Former worker at the Twentieth Century Motor Company, walked out on the night that the 'plan' was voted in by the 6,000 employees, saying that he would "stop the motor of the world". He is the inventor of the motor that Dagny and Hank stumble upon, and which Dagny becomes so focused on investigating.
In the exodus, Galt is the "handyman, I guess," his responsibilities including being "on call whenever anything goes wrong with any of the installations -- with the power system, for instance." The 'power system' being the practical application of the concepts behind the motor he invented.
He has been watching Dagny for years, and has actively acted against what he knew to be her own actions -- i.e. getting to Quentin Daniels when he knew that Dagny was on her way to him herself to plead with him not to quit.
Galt is the first person to 'strike'.