Martin Keamy is the main antagonist of the fourth season of the ABC TV series, Lost. He is an assassin hired by Charles Widmore to kill Benjamin Linus, and then burn the Island, which would kill everybody there, including the innocent survivors.
His team arrives on the Island, and kill Karl, Danielle Roussaeu, and takes her daughter Alex hostage. Knowing that Ben cares for Alex, because Ben raised Alex, Keamy attempts to negotiate for Ben's surrender in exchange for the safe release of Alex. Believing that he is bluffing, Ben does not comply, and Keamy shoots Alex dead.
Upon returning to the freighter, Keamy unsuccessfully attempts to kill Michael Dawson, whom he has discovered is Ben's spy, then obtains the "secondary protocol" from a safe. The protocol contains instructions from Widmore for finding Ben if he finds out Keamy's intention to torch the island, which he apparently had. The protocol contains details about a 1980s research station called the "Orchid" that was previously run by a group of scientists working for the. Keamy is also informed by Captain Gault that Keamy and his mercenary squad may be suffering from some sort of mental sickness, a notion Keamy dismisses. Later in the day, Omar straps a dead man's switch to Keamy, rigged to detonate on the freighter if Keamy's heart stops beating. That night, Frank refuses to fly the mercenaries to the island. In a display of power, Keamy slits the throat of the ship's doctor Ray and throws him overboard and later outdraws and shoots Captain Gault during a tense standoff. Frank Lapidus flies the remaining five mercenaries back to the island. The team apprehends Ben at the Orchid and takes him to the chopper where they are ambushed and killed by Ben's people and 815 survivors Kate Austen, and Sayid Jarrah. After a chase to recapture Ben and a brawl with Sayid, Keamy is shot in the back by Richard Alpert, who leaves him for dead, unaware of Keamy's bulletproof vest. Later, Keamy descends into the Orchid's underground level via its elevator to stalk Ben, who hides in the shadows. Goading Ben with taunts about his daughter's death, Keamy is ambushed by Ben, who beats him into submission with an expandable baton before stabbing him repeatedly in the neck. Though Locke attempts to save his life for the sake of the freighter, Keamy dies and the dead man's trigger detonates the explosives on the freighter, killing nearly everyone aboard.
Keamy is featured in the "flash-sideways" world in Season 6, portrayed as a business partner of Sun's father Mr. Paik. He was to receive payment for killing Jin from him, who had brought $25,000 to America, but it was taken away at the airport. Jin is put in a freezer in restaurant, and Sayid is brought to the same restaurant to be told that his brother failed to pay back Keamy some money for a business. Sayid retaliates and shoots Keamy, supposedly killing him.