Kate is a fugitive. She killed her father because of the abuse he inflicted on her and her mother. She has been on the run ever since. While on the run, Kate has acquired a few more charges including resisting arrest.
Kate is found by a bounty hunter in Australia and is being extradited to the United States. Both she and the bounty hunter are on Oceanic Flight 815. Upon crashing, the bounty hunter is killed and Kate is able to keep her fugitive status a secret.
Kate ends up becoming one of the Oceanic 6. She takes Aaron off of the island with her, as Claire has disappeared. She tells the world that Aaron is her son. She finally stands trial and is found not guilty. She ends up making a life with Jack and Aaron and for the most part, is happy. She raises Aaron as her own for three years, but thanks to Ben, she has no choice but to turn Aaron over to his grandmother.
She returns to the island for one reason; to find Claire and bring her home to Aaron. Kate finds out that she was a candidate, but that her name was “crossed off” because she became Aaron’s mother. Upon finding a mentally disturbed Claire, her resolve to reunite the two is stronger.
Kate is able to escape the island with Claire, Sawyer and a few others. We can only assume that the reunion between Claire and her son was all that Kate hoped.
In the flashsideways, Kate is still a fugitive and is incarcerated by Sawyer. After meeting up with Desmond and delivering Claire’s son, she remembers all. She helps Jack to remember and they head to “the light” together; full of happiness.