.
Faking her death to escape her fate during the Clone Wars showed resilience. Reva showed talent when she made her way to the Grand Inquisitor without losing control of her urge and hatred. Considering how Palpatine "nurtured" Anakin's dark side to his use, it seemed possible that Vader might have found a person of interest who he would like to train and guide to the dark side.
As seen in the original trilogy, Vader tried to convince Luke to join him in the dark side after learning Luke was his son. After all, Reva was already halfway there when she kidnapped and threatened Leia for a personal agenda. On the other hand, both Vader and the Grand Inquisitor seemed to take her revenge as a joke. It was also possible that Vader didn't see the necessity of finishing her off. How would Reva strike again after leaving her with mere breath?
Leaving Reva alive undoubtedly created more suspense in the series. The end scene in Part V also saw her accidentally stumbling upon one critical piece of information to figure out Obi-Wan's biggest secret. The twist in the plot hinted at the possibility of Reva using Vader's children against him should she find out about Luke Skywalker and Leia. However, Reva was a character who only existed on Obi-Wan Kenobi. Her lack of presence in the original trilogy sealed her destiny and almost guaranteed that the character would not substantially impact the storyline.
Season 1 of Obi-Wan Kenobi is streaming now on Disney+.