Doug Rattmann

Doug Rattmann, better known as the 'Rat Man', is an Aperture Science employee who survives GLaDOS's attempt to poison every employee in the facility with neurotoxin when she is switched on. He does not appear in-game, instead leaving behind paintings and murals which can be found throughout the Aperture test chambers in Portal and Portal 2. A paranoid, he is dependant on anti psychotic medication to keep him sane.

Aperture Science career
In the Portal 2: Lab Rat comic he is revealed to be an Aperture Science employee who worked on the Handheld Portal Device .He expresses doubts about GLaDOS and the effectiveness of a morality core, remarking 'You can always ignore your conscience'. His fears turn out to be justified when she poisons every employee in the facility when she is switched on during 'Bring your daughter to work' day. Prepared for the outcome, he survives the initial attempt to kill him and proceeds to sneak through the facility, avoiding GLaDOS who is actively hunting him. Acting on a 'hunch', he breaks into the test subjects file room and moves Chell to the top of the test subject order list, leading to the events in Portal.

Portal
Delusional, running low on medication and traveling with a talking Weighted Companion Cube, he watches from the shadows as Chell is tested by GLaDOS. Although never seen in Portal, his 'dens' can be found by the player, containing crude bedding, empty cans of beans and scribblings and dioramas on the walls. It is in one of these dens that the words 'The cake is a lie' is found scribbled on a wall, resulting in the well known meme. Following Chell's destruction of GLaDOS at the end of Portal, he follows the sound of the explosion and finds a route to the outside world. His joy is short lived as he witnesses the unconscious Chell being dragged back into the facility by a bot. Feeling guilt, as it was his actions that resulted in her being the first test subject, he once again enters the facility and finds Chell has been put in long-term relaxation. He then finds out all the cryo-chambers are offline due to the massive explosion caused by GLaDOS's death. He proceeds to save Chell's life by putting her into indefinitely long sleep, injuring himself in the process when he is shot by a Turret. He then enters a cryo-chamber himself and falls asleep. His fate is unknown.

Trivia

 * Doug Rattmann was behind the Aperture Image Format, used in the Portal ARG.