Book: The Exo Stranger.
Stories of an Exo who walks in the Darkness without a Ghost have long haunted the Tower. Legends say this anomaly dissolves in and out of the world, intangible and elusive, as if she is a visitor from somewhere beyond.
Some believe she's the last of an ancient Exo squadron, fighting a long-forgotten war. Others dismiss her as a hallucination caused by exposure to Vex technology. But there are those who maintain that her intervention saved their lives - or averted unspeakable catastrophes.