Prior to the crash of Oceanic Flight 815
The cryptic Kate is not quite the innocent figure she initially seems to be.
She grew up in Iowa, the daughter of a soldier and a diner waitress. As we learn in flashbacks in What Kate Did, her parents divorce and Kate's mother marries a man named Wayne, an alcoholic who physically abuses Kate's mother (Diane) and may or may not have abused Kate. When Kate is making a scrapbook as a present for her father, she discovers that the man she thinks is her real father was still in Korea when she was conceived, and she then realizes that Wayne is her biological father. Kate takes out an insurance policy and blows up the house with Wayne in it, meaning to benefit her mother. She explains all this to her mother and then flees, but is captured at a bus station by US Marshal Edward Mars, who claims that Kate's mother went to the authorities with what Kate had told her. She later escapes from him, while being driven to the indictment hearing for Wayne's murder, after a large black horse appears in front of the Marshal's car, causing him to swerve off the road. While the Marshal is still stunned from the deployment of the air bag, Kate is able to take his keys. After a brief tussle in the car, Kate knocks him out of the car and drives off.
In the flashbacks in Born to Run, Kate travels around the country with a car trunk full of license plates from various states, using hair dye to disguise her appearance.
When she receives a letter that her mother is dying from cancer, Kate takes a chance and goes home. There she meets and reignites the romance with her childhood boyfriend Tom Brennan, now married and a doctor at the hospital. The two dig up a time capsule they had buried 15 years earlier, which contains a tape recording and a selection of Tom's toys, including a little toy airplane. Kate convinces Tom to arrange a visit with her mother, who is being guarded, apparently in case of her daughter's appearance. However, once Kate reveals herself to her mother, Diane begins screaming for help, and Kate is forced to flee. She asks Tom for his car keys, but he demands to go with her, saying that the police will be easy on her if she cooperates. Kate speeds through the parking garage, directly at a police car blockading the entrance, but an officer fires off a few shots as she passes, and she then crashes into another car. Kate turns to Tom and sees that he is dead. Kate flees the scene, leaving behind the toy airplane.
Subsequently, Kate is involved in an elaborate bank robbery in New Mexico, in which she presents herself to the bank manager as an innocent bystander, while she is actually romantically linked with the bank robbers' supposed leader. Posing as a "Maggie Ryan" applying for a loan, Kate is rounded up by the robbers as one of the hostages.
When another hostage overpowers a robber, Kate grabs one of their guns, but she immediately claims not to be able to use it. The lead robber then takes Kate into a back room, and after a quick kiss, hits her to continue the ruse. The robber brings in the bank manager and claims he will kill the seemingly innocent Kate, unless the vault is opened. Thinking he is saving her life, the bank manager complies. In the vault, the robber reveals that Kate has actually planned and organized the whole operation. Appearing intent on shooting the manager, the robber is stopped by Kate, who instead shoots the leader in the leg, then forces the manager to open a safe deposit box. Inside is a single item: Tom's small toy airplane.
Sometime later, Kate turns up in Australia as a fugitive. Using the name Annie and claiming to be Canadian, she arrives at the house of an Outback farmer, who, in exchange for help around his farm, offers her a wage and a home. Months later, in the middle of the night, Kate attempts to sneak out of the house. The farmer interrupts her in this attempt, and insists he drive her to the train station the next morning.
On the road, their truck is followed by Marshal Mars. The farmer admits that he saw Kate's picture on a wanted poster at the post office and has turned her in for the cash reward. Kate grabs the wheel and drives off the road, rolling the truck. Despite being free to escape, Kate pulls the injured farmer away from the wreck, giving the Marshal time to capture her.
Immediately prior to the flight, the Marshal reveals to an Australian law enforcement officer that he has been chasing Kate for three years, and that he had put the toy airplane in the safe deposit box as bait. He has the toy airplane in his possession, presumably having recovered it from Kate's possessions after capturing her.
On board Flight 815, she is in handcuffs, sitting next to the Marshal. Just prior to the crash, she asks the Marshal a favor, which she later claims was to make sure the farmer received the reward for turning her in. As the plane begins to break apart, the Marshal is injured. Kate steals the keys to her handcuffs, but makes sure to apply the marshal's oxygen mask before putting on her own.
On the Island
Since arriving on the island, Kate has been involved with most of the major developments that occur to the survivors, from the recovery of the transceiver, to the discovery of the repeating transmission, the polar bear, the cave-in, and the decision to move to the caves. This is due in no small part to her friendship with Sayid, Charlie, and Hurley, as well as her apparent feelings for both Jack and Sawyer, although her relationship to Jack suffers because of her continued manipulation and equivocation.
