The Builder (aka Steve Sissay) is a demolitions expert who works for The Ministry of Recovery. He contracted for the Department of Defense before the apocalypse, when Sigrid Hackenham was the permanent private secretary. Sigrid relied on him for "off the books" operations when she needed plausible deniability. He believes he was Sigrid's first call on Day Zero.
Steve and Amelia apparently journeyed to Cardiff a year before season 4 to secure radio communications and signal blocking equipment out of a government facility. It is implied they had a tryst, as Amelia states that the only things she likes about Steve are his high explosives, his upper arms, and "that thing [he] did in Cardiff."
Steve is interested in Dr. Lobatse, but she quickly turns cold when she realizes Steve was a mercenary in Sudan.