Sean Ramsey was a British navy officer. After most of its crew got sick and died, he took over the submarine HMS Achilles as its Captain/Commander. He then became leader of the Immunes. He came into possession of the Achillies after all crew members died of the Red Flu except for him and his brother Ned.
He believed strongly in that he and all Immunes were specially chosen to carry the torch for humanity and would stop at nothing to see this vision come true. He created a vast network for the Immunes and recruited fighters from Europe and the US to his cause. He skillfully manipulated people into following him and even turned many non-immunes against the crew of the Nathan James that were trying to help them. He was also responsible for the death of hundreds if not thousands of people in his crusade to build a new world for his immunes. He was last seen trapped on the ocean floor after the Achilles was sunk by the Nathan James. He presumably ultimately died in his sinking sub as he had no way out or back to the surface.