Jeff was a crew member aboard the freighter Kahana. Sayid and Desmond discovered Michael in the engine room, where he was working with Jeff to fix the engines. Soon after Sayid and Desmond entered the engine room, Michael sent Jeff to fetch a pressure valve from the supply room. Jeff did not witness Michael's retelling of the circumstances that led to his being aboard the freighter. (4x08)
Jeff presumably was killed when the Kahana blew up as a result of Keamy's death. (4x13)
In the press release for "第4季 第8集", he was credited simply as "mechanic".
The actor that portrays Jeff has a very meaningful name in the Lost universe. He shares his first name with Sawyer's birth name, and his last name is the same as John and Emily Locke.