In the origin vision of LOST, Kate Austen was supposed to be the de facto leader of the survivors. Dr. Jack was supposed to be killed by the smoke monster at the end of the pilot in order to maximize the horror and mystery of the island. Kate was supposed to be traveling with her husband, but gets separated by the plane crash (this would be reworked into the Rose and Bernard story arc). But the network executives wanted to keep Jack's likable character in the show. But in the final tally, Kate was the character with the second most screen time.
As background, Kate was born in May, 1977 in rural Iowa. She grew up with her mother, Diane, and Sam Austen, her stepfather. Kate did not learn of Sam's status until Wayne moved back into Diane's house after her divorce. It appears that Kate grew up as a rebellious tomboy. In 1989, she was caught with her friend, Tom Brennan, stealing a lunchbox from a store. Jacob interceded with the store clerk to pay for the lunchbox, tapping her on the nose to tell her "to be good." Kate and Tom turned that lunchbox into a time box of a tape message, toy airplane and a baseball. Kate and Tom talked that one day they would get married and have kids.
In 2001, Kate learned that Sam Austen was in the service so he could not be her biological father. She was upset that creepy, drunk and abusive Wayne was her father. Kate decided to "save" her mother from Wayne by blowing up the house (after taking out a large life insurance policy). Her mother called the police on her, which led to years of estrangement.
When Kate was hiding out from the law, she moved to Florida where she married a police officer named Kevin Callis. He knew nothing of her criminal past. Kate called Marshal Mars to beg him to stop chasing her, that she had a new life. The marshal remarked that Kate could never settle down. After a pregnancy scare, Kate started to believe she could not handle being a housewife. When Kevin got honeymoon tickets, Kate knew her lie would soon be exposed because she had no passport. She fled Florida.
In 2002, Mars contacted her to say he found "Tom's toy airplane" in a New Mexico bank vault. Kate went to New Mexico with her new boyfriend, Jason, and robbed the bank in order to get access to the box. Afterward, she killed her boyfriend and fled. She somehow wound up in Australia to work on Ray Mullen's farm, until he betrayed her by turning her over to the authorities. She was on Flight 815 with the marshal on route to the U.S. for trial.
We first see Kate wandering out of the jungle to startle an injured Jack, who asks for her help in sewing up his wound.
There are three ways to view Kate's character. Some people loved her scrappy style. Some people loathed her wishy-washy personality and runaway mentality. Some people neither liked or hated her and treated her more as a supporting character than a main one.
But most people would agree that Kate's back story was a horrible mess: the facts and circumstances have no basis in reality. Throughout the series, Kate's story line is rambling fiction, especially in regard to the legal issues presented in the series. Character's character posed a deep enigma among the fan base.
CHARACTER TRAITS
Kate's main character traits could be described as being wild, impulsive, aggressive, and sentimental. Her best defense was to run. Run away from her problems. She was not analytical in her problem solving but more emotional. She appears to have little moral compass in her decision making. She has a habit of using people, especially men with her cute charm, to get what she wants in life.
KATE'S JOURNEY
One has to remark that Kate's journey through the island to the sideways conclusion did not change her core. She lost her opportunity to become a stable influence as an O6 survivor raising Aaron in LA. Domestic life did not suit her. She could not keep Jack. She was still haunted by her past. She made up the excuse that she had to give up Aaron in order to go back to the island to find Claire. It was not a noble act, but a selfish one.
On the island, she manipulated both Jack and Sawyer, the perceived strong leaders of the group, in order to hide her deep criminal secrets. Even though Jack told her that everyone had a fresh, clean slate on the island, she refused to let go of her past. She continued to manipulate her relationships in order to get a personal advantage. She was with Jack when Jack was in charge; she went to Sawyer when Jack was no longer in a strong position. She went back to Jack, but he rebuffed her. She became jealous of Juliet because Juliet represented stability to both Jack and Sawyer.
Of all the characters in the show, Kate probably changed the least. In the end of her stay, she shoots a kills another man, MIB. And when she leaves the island, she leaves Jack behind. We don't know what happens to the Ajira passengers after they left the island, but we can only assume that they did not live a meaningful or productive life if Christian's words were correct, that the most important time they had was on the island.
SIDEWAYS KATE
While other characters had a more pastoral fantasy life in the sideways world, Kate was still on the run for murdering a man. She was caught and brought back to the states to stand trial for murder. She tried to rope in Jack for her escape, and tried to con detective Sawyer in letting her go. There was going to be a harsh end for Kate in the sideways story, until awakened Desmond broke her out to go to a concert. At that concert, she re-witnesses Aaron's birth, which makes her "remember" her island past. But what good memories were there on the island?
Kate "survived" a plane crash. She lied, manipulated and killed people. She played with other people's emotions, including Jack and Sawyer. She was tortured. She was attacked by the smoke monster. The only problem she solved in the end was getting Claire back on the plane for an alleged reunion with Aaron and his grandmother. But in an illogical twist, one cannot say that reunion ever happened - - - since Aaron was "reborn" in the sideways purgatory.
In fact, the improbable legal events portrayed in the Kate story puts into doubt whether the island and back story time lines were also fantasy places like the sideways realm. Many viewers do not want to believe that the series was all fantasy, because it would dilute the characters.
Then, there is the improbable conclusion that Kate winds up with Jack. Jack, it seems in his own fantasy world, wants Juliet as his soul mate. And if reality is truly a good barometer of her real feelings, Kate rejected Jack during the O6 days. And in the 1977 Dharma flashback, Kate was a quiet mouse with no real rekindled relationship with Jack. So why at the church, when the light bulb goes on, Kate is steadfast with Jack?
Remember, Kate is a married woman. When do adulterers get rewarded in the afterlife? She had strong enough feelings to marry Kevin. Why would he not be the love of her life? Would she not have strong feelings for her childhood crush, Tom, whom she caused to be killed in 2002 while fleeing a police chase. He certainly had strong feelings for Kate to put everything on the line for her. But the hazy on-and-off relationship with Jack trumped all in a soap opera ending.
Kate's character never fit into the main themes of the show: science versus faith. In fact, she is the antithesis to both propositions. Throughout the six season journey, Kate kept repeating her past (including past mistakes), including winding up with Jack in the front row of the church. It is hard to believe that is what Kate always wanted since all she was shown to want was an outlet to runaway.
Did anything in the sideways world indicate any change in Kate's character? No. Even Sawyer's good side was revealed as being a lawman instead of a law breaker. Even Jack's father relationship evolved with his fictional son, David, as a means of personal growth or understanding. Kate shared no growth of any kind: she did not represent herself in a different life (domestic bliss in Florida), repentant in a jail cell or having any maternal instincts of her own.
Kate's overall journey to the after life would be considered a nullity. There was no personal sacrifice. There was no personal redemption. There was no punishment for bad behavior. There was no true change in her personality or character traits. Kate continued to be Kate. And that is why so many fans have harshly polar opinions of her.