Honestly, I see him in the same boat as Han Solo. No, he is not a Jedi, but he is absolutely not dark side, either. As a clone of the original Starkiller, he is inherently good, and shows that through the canonical story of TFU.
Yes, he was trained by Vader at the start of the game, but he escapes and ends up helping the rebellion, much like Han did when he was a pilot in the Imperial Academy. This version never actually kills anyone before the point where this costume is shown, and in fact, is to be destroyed by Vader's orders for not killing a test droid that backs the fact that he is not dark side. The story writes this clone of Starkiller as good, whether the player picks light or dark through gameplay (much like Revan and his "canon" storyline places him as back to light side, whether you finished the game as dark or not.
By those details, I would put him in the RL. Under what, I'm not sure as I'm not familiar with the breakdown of their designations, but he is very much a "good guys" character at this point and not a "bad guy" as the 501st holds. Much like Mara Jade has both a dark and light side version in the different Legions, so should Starkiller/Galen Marek.