Dr. Jack Shephard is a fictional character and the protagonist of the ABC television series Lost, played by Matthew Fox. Lost follows the journey of the survivors of Oceanic Airlines flight 815 on a mysterious island and their attempts to survive and escape, slowly uncovering more of the much broader island history they are a part of. The character was originally conceived by creator J. J. Abrams, though the direction of storylines owes more to co-creator Damon Lindelofand fellow showrunner Carlton Cuse. J.J. Abrams, the creator of Lost, once told Entertainment Weekly, "Jack Shephard may be the greatest leader in any television series." Actor Matthew Fox would have some influence on the character during the course of the series' production. For example, Fox's own tattoos were incorporated into the character's backstory. Although at an early stage in the show's development, the character was originally intended to die in the pilot, the writers soon changed this plan, and Jack became the show's main character from its pilot episode onwards.

The de facto leader of the crash survivors, in the show's first four seasons, Jack is very much a man of science. He serves as the antithesis of man of faith John Locke; however, his experiences in season five mold Jack into a believer, to the point of becoming John Locke's spiritual successor in the final season. His leadership role culminates in him taking briefly over as protector of the island from its immortal guardian, Jacob, and being the one to engage one of the series' villains, the Man in Black, in a climactic battle to the death. Jack's storylines have included the exploration of his relationships with various love interests: fellow survivor Kate Austen, and Juliet Burke, who is initially a member of the mysterious Others, who are the series' visible antagonists for the majority of its run. Other key relationships in the show involve Claire Littleton, whom he discovers is his half-sister; Hugo "Hurley" Reyes as his right-hand man; and Sawyer, his rival for the affections of Kate, even when at some points, Sawyer calls him the nearest thing he has as a friend.

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