Martin Shepard was a hitman who was brainwashed to kill. He unknowingly was the man who murdered Danny Hecht.
After losing his memory and becoming distraught due to his brainwashing, Shepard checked himself into an asylum that was run by K-Directorate. According to SD-6, Shepard would do whatever someone asked if the person recited a certain type of poem and would instantly forget what he did when it was recited again. Sydney was sent into the asylum to make Shepard remember what happened to a man he killed and to help him remember where his body was buried so SD-6 could find it. However, Shepard knew what Sydney was doing so he attacked her and demanded to know who she was.
After the fight, Sydney told Shepard that if he remembered where the man was, then the head of the asylum would let them go. If he didn't tell them, the doctor's guards would kill them both. Shepard said Sydney had a problem, since he preferred death as opposed to living in the state he was in now.
Somewhere down the line, Shepard's brainwashing wore off and he remembered everything that happened to him in his past. Shepard got out of his room and found Sydney ready to be executed by the head doctor. At the last moment, Shepard rescued Sydney and both of them escaped intact, retreating to a safehouse.
While they were there, Martin slowly began to remember his past, including how he found out what Sydney's real name was. After explaining how he saw her name on a picture frame in her house, Shepard told Sydney he was sorry, because he was the one who killed her fiance. Even though Sydney was deeply saddened and shocked at her discovery, she still felt sorry for Shepard, as he wasn't a bad man by nature and SD-6 used him the same way they used her. Shepard, hoping to get back at the organization that did this to them, told Sydney the location of the man's body, which was in Lousiana marshlands on the border of Texas.