When Disney purchased Lucasfilm, the entire Star Wars Expanded Universe was removed from canon. However, the existing Clone Wars remained, and that’s exactly where Disney is pulling from for Star Wars: Rogue One.
After revealing the movie’s new cast of characters, Entertainment Weekly shed light on Saw Gerrera, described as an “old-time insurgent” from the Clone Wars universe. He actually fought alongside Anakin Skywalker when he was still a young Jedi and was unofficially trailed by Anakin, Obi-Wan, and the padawan Ashoka Tano.
You can actually see what happens to Gerrera in the Clone Wars, which you can watch on Netflix right now. Suffice to say he pays a big price for his involvement with the resistance, turning him into the weary character seen in the Star Wars: Rogue One trailer.
Despite what he’s been through, he continues to fight.
He has continued to fight; and, as he suggested in the trailer, he has become something—and it’s not quite a hero. Rather, he’s a man who has tried to do the right thing by occasionally doing questionable things.
According to Lucasfilm president Kathleen Kennedy, he’s on the fringe of the Rebel Alliance. “Consider him king of a battered veteran who leads a band of Rebel extremists,” Kennedy said.
You can learn much more about the character at the official Star Wars website.