Jin

Pre-Crash: Jin lost his job as a doorman when he let a man take his son into the hotel to use the bathroom. He met Sun after bumping into her and the two formed a relationship, despite the fact that Jin was poor and Sun was well off.
Sun knew it would be a problem with her father if she married Jin, but Jin asked him anyway. He said it was okay as long as Jin worked for him. Jin agreed, but quickly discovered that Mr. Paik was into shady dealings and Jin was expected to use violence in order to enforce Mr. Paik's wishes.
Island: Speaking only Korean, Jin could not understand the other survivors and wanted he and Sun to keep to themselves. He got into a scuffle with Michael over a watch that Michael found that Jin thought he had stolen. Because of the language barrier neither understood what was really going on and they got into a fight. Sayid handcuffed Jin to a piece of the wreckage.
When the raft burned, many fingers pointed at Jin, and Sun revealed the fact that she speaks English in order to save him. He hadn't known she could speak English and it caused a rift.
A new raft was built and Jin sailed away in search of rescue with Sawyer, Michael, whom he'd become good friends with, and Walt. Walt was taken by pirates and the raft blown apart. Jin washed ashore and was captured by the Tailies. The Tailies later captured Sawyer and Michael and they discovered that they were all on the same flight.
Jin, Sawyer, Michael, and the Tailies made their way to the other survivors' camp and Jin and Sun reconciled.
Jin had always wanted a baby, but Sun couldn't have children. Sun revealed to him that it is he who can't have children, yet she is pregnant. She swears that she was never with another man.
Strong Relationships: While rocky at times, overall Jin and Sun have a solid relationship.
Rocky Relationships: Jin and Michael were at first enemies, but ended up being really close friends.