S129 is an entity that originated from the Void. He was found by a group of reapers wandering aimlessly. It's theorized that he originated from one of the illegally run creation labs.
He is very fond of Paradigm for unknown reasons. He lived with her for an extended period of time. He'd make tea for her, water her plants, and other household duties.
When Kie Amina appeared in the Middle Level, Paradigm gave S129 the job of watching over her.