Is Galen Marek considered part of the Star Wars canon? More specifically, does he appear in any canon property?
On April 25, 2014, a StarWars.com press release confirmed that the films of the sequel trilogy would not adhere to the post–Return of the Jedi Expanded Universe, with further comments from LucasBooks Senior Editor Jennifer Heddle confirming that the EU as a whole is no longer considered canon. The EU has been re-termed "Legends," with related publications remaining in print under that banner.
Since then, the only previously published material still considered canon are the six original trilogy/prequel trilogy films and the Star Wars: The Clone Wars television series and film. Most material published after April 25—such as the Star Wars: Rebels TV series and all novels beginning with A New Dawn—is also considered part of the new canon, on account of the creation of the Lucasfilm Story Group, which currently oversees continuity as a whole. Characters under the Legends banner are still available for use as needed, even if events concerning them are no longer canon.
Given that Starkiller's appearances are restricted to the video games, EU Novels and comics, his presence is officially part of the Legends banner and as such, not canon.