For the most part Rogue One: A Star Wars Story consists of brand new characters, but given that it takes place just prior to the events of 'A New Hope', there are a few exceptions. We already knew Darth Vader and Mon Mothma would return as part of the ongoing conflict between the Empire and the Rebel Alliance, and now a key character on the side of good has been confirmed.
While Jimmy Smits was guesting on CBS' The Talk (But...but why!?!?) he revealed that his Bail Organa would make a cameo in Rogue One: A Star Wars Story. Organa is the Senator of Alderaan and a founder of the Rebel Alliance, which is why his adoptive daughter Princess Leia Organa was such a key figure. He was killed when the Death Star blew up the planet, but this story takes place prior to that occurring. Smits played the character in 'Attack of the Clones' and 'Revenge of the Sith', but his story is really fleshed out in the animated shows Star Wars Rebels and The Clone Wars.
Rogue One: A Star Wars Story opens December 16th.