We've had our suspicions that there may be more to Reva than meets the eye, but the latest episode of Obi-Wan Kenobi finally pulled the curtain back on the villain's true motivations. During a conversation with Ben, it was revealed that she was among the Younglings in the Jedi Temple the night Anakin Skywalker turned to the Dark Side and slaughtered the children there.
Reva was attacked by Anakin, but managed to play dead before being taken by the Sith Inquisitors (that we didn't get to see, but the implication it happened shortly after is apparent, even if it was a few years later). This explains how she knows who is beneath Darth Vader's mask, anyway, and it's then we learn that she's waiting for her moment to strike and kill the Sith Lord.
That doesn't make her a hero as her quest for revenge is one that's led Reva down a dark path. Not even being named the new Grand Inquisitor deters Reva from her mission, and after Obi-Wan and the Rebels escape, she takes the opportunity to try and kill Vader.
However, he sees it coming and fends Reva off with the Force alone before a brief lightsaber duel ends with Vader stabbing her through the stomach (just as Anakin did to her when she was a child).
It turns out Vader always knew who Reva was and had only been using her anger as a tool. With that, the true Grand Inquisitor returns, explaining that while her rage was once useful, it is now tiresome. He and Vader leave the fallen Sith Inquisitor to die, but she finds Obi-Wan's communicator in the dirt (dropped by a returning Haja earlier in the episode) with a message from Bail Organa mentioning Tatooine and the fact that Luke and Leia are "his" children.
Ben senses a disturbance in the Force, and while he still has Vader to deal with, something tells us Reva will be looking to find Luke either to make Vader pay by murdering his son or to train him to become the one who is finally able to enact the revenge on Anakin Skywalker she wants.