I think she is from a future where the Vex won...yes the Vex not the darkness. She says in the strangers call mission when you ask "why are we here?" she says "they brought us here. Evil so evil it despises other evil." so perhaps she is trying to change the future by going back that is why she set you on the course to destroy the heart

Weapon implies she's from the future. You're friend's theory is unlikely because she states she isn't like us ("forged in light"), we were brought back to life via Traveler's light but she pretty much states she wasn't. One theory is that she was with the War Cult (that eventually grew to being the Future War Cult) helping conduct experiments on the Device. The woman speaking did not seem to own a Ghost but she knew what one was and had found/repaired one. There was a hint she wasn't a human so its possible she was an Exo. The Device was hinted at being able to do some sort of time travel when she asks the Ghost if its well enough to travel. She also referred to the Ghost as "little Ghost" which may or may not hint to the Stranger calling our Ghost "little light". Here is the Grimoire Card: "RECORD 343-CHASM-7887 Subject twenty-two. Admitted to the Inner Circle at 24:00. A promising postulant - I regret to say he performed poorly. He was administered the standard medication but refused to enter the Device. Aren't people unpredictable? I suppose there'd be no point if they weren't, would there? He knows to keep silent. END RECORD RECORD 343-CHASM-7888 Subject twenty-three entered the Device at 11:00. A clever girl from the Core District; an artist, before she joined the War Cult. At 11:03 she reported a sensation of floating. At 11:06, a sensation of lights within the darkness of the Device. Between 11:06 and 11:32 she reported these lights variously as white, golden, and blood-red. At 11:32 she reported a sensation of someone taking her hand; a stranger, but also herself. Twelve subjects have reported similar experiences. At 11:33 she reported the sensation we have called "The Opening Of The Veil." The Device recorded temporal displacement of her consciousness to the order of six degrees. At seven she began screaming. Brainscans near-death. Removed from the Device at 11:34. She believes without question that the Device granted her a vision of the future, and that it was one of utter Darkness. She thanked me for this enlightenment. She says it will make her stronger. Little Ghost, there in the corner of the Sanctum - I see you blinking. Are you listening? Are y - END RECORD RECORD 343-CHASM-7889 the Device at 12:22 and immediately the Device reported displacement of his consciousness. Visions of war and the City in flames. Subject twenty-nine worked the supply channels on the Slip before he joined the War Cult. By 12:27 he was babbling and by END RECORD RECORD 343-CHASM-7890 We have applied certain refinements to the Device. Novarro found records of a prototype of the Device at a Golden Age laboratory in Tibet, and Hari's team retrieved what was left of it. We are the first to see it operational in who knows how long. Too many subjects come back damaged. Mad. We are grasping at straws. What do you think, little Ghost? END RECORD RECORD 343-CHASM-7891 Forty-seven human subjects; eleven report timelines in which the Darkness has already prevailed, thirteen report timelines in which the City has fallen. Twenty-three babbled madness. Hopeless. Trapped. No wonder the Device was abandoned. The human mind is too weak for it. Too weak to look into the Future, or to understand what it sees. What the situation calls for, little Ghost, is a better sort of witness. We found you in pieces in Siberia, and repaired you as well as we could. What do you say? Are you well enough to travel? END RECORD"

I personally think she's the traveler itself. Maybe in a exo body but still traveler. The way the teleports at the end towards the traveler? My theory

Well the keyword in The Stranger's Rifle's description is "yet". Meaning she's from the future.

but that would make her from the past. (If she's from the golden age) Yet the gun is from the future....