 First Sergeant Martin Christopher Keamy is a fictional character played by Kevin Durand in the fourth season and sixth season of the American ABC television series Lost . Keamy is introduced in the fifth episode of the fourth season as a crew member aboard the freighter called the <unk> that is offshore the island where most of Lost takes place . In the second half of the season , Keamy served as the primary antagonist . He is the leader of a mercenary team hired by billionaire Charles Widmore ( played by Alan Dale ) that is sent to the island on a mission to capture Widmore 's enemy Ben Linus ( Michael Emerson ) from his home , then torch the island . 
