U.S. Army Captain Edwin C. Steen grew up in an affluent Seattle neighborhood, where he lived alone with his father after his mother died when he was very young. A quite, sensitive child, he lived alone with an abusive, workaholic father, whose abuse made Lifeline shun violence in his life. He learned the non-violent, defensive martial art of Aikido and decided he wanted to work in a field where he could help to heal others. He began working as a paramedic for the Seattle Fire Department, but when he learned he could jeopardize his pension if injured while providing care off duty, Lifeline decided to enlist in the Army where he would never be off duty. After basic training at Fort Dix, New Jersey, he attended X-Ray technician school at Fort Sam Houston, Texas. After a few years, he was offered a position on the G.I. Joe team.. As a combat medic he was officially a non-combatant, and intended to stay that way.
On one of his earliest missions, for the Joes, Lifeline was constantly berated by the marine Leatherneck for his unwillingness to fight, even in the face of enemy fire and injured teammates. Lifeline didn't let the insults bother him, and tended to the others' wounds, even Leatherneck's. As in many other missions, Lifeline was more than willing to lend a hand at other non-medical tasks, as long as he didn't have to use a weapon. At the end of the missions, Lifeline gave up the Joes' primary objective in order to save the lives of the Joes and members of the October Guard, ensuring that he and Leatherneck would never settle their differences. Lifeline continued to participate in many more missions, including the Cobra Island civil war, which the Joes entered on the side of Serpentor. A short time after the war, Lifeline and a number of Joes flew a Tomahawk rescue chopper into Southeast Asia to pick up four Joes who had been on a long mission since before the war. The aircraft went up against Russian attack helicopters and was nearly shot down before the team reached their objective and picked up their teammates. In subsequent years, Lifeline was one of the Joes who met the original G.I. Joe and later heped the Joes fighting to stop Darklonian terrorists in the heart of New York City.
Lifeline remained with the Joes until the death of the team's medical doctor, Doc, prior to the Battle of Benzheen. He was given the opportunity to go to medical school and attended John Hopkins University. Before he could complete his schooling, the Joe team was shut down in 1994. He worked at Walter Reed medical center for the next several years until 2001, when he was assigned to the reinstated G.I. Joe team. Lifeline's first task with the new team was to try and discover a way to stop the Cobra nano-mites that had infected many of the Joes. With Mainframe's help, he figured out how to save them. He later headed to the Arctic circle to investigate a secret laboratory where experiments had gone wrong and fought the mutated humans trapped inside.