Desmond David Hume was the lone resident of the Swan at the time of Oceanic Flight 815’s crash.
Before coming to the Island, Desmond’s life seemed to be defined by failure. He was fired as a monk, sadly broken up with his girlfriend Penny Widmore, dishonorably discharged from the Royal Scots Regiment, and regarded with contempt by Penny’s father Charles Widmore. In an attempt to regain his honor, Desmond entered Charles Widmore’s sailing race around the world and consequently became shipwrecked on the Island.
Once marooned on the Island, Desmond was found by Kelvin who recruited Desmond to push the button at the Swan station for three years, until he discovered that Kelvin was secretly planning to escape on Desmond’s boat. Desmond confronted Kelvin and accidentally killed him by tripping him into a rock. Desmond returned to the Swan but pushed the button too late, initiating the electromagnetic incident which caused the Oceanic 815 crash. He lived alone in the Swan for 44 days until the Oceanic survivors discovered the Hatch. After failing to leave the Island on his sailboat, Desmond joined the Beach camp.
Desmond soon agreed with Locke that nothing would happen if they failed to push the button every 108 minutes, but after realizing his previous failure had caused the crash, Desmond turned the fail-safe key which discharged the energy, showed the Island to the outside world, and imploded the Swan. After the implosion, Desmond gained a mysterious ability to see flashes of other times and timelines. He predicted Charlie’s imminent death and prevented it on many occasions. He later had a vision of a woman parachuting onto the Island, claiming to be working for Penny and revealing her boat was near the Island. Desmond and Sayid traveled by helicopter to the nearby freighter, during which Desmond’s consciousness from the past (1996) began drifting to the present (2004) until he became anchored by Penny, his constant. Desmond, along with helicopter pilot Frank Lapidus and the Oceanic 6, escaped the doomed freighter when the Island was moved, and were eventually rescued by Penelope herself.
He and Penny went into hiding, had a son, and lived on their boat until Desmond had a vivid dream that he referred to as a “memory” which told him to find Daniel Faraday’s mother in order to help the people left behind on the Island. He then seeks out and finds Eloise Hawking, who tells him that the Island is “not done” with him yet, but Desmond is reluctant to return there.
Benjamin Linus found them and tried to shoot Penny, but Desmond defended her and took the bullet. He fell, then got back up and threw Ben into the water. Desmond was taken to a hospital and survived, telling Penny he would never leave her again.
Desmond was then drugged and forcibly taken back to the island by Charles Widmore, who referred to him as “the package”. Once there, Widmore’s team performed an experiment on Desmond which gave him knowledge of the flash-sideways timeline. After agreeing to work with Widmore, Desmond met with the Man in Black who threw him into a well. Sayid was later sent by the Man in Black to kill Desmond, but he had a change of heart and let him live. Soon after he was freed by Rose and Bernard, but Locke/the Man in Black soon found them and coerced Desmond into helping him.
Desmond went to the source with the Jack and Man in Black in order to release the energy within. He removed a large stone cork, draining the water from the Source’s pool, thereby disempowering Jack, Locke, and the Island itself. After the stone was restored, Desmond was presumably taken home by Hurley, the Island’s new protector.
In the flash-sideways timeline, Desmond was the first to remember his previous life and he embarked on a mission to help everyone else remember. Later, once they had all remembered and became aware that they had died, the entire group “moved on” in a church.