Lucasfilm and Disney XD's Star Wars Rebels animated TV series takes place between the events of Star Wars: Episode III (Revenge of the Sith) and Episode IV (A New Hope), following a small group of resistance fighters (who reside on the spaceship Ghost) on the planet Lothal. The show's first season concluded with the Ghost's crew formally joining forces with the growing rebellion against the Galactic Empire; then this past summer's season 2 one-hour premiere special, titled "The Siege of Lothal", saw Darth Vader (James Earl Jones) become personally involved with the hunt for the Lothal rebels.
Rebels season 1 antagonist Imperial Agent Kallus (David Oyelowo) should be back in the upcoming season 2 episodes, though the mysterious Inquisitor (Jason Isaacs) from season 1 will not - having committed suicide, after he lost a lightsaber duel with Kanan Jarrus (Freddie Prinze Jr.) in season 1. However, it's now been confirmed that another villain skilled in using the Dark Side of The Force will be debuting in Rebels season 2 - a character voiced by Prinze's wife and Buffy the Vampire Slayer herself, Sarah Michelle Gellar.
Gellar's Star Wars Rebels character is known as the Seventh Sister Inquisitor, another member of the Inquisitor line, who uses various tools (ranging from lightsabers to Mimic droids that can imitate others) in order to track down the TV show's merry band of heroes. Buzzfeed has the exclusive on the reveal, with Gellar teasing that the Seventh Sister "definitely plays cat and mouse with Kanan," much as Isaac's Inquisitor did with the Jedi Knight back in season 1. You'll have to head over to Buzzfeed for the time being, to see additional video interview footage with Gellar, in addition to fresh material from season 2 that shows the Seventh Sister in action.
Below, however, we have screenshots of the Seventh Sister from Star Wars Rebels season 2 (with and without her mask on):
One of the reasons we gave for why Star Wars Rebels is essential viewing for Star Wars fans is that the series (like Star Wars: Clone Wars before it) better explains and expands The Force - both the Light Side and the Dark Side, at that. It has already been confirmed that the upcoming Star Wars: The Force Awakens will feature a villain in Kylo Ren (Adam Driver) who, like the Inquisitors, is not actually a Sith Lord, yet is skilled in the Dark Side. Clone Wars previously established greater nuance within the Dark Side, while Rebels has gone a step further by revealing that Dark Side orders other than the Sith exist in the new Star Wars canon, before fans are introduced to the Knights of Ren in Force Awakens.
The Seventh Sister may be from a group similar to The Nightsisters/Witches of Dathomir. Clone Wars antagonist Asajj Ventress was a Dathomirian herself - trained in the Dark Side of the Force by Count Dooku - but there are enough differences in her appearance to suggest the Seventh Sister comes from a different background than Asajj.
Another element of Rebels that we've praised before is how the show (like Clone Wars) highlights great female characters. Gellar, in her video interview, likewise praises Rebels for having two well-developed main female characters - the Ghost's pilot Hera (Vanessa Marshall) and warrior/artist Sabine (Tiya Sircar) - and, fingers crossed, the addition of the Seventh Sister means the show will now boast a solid female antagonist, much like Clone Wars had thanks to Asajj Ventress.
Furthermore, Anakin Skywalker's former Padawan-turned Jedi, Ahsoka Tano (Ashley Eckstein) from the Clone Wars TV show, is now formally part of the Rebels cast - in turn, setting her on a path towards an eventual confrontation with her former Jedi Master - and is working with Ezra Bridger (Taylor Gray) and the rest of the gang, to battle the oppressive Imperial forces on Lothal. As an additional bonus, it turns out Ahsoka isn't the only Clone Wars fan-favorite character who will be play a role in the Rebels season 2 over-arching narrative, either...
Star Wars Rebels continues with "The Lost Commanders" on October 14th, 2015.