Glad you picked up on the American flag clue in the adaptation.
I really haven't developed too much of an idea on his backstory because I feel there are so many fascinating angles to approach his story that to come up with a "definitive" telling makes one lose the mystique of him just a little. I do think though that he likely was part of an expedition that perhaps got sucked into some kind of black hole or through some phenomenon that hurled him many thousands of light years to a region of space that would have been totally unknown to him, and with all his fellow astronauts killed, he suddenly finds himself in the very terrifying situation of being all alone in this distant environment. In a way, the sight of other human beings so far from his own world, but speaking in what has to be an alien tongue that he can't understand a word of, likely compounds his inner misery and in a sense makes it easy to understand why he becomes a "Silent One" who would take to sketching on his prison wall as his only way of keeping his sanity. In all likelihood, he probably scribbled his name on the wall but the characters would not have been recognizable to Starbuck.
That's my general theory, but expanding it into something more detailed I'm not quite ready to do yet. Thanks again!
