The final member of the Ghost's crew in Star Wars Rebels is Hera Syndulla, as revealed first on EW.com. Hera is a Twi'lek, a race that has featured heavily in the expanded universe, with the most prominent being Aayla Secura, a Jedi who appeared multiple times on The Clone Wars and even made it to live-action (for a few moments).
Hera is the pilot of The Ghost, the ship that Kanan, Ezra, Chopper, Zeb, and Sabine, all previously officially revealed, will be journeying through the nascent empire in, trying to begin a Rebel Alliance. The Ghost is her ship, in fact, as revealed in the video below. Described as "the heart" of The Ghost, Hera is voiced by geek voice acting veteran Vanessa Marshall, who has previously played characters like Wonder Woman, Black Canary, Black Widow, and many more familiar to the fanbase. She was first revealed through LEGO packaging, with her minifig included in The Ghost pack, which also includes Kanan and Zeb.
In Hera's debut video, Producers and staff said the character "makes flying the Ghost look easy." Dave Filoni describes her as a "very strong-minded character," while Marshall says she's also "nurturing." She "guides" the team - while Kanan may be looked at as the leader at times, she's still the "commander of this ship."