Simon Phillips was a Cortexiphan test subject. He could read minds due to the experiments, which was not a direction that Bell and Bishop had in mind for his powers. This caused Bell and Bishop to cancel his progress in the Cortexiphan Trials, because he could find out about Peter being from the other Universe, and the tax it put on his brain from being around crowds of people. The Fringe Team approached him; including Walter Bishop, who helped make him read minds. He was upset at him because this forced him to be in the smallest of crowds, as big crowds results in overloads to his brain.
Upon meeting with Olivia, he quickly became good friends with her. He was happier due to Olivia Dunham being a fellow Cortexiphan Subject, as he couldn't read her mind. Even though he was reluctant, Olivia convinced him to read the mind of Aaron Downey, which he succeeded and told them of what he discovered. His information lead him and the Fringe team to discover a bombing attempt at a party which involved blue powder that disintegrated bones.
Olivia was able to help him through the overload, and he discovered who the bomber was. After this, he informed Olivia that he read Peter Bishop's mind, and told her that Peter still has feelings for Olivialternate. ("Concentrate and Ask Again")