Board Thread:Spoilers!/@comment-1916997-20160915154417/@comment-24674051-20160915171246

And I thought I did a lot of overanalysis?! XD

But, if the musketeer is in fact one of the famous 4, I would agree that Athos would be the most likely candidate. Of course, if they pick up the story of the Musketeers later in their lives (more towards the time of "The Man in the Iron Mask" for example), then the lone musketeer could in fact be D'Artagnan, and would not need to be portrayed as a younger man.

But for now, I'll agree and say it's most likely Athos.