Doug Rattmann, commonly nicknamed as the Rat Man, was a scientist at the Aperture Science Enrichment Center. Prior to the events of Portal while GLaDOS began flooding the entire facility with neurotoxin, Rattmann is the only known employee to have survived. A paranoid schizophrenic, he is dependent on anti-psychotic medication as a means of keeping him sane.

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 all the staff in the Enrichment Center with neurotoxin upon activation during the company's Bring-Your-Daughter-To-Work Day. Prepared for the outcome, he survives the initial attempts to kill him and proceeds to sneak through parts of the facility, avoiding GLaDOS who is actively hunting him. Acting on a 'hunch', he breaks into the test subjects' filing room and arranges Chell's name to the top of the test subject roster, leading to the events in Portal.

Learn more about Doug Rattman: http://theportalwiki.com/wiki/Doug_Rattmann