She could be the Traveler or from the Traveler, although I don't think so. The good news is that the Stranger is one of the few plot points they bothered putting into the original Destiny so I'm pretty sure we're eventually going to find out who she is. But we don't know yet. When she says "I was not forged in Light," I think she means she was never a Guardian and did not receive Light from the Traveler, not that she herself was born in the Darkness. My interpretation is that she saw other things that were born out in the darkness. No theory seems more likely than not, but there are theories that are more likely than others. My favorite is that the Stranger is Maya Sundaresh. She was a scientist from the Ishtar Collective who helped create what were possibly the first exos with copies of her teams' own consciousnesses and also likely founded the Future War Cult (see Ghost Fragment: Vex 1-5). She spent her whole life studying the Vex and--if she founded the Future War Cult--attempting to look into the future(s?) which may explain why/how she possesses a gun with parts that shouldn't exist yet. If grimoire card Rasputin 4 is Rasputin addressing the Stranger (the passage, "you’re certainly not MINE although once you must have been," seems to imply Rasputin is addressing an Exo) she can also step through time which is a trait of the Vex or possibly someone who figured out how they do it (like a scientist who spent their original lifetime studying the Vex and many more as a consciousness inside of an Exo). If it's Maya, the Stranger was around pre-collapse and either through Future War Cult rituals looking into the future(s?) or from stepping through time some other way she has seen things no one else has. Like terrible things born out in the Darkness.