I literally just thought about this but the Exo Stranger is from The Golden Age of humanity. Within the games story the ghost says [quote]you're not a guardian[/quote] to the Stranger. She replied saying[quote]I was not forged in light.[/quote] Obviously only the humans off The Golden age could have made them amd since she is no guardian, she must've survived all this time fighting the Vex with unknown allies (possibly Osiris). I know this is short and not too detailed but if you think about it, it kinda fits.