Simon Walker was a freelancing mercenary who accepted a contract from Julian Sark, as well as other members of the Covenant. He was usually the leader of the team he commanded. He also had both job and intimate relationships with Julia Thorne.
Simon was first seen in a bar, greeting Sydney by kissing her on the hand and calling her Julia. Sydney goes under that alias and the two begin to talk about the previous two years of their lives, and how Simon was wondering why Julia changed her appearance. The rest of the night was spent remenicsing about their assignments and their working relationship. Upon hearing of Julia wanting to return to the group (after their previous member, Laszlo Bogdan died) he accepted her offer and she was added to the team.
However, one of his team members found photos of "Julia" talking to Michael Vaughn. While in bed, Simon began to strangle Sydney and demanded to know who the man was and Sydney revealed his real name at that he was her supplier. After confirming the information, Simon was satisfied to keep her on the team. Simon and his team infiltrated a storage facility to acquire a biological element for Julian Sark. As they were leaving, they found Michael trailing them and uncovered the truth: he was working for the CIA. Simon forced Sydney to stab Vaughn and then kicked his body down a hill.
After going through the deal with Sark the next morning, Simon returned to his apartment to find Sydney in the shower. He told her that he was leaving for another assignment and at night, a contractor (who was really Jack Bristow) requested a job from him and Simon accepted it.
During the money exchange, Javier called Simon and told him that the contractor was a CIA agent. Just as Simon was going to kill Jack, he shot him in the chest. Jack told Simon he would lend health only if he revealed who Julia Thorne was. Instead, Simon tried to shoot Jack and he was forced to shoot Simon in the chest again, this time killing him.