He has been revived by Brigid Tenenbaum and Charles Milton Porter, who request assistance and work with Sigma during the course of his adventure. He is the eighteenth Alpha Series test subject, as noted by the Sigma symbols on his hands. Sigma is also a symbol commonly used in mathematics to represent summation (total) of a set of numbers/data.
It is revealed that Sigma is Charles Milton Porter, the creator of The Thinker. Porter had come to Rapture after his wife Pearl was killed in World War II. Adapting to Andrew Ryan's ideas, he met Reed Wahl, a brilliant engineer, and the two worked together to create The Thinker for certain operations in Rapture, like dispatching trains and phone calls.
Porter and Wahl began to disagree on how to use The Thinker after creating it. While Wahl used it to predict outcomes of ball games or stock prices and profit from it, Porter began to use it to replicate his dead wife's voice. After Wahl started splicing, which drove him insane, he framed Porter by creating a fake Audio Diary that suggested he was working for Frank Fontaine. Ryan had Porter arrested and converted into an Alpha Series, while Wahl took over Minerva's Den and became obsessed with using The Thinker to solve the 'predictive equation'.