Even though Lily Shen serves as Chief Engineer for XCOM, her ties to Earth's supreme defense force started at a much younger age. She moved to California from Taiwan when she was in grade school, all under the guise of her father accepting an academic position at Caltech. This turned out to be a cover story, as her father secretly accepted the position of director for XCOM's entire engineering division.
When the aliens attacked, Shen was only eight years old. She said goodbye to her father as he departed for XCOM HQ, and would not see him again until XCOM lost the first war. After surviving the initial conflict, the two stayed away from the cities and tried their best to avoid ADVENT while she completed her education. It was exceptionally difficult for Lily when her father passed away, especially since he had yet to complete his life's work: defeating the alien invaders once and for all.
Chief Shen is a results-oriented individual who rarely shies away from conflict and finds it impossible to be complacent; a moment not spent working with her hands is a moment wasted.