Jin-Soo Kwon, known as Jin, was one of the survivors in the Oceanic Flight 815 crash along with his wife Sun. He knows no English but would later begin to learn it. At first he separates from the others along with his wife. We see him as being very dominating over his wife Sun. Although later in the first episode, "Pilot", we see him sharing his sushi with others on the beach, even stopping to feel Claire's baby kick.
We find out in a series of flashbacks about Jin's past. He started by serving in the Korean army, then moving to Seoul to work in various hotels. As a doorman he meets Sun. He then goes on to catch the attention of Sun's father, eventually working for him in Paik Automotive. The working relationship changes dramatically when Jin's own mother blackmails Sun by threatening to tell Sun's father of his past. Jin tried to have her give the money back but Mr. Paik made him pay off the debt anyway by becoming his personal assistant of sorts. He was to be a messenger and soon discovered the real meaning of this job. He was to be a hired killer.
Home life was not great for Jin and Sun. Sun was growing increasingly concerned with his work after finding out exactly what sort of work he was doing for her father. Jin was also having trouble coming to terms with the jobs Mr. Paik had him doing. Mr. Paik never seemed pleased with Jin and that put a great strain on his marriage. Jin and Sun tried to have a baby but then find out Sun is unable to conceive children. It was around this time Mr. Paik gave Jin a gold watch to deliver. Jin is ordered to board a flight to L.A. with Sun to deliver this watch. That flight never makes it to L.A.
This watch was lost during the crash and later found by Michael, another survivor. He wore this watch without hesitation. When Jin discovers the watch on Michael he attacks him. This provokes the other men to detain Jin. At this point, Sun intervenes and explains the watch. After this incident, Jin and Sun separate themselves from the others by moving to the caves Jack discovered.
Jin continues to be demanding and strict with his wife Sun until the day the raft the losties were building caught on fire. Jin tried to put out the fire succeeding in only burning his hands. The others instantly blamed Jin for the fire and Sun screamed at them in perfect English surprising them all especially Jin. He was so upset at her he did not speak to her again for some long time. He breaks his silence only when he helps Claire deliver her baby.
He helps to finish the raft and they all set off together after coming to a better understanding of each other. The raft would then crash and Jin would end up on the other side of the island where he meets other survivors. Finally he was able to convince the others they too were Oceanic survivors. After several months time they made it back to the other side of the island where Jin and Sun are reunited.
Jin and Sun were now back together again and their relationship was better. They decided to use a boat they found to circle the island. There they see a four-toed statue. They decided to use this opportunity to fight the others. It did not go well and they abandon the mission with barely their lives.
Jin then tries to get Sun to teach him English. He was upset that she had kept the secret and argued with her about it. Jin had been previously learning English just by being around the other losties. Jin was there when they discovered Naomi. They decided to keep her a secret. Jin was also untrusting of Juliet because of her involvement with the mothers on the island. It was not until Sun confessed that Juliet had shown her their child in ultrasound that Jin let his distrust go.
The losties again would divide and Jin would side with Jack this time. Shortly after a dead body comes ashore and Jin and Sun go to the Staff station to collect medical supplies. This is where Charlotte admits she understands Korean. They travel out to the freighter that is supposedly waiting to take them all home. It is here Jin discovers the control room is wired with C4. He stays behind to try to slow the detonation and as the helicopter takes off the freighter is blown up with Jin on it.
After the losties leave the island they believe Jin to be dead. Sun has their baby and mourns his death. Sun is bitter towards Ben for Jin's death believing it is all his fault. She goes on a mission for revenge.
Jin actually survives the explosion and washes ashore the island. Here he meats Danielle Rousseau. He promises to lead Danielle and her team to the radio tower but they get sidetracked in the jungle next to the Temple. It is here Danielle's men become infected.
Danielle was about to shoot Jin but the island skips in time again shooting them to a different time. After recovering from the jump he is discovered by Sawyer, Locke, and the rest of the group. They go to the Orchid. Another jump in time and the losties find themselves in 1972. Jin and the others join the Dharma Initiative.
Three years later they discover Jack, Kate, and Hurley. They decide to find a way to blend them into the Dharma Initiative. They live like this until their cover gets blown. The others want to leave the island but Jin wants to stay in case Sun should arrive. They then decide to activate an atom bomb core in the center of a hole the Dharma Initiative is building for a station. The bomb does not exactly work sending them all into the future.
Jin was one of the captives brought to the Temple. They watch as Sayid drowns and is then brought back to life. The smoke monster gains access to the Temple and everyone splits to escape. Jin hurts his ankle and is found by a now very crazy Claire. Jin meets the a now changed Locke and realizes that all is not what it seems. Frightened he lies to Locke and Claire to avoid being killed. Locke promises Jin if he follows him they will find Sun. Jin complies with their wishes.
Several days later Jin and Sun are finally reunited on Hydra Island. Widmore imprisons them in cages. Locke attacks the camp and Jin and Sun are allowed to escape. They enter the submarine to get off the island. They discover a bomb on the sub and it explodes trapping Sun in some debris. Jin refuses to leave her alone and he dies with her in his embrace.