Is Galen Marek Really Luke Skywalker?

Answer 1

No, he is not. In-universe it isn't possible due to the fact that Luke was raised on Tatooine with Obi-Wan Kenobi watching over him, albeit from a distance, although even he would have noticed if Luke disappeared right out from under his nose to go train as a Sith apprentice.

Galen Marek's codename in the video game The Force Unleashed was Starkiller, which was supposed to be the surname of Star Wars' main character, Annikin Starkiller, in an early draft of the first film before becoming Luke Skywalker. Adding to the confusion was a comment made by Galen Marek's actor, Sam Witwer, where he stated that "[Galen Marek] is what would have happened if Luke had joined Vader." The Galen character is suppose to be a dark mirror of the Luke Skywalker character, both of them were strong with the force but one was raised without it while one was raise engulfed with the darkside.

Rest assured, though, that Luke and Galen are two completely different people. Despite George Lucas' penchant for retconning canon to include improbable coincidences (such as Darth Vader building C-3PO, leaving him on the Lars Family's farm for about a decade, yet later on Owen Lars purchases the very same droid and has no recollection of the ten or so years he spent on that very farm...droids get mind-wiped regularly, but so far he hasn't shown the same happen to humans...), I think Luke and Galen being the same person would be going too far, even for him.