Ian Sinclair was a scientist assigned to the Mars City Underground on Mars. Because of the hallucinations experienced by many workers and rumors about the Underground being haunted, Ian was jumpy during his job. He had special access to the Processing Unit area.
When the player meets Ian Sinclair in Doom 3, he is working at a console that controls the Filtration Routing System. After expressing annoyance at the player for "sneaking up" on him, Ian remarks that the equipment is being affected by something unseen. However, he does not have time to report this before the demon invasion.
During the demon invasion, when the player passes through the area where he last saw Ian, it is revealed that he has been transformed into a zombie and his face has been torn off. The player is then forced to kill him.