Ana Lucia

Pre-Crash: Ana-Lucia worked at the LAPD as an officer. Her mother was the chief. While newly pregnant and on duty, Ana-Lucia was shot and lost her baby. She later shot and killed the man shooter. Her mother suspected and said she would help, but instead, Ana-Lucia resigned.
Ana-Lucia went to work in an airport as a security guard. One day in the bar she met Jack's father, Christian, and he persuaded her to escort him to Australia as his body guard. After going to Australia and seeing a run-in Christian had with a woman, Ana-Lucia decided to return to the states, putting her on Flight 815.
Island: Ana-Lucia is a tail section survivor. She quickly became the leader of the group and eventually led the remaining survivors across the island to meet up with the survivors of the fuselage section.
As they got close to the other camp, they heard whispers and Ana-Lucia accidentally shot and killed Shannon Rutherford.
Ana-Lucia later made amends with Shannon's boyfriend, Sayid, but she never really fit in with the group. After 65 days on the island, she was shot and killed by Michael who had made a deal with the Others in order to get his son back from them.
Strong Relationships: Jack, who had met her before the crash, and Libby, who was also a tail section survivor.
Rocky Relationships: Sayid, after the shooting of Shannon they were enemies, but Sayid later told Ana-Lucia that he didn't blame her for Shannon's death, he blamed the Others.