There's a skeleton hidden in every closet. Especially when that closet is the entire Valley. Mr. Faust's name is so rarely pronounced there, that not every Ivo XO's subordinate knows about the existence of one of the most talented Wasteland engineers.
A longtime friend of the Mentor, he was a member of the first ENgineers, chose Ivy's side in the conflict with the Founders and stayed with her afterwards. Doing the most "delicate" work of designing and repairing vehicles, he hasn't left the Engineer base for a long time, having become practically invisible to those around him.
As time when by, he gradually became forgotten not just in the Valley, but within the organization itself. But the attack of the Ravagers and the alliance with the Founders prevented Mr. Faust from disappearing for good. His past life acquaintances and the appearance of advanced technology that could be captured forced him to resurface, which means that the skeleton has emerged again.