Kate agrees to kiss Sawyer only because that is his declared price if he is to tell where Shannon's inhalers are located. After they kiss, though, he tells her that he actually does not have the medicine.
Kate goes with Jack to find Charlie and Claire after they are kidnapped by Ethan. They eventually find Charlie hanged from a tree, and Jack revives him. She is with Jack when they confront Ethan after Claire mysteriously returns.
During a game of "I Never" in Outlaws, Kate reveals to Sawyer that she was married previously, though she says the marriage didn't last very long. She also reveals that she has killed a man, although she may have been referring to Tom, as she also tells Jack that the toy airplane "belonged to the man I killed" - although Tom was not the only man she killed, her father Wayne being the first.
Kate delivers Claire's baby boy in the middle of the jungle, while Jack is in the midst of trying to save Boone's life.
When Sawyer dumps out the contents of her backpack, she is carrying Joanna's - a survivor who drowned a few days after the crash - passport, presumably to use for future false identification, in Born to Run.
She is present when the hatch is finally opened with explosives, and is the first to enter it. Desmond captures her and is able to escape after Locke ties her up and puts her in the food storage room.
Because Sawyer reminds her of Wayne, Kate confesses to Sawyer that she killed Wayne. She says that this remembrance makes her feel sick inside when she looks at, or feels anything for Sawyer. She and Sawyer witness a black horse on the island. At the beach, she gives Sawyer a haircut.
After Sawyer, Jack, and Locke decide to go after Michael, Kate demands to go with them. Jack denies her request and she disobeys his request by following them. The Others end up capturing her and tie her up. Then, they point a gun at her head after she is brought out by an "Other" named "Alex". Jack decides to drop his guns to save Kate's life. Kate tries her best to say that she is sorry for following them. She only wanted to help. When they get back to camp Sawyer tries to comfort her by telling her that he would of done the same thing she did.
Later, Claire and Kate go out to find Rousseau so they can find the place Claire was taken to. When Claire asks Kate about Rousseau, she is reluctant to tell Claire about Rousseau killing her team because they were sick. They happen upon another hatch, which seems to be an abandoned medical station (dubbed "The Staff" on a map viewed by Locke in a later episode). Kate opens some lockers to discover raggedy clothing, a fake beard, and glue.
In "S.O.S," Jack enlists Kate to go with him to recover Walt in exchange for Henry Gale. Kate is flattered that Jack has welcomed her back into the club, but Jack merely states that he has chosen her because Sayid refused him. During the venture into the forbidden line, Kate apologizes for kissing Jack. Jack says he doesn't feel sorry about the kiss. As the two are about to talk about their relationship in further detail, an exhausted and possibly injured Michael stumbles toward them, thus halting any more attention regarding Jack and Kate's tempestuous relationship.
In "?", Jack asks Sawyer for the heroin he has stashed away in order to ease Libby's pain before she dies. Kate goes with him, and discovers that Sawyer's hidden stash is in his very tent, buried underneath the sand. She also is the one who tells Hurley that Libby has been shot. During Libby's death Kate is overcome by saddness and weeps to herself. There, Sawyer comes to comfort and weep with her. Kate then aids Hurely in digging the graves for the dead Ana-Lucia and Libby.
During "Three Minutes", Kate agrees to go with Michael and the other people to go and get Walt. She is unaware that in fact the Others told Michael that she is to be one of the people that has to be brought to them.
In the season 2 finale "Live Together, Die Alone" Kate is suspicious about Michael's claim of the Others being primitive "hillbillies," mainly due to her prior discovery of the clothing in The Staff. As they are walking she discovers that they where being followed by two members of the Others. She and Sawyer begin firing at them hitting and (presumably) killing one of them. Jack then tells them he knew that they would be following them and gets Michael to tell them about his plan.
The Others trap the party when they stop to investigate a pile of tubes in a field. While trying to escape Kate is hit with a powerful dart rendering her unable to move. She falls down in a comatose state. Jack then tries to carry her but is also shot by another dart and both are rendered immobile. They are taken to a Pala Ferry bound, hooded, and gagged where Michael is reunited with Walt and they leave. Kate gets the Other known as Tom (also known as Mr. Friendly) to admit that his beard is a fake. She then waits as Henry Gale comes to give instruction to The Others. Shortly after a mysterious light and sound imminates from the hatch, Kate, Sawyer, and Jack are taken away. (credit)