Karl Martin was a young man who was one of the Others. His origins are unknown, but he seemed to have been desperately trying to escape. Karl was Alex's boyfriend, but Ben, fearing he would get Alex pregnant, would not allow this relationship and locked up Karl. Afterwards, Karl escaped and joined the survivors with Alex. He, along with Alex and Danielle, was en route to the Temple before he and Danielle were assassinated by an unseen foe. Karl was shot in the chest; his death was later reported by Alex, and his body was found by Miles, Sawyer, and Claire. ("The Shape of Things to Come") ("Something Nice Back Home")
On the Island
Some time between then and the kidnapping of Jack, Kate, and Sawyer by the Others, Ben became concerned that Karl would get Alex pregnant and decided to prevent this by having Karl imprisoned at the Hydra Station. ("Through the Looking Glass, Part 1")
Days 68-91 (Season 3)
When Sawyer woke up in his cage, Karl did not respond to his questions about where they are and who he is. When Sawyer decided to investigate the cage and its buttons, Karl tried to warn him about the shocking device in the cage's food delivery system. Right before picking the lock on his cage, Karl asked Sawyer about the other survivors from the Oceanic Flight 815 (thus, Karl knew of the crash). (It should be noted that he asked Sawyer the walking distance to where he came from, making it sound as though there was no second island.) He broke out of his holding cell and freed Sawyer as well, apparently to serve as a distraction while he escaped.
Karl was captured by Tom and Juliet, who also captured Sawyer. He was forced by Tom to apologize to Sawyer for using him in his breakout attempt. Then he was taken away, presumably for punishment. ("A Tale of Two Cities")
After Karl was taken, Kate was put in his cell. While Kate and Sawyer were digging up and moving rocks, Alex, who was trying to remain hidden from the rest of the Others, asked Kate about Karl's whereabouts and whether she had seen him specifically in that cage. ("The Glass Ballerina")
During Kate and Sawyer's escape from The Others, Alex met up with them, asking them to help her find Karl, who she said was her boyfriend. At this point, Sawyer realized that Karl was the person originally in the cell next to his. If they agreed to help her, Alex promised, she would give them a boat to leave Hydra Island. ("Not in Portland")
The three arrived at the small building bearing the Hydra logo on the main doors. Inside, Karl was found in Room 23, restrained to a chair and wearing LED glasses, being forced to watch a bizarre video with flashing images and words, as well as a loud industrial-sounding soundtrack that seemed to be a form of brainwashing. When Kate and Alex freed him from the chair, Karl was not fully conscious and had to be carried out by Sawyer. ("Not in Portland")
Just before escaping, Juliet stopped them, forcing Alex to stay on the Island by saying that her father would need to see her when he came out of surgery, and would never allow Karl to live if Alex left. Juliet allowed the half-conscious Karl to leave in the boat with Kate and Sawyer. ("Not in Portland")
While the three were on their way to the main island, the still-groggy Karl repeated a phrase from the Room 23 video: "God loves you as he loved Jacob." Once they reach the Island, he briefly talks to Kate about the Others. He says that they use the Hydra Island for "projects" (but live on the main island) and that the children who were taken have been given "a better life" by the Others. He also reminisces about when he and Alex were younger. The following morning, Kate and Sawyer find Karl crying by himself. Sawyer talks to him alone, and suggests that he go back for Alex. Karl fears that he will be killed if he's caught again, but ultimately, he seems to be convinced that she's worth the risk. ("Stranger in a Strange Land")
Karl was later reunited with Alex and followed her path as she traveled with the Others. Hiding nearby but beyond the Others' detection, Karl received food and care from Alex in secret. When Ben decided to attack the survivors' camp earlier than planned, Alex urged Karl to warn them, pointing out that he owed Kate and Sawyer for saving his life earlier. Karl consented, taking the gun Alex gave him, and then ran to the shore. He hurriedly paddled the outrigger to the shoreline of the survivor camp, and there informed them that, once again, the Others were coming--earlier than the losties had thought.
He volunteered the gun to them, proving that he could be trusted as Sawyer and Kate had vouched for him. ("Greatest Hits")
On their way to catch up with Jack, Ben reveals to Alex that the reason he disapproved of her relationship with Karl was because he wanted to prevent her from getting pregnant, and having her life at risk on the Island, at all costs. Karl accompanied the main group of survivors to the radio tower, and reunites with Alex on the way. After Ben gets captured, Karl witnesses Alex's reunion with her mother. Karl then holds the rope which Ben is tied to, and guards him during Locke's confrontation with Jack. ("Through the Looking Glass, Part 1")
Days 91-97 (Season 4)
Following Jack's call for rescue, Alex and Karl both celebrated both the survivors' rescue and the fact that they were reunited, without Ben's interference. That night, when the survivors all met and split into factions he followed Alex and sided with Locke and proceeded with the rest of the group to the Barracks. ("The Beginning of the End")
In a moment of rest, Ben's taunts of his relationship with Alex resulted in Karl getting enraged and menacing his former captor with a gun, but he was told that "he's just trying to get in your head" and to take a walk by Sawyer. ("Confirmed Dead")
He was present, along with Alex and Danielle, when Ben told everyone that his spy on the freighter was Michael. Ben then told them that the freighter people's mission was to capture him, then kill everyone on the island. After the rest of the survivors left, Ben asked Alex to stay behind. He told her to go to The Temple because it was a "sanctuary" and possibly the only safe place left on the island. Karl and Danielle accompanied her.
On the way there, the trio stopped for a short break. As Karl told Alex that he was getting a bad feeling, a bullet went through his canteen. He stared blankly at it before he was shot in the chest. He fell over, dead. As Alex was screaming for him, Danielle claimed that he is "gone". ("Meet Kevin Johnson")
Before she was shot by Keamy, Alex told Locke's group that the freighter mercenaries killed Karl. ("The Shape of Things to Come") Miles later found his body while walking through the jungle (next to Danielle Rousseau's) and Sawyer was visibly anguished by confirming the young man's fate. ("Something Nice Back Home")
- Karl's surname was not revealed until the memorial video at the Lost panel at Comic Con 2009. 
- Karl has met all the main characters except Frank, Nikki, Shannon, Libby, Ana Lucia, Paulo, Eloise, Penny, Pierre, Christian, Eko, Daniel, Ilana, Michael, and Boone.
- When he first appears and his name is not yet known, Sawyer uses the nickname "Chachi" for him. This is a reference to the character from the 1970s/1980s television shows Happy Days and Joanie Loves Chachi. Presumably, Sawyer thinks that Karl bears a visual resemblance to "Chachi." Later on, he calls him "Bobby", in an admitted reference to The Brady Bunch. Karl, confused, asks Sawyer "What the hell is The Brady Bunch?" indicating that he has grown up unexposed to American pop culture.
- This is also a tongue-in-cheek joke referring to Karl's appearance on the show. Chachi, like Karl, was a new addition to his show several seasons into the series run.
- "Greatest Hits" contains a brief flashback from Karl, albeit a recap told by him to the survivors.
- Karl and Alex's love story could be a reference to Willa Cather's "O Pioneers!" The main character, Alexandra, falls in love with but is unable to have a life with her neighbor, Carl.
- Witnesses of death: Alex, Danielle Rousseau, several soldiers.
- Last words: "Well, at least we've got something in common."
- For fan theories about these unanswered questions, see: Karl Martin/Theories
- What was his status in the Others' hierarchy?
- How did he come to be on the Island?
- Who are his parents?