The following contains significant spoilers for Obi-Wan Kenobi Parts I and II, streaming now on Disney+.

The Imperial Inquisitors first debuted during Star Wars: Rebels, introducing fandom to the Force-sensitive black ops zealots tasked with hunting down fugitive Jedi. They were led by the Grand Inquisitor, who was originally responsible for their training and deployment before Lord Vader usurped his role and the Inquisitorius came under his direct purview. Almost all the Inquisitors were originally Jedi, though their specific identities have been obscured since their indoctrination as agents of the Empire. Now known by appellations like the Ninth Sister and Second Brother, they are anonymous vessels of terror who will stop at nothing to execute their prey.

Obi-Wan Kenobi focuses on one of the Inquisitors in particular. A woman known as the Third Sister, who also in direct contradiction of the aforementioned precedent goes by the name Reva, seems to have a personal connection to the former Jedi Master. In the first two parts of the series, the nature of her animus is still unknown, but it seems specific to Kenobi as opposed to the eagerness to rid the galaxy of Jedi as a whole. As much as she seems to abhor Obi-Wan, she appears hungry to appease Darth Vader. Their connection is also unclear at this point and will certainly be revealed by the sixth and final episode but in the interim, here are some prevailing theories as to her 

Darth Vader's identity as Anakin Skywalker has always been a closely held secret accessible to only a handful of people in the entire galaxy. In the year 9 BBY (Before the Battle of Yavin) and the year that Obi-Wan Kenobi takes place, the only people previously known to be aware of Vader's past are Emperor Palpatine, Yoda and Obi-Wan Kenobi. Admiral Thrawn may also have known at this time based upon another canonical resource. The book Thrawn: Alliances, written by Timothy Zahn, described the secret in the following way from Darth Vader's perspective. "Vader stared into those glowing red eyes. No—it was impossible. The relationship between him and The Jedi was one of the darkest and most impenetrable secrets in the galaxy. It was unthinkable that the Chiss could have found his way through the barriers."