The actor revealed it by writing the name “Galen” on a fan’s Star War’s poster below his signature.The actor has mentioned before that his character is not an antagonist and is very important to the movie.
Galen Marek is a character in the Star Wars video games. This is his character description:
Galen was Darth Vader’s secret apprentice, who ultimately became instrumental in the formation of the Rebel Alliance when he gathered various wealthy enemies of the Empire together (in a failed scheme from Emperor Palpatine to arrest, interrogate, and execute the future Rebel Alliance leaders).
Source: Star Wars 7 news