Kaitlin is the younger sister of the popular : Marissa Cooper, the daughter of Julie and her ex husband : Jimmy Cooper, the older half sister of a child whose name is unknown. Kaitlin grew up with her wealthy family, in Newport Beach, spending her time riding her pony named China. Her house, is next to The Cohen Family property.
Story of the character
During the first season Kaitlin was seen mostly at events with her family, or when she comes and goes from riding her pony, but she was not an important character anyway. Her life is then overwhelmed by the scandal of her father's misappropriation, and so the child has continued to live with her mother in her childhood home. But when her mother married Caleb Nichol, it is assumed that she had moved in the house, which the man had built for her mother.
In this season, Kaitlin is only mentioned a few times - once during a conversation between Caleb and Julie, another in a conversation between Marissa and Jimmy, and another between Julie and Jimmy. During a particular conversation it is revealed that Julie has sent Kaitlin away to a boarding school, to get her away from all the craziness that is going on with Marissa. Kaitlin is not really seen or mentioned again until the third season.
At the beginning of this season she shows up on the Cohens' front step looking for her mother. When Ryan and Seth first open the door they do not recognize the girl as Kaitlin, but they soon realize who she is and are surprised by how much Kaitlin has grew up in the time she has been absent. Seth puts it best when he says "Mini-Coop not so mini".
Not too long after her arrival, Kaitlin begins to stir up trouble for her older sister and starts to flirt with Johnny Harper, a surfer who has feelings for Marissa. She also causes trouble when some boys show up looking for her, and money she stole supposedly to help with a friend's abortion. As much trouble as she causes, Kaitlin does help in forcing Marissa to deal with her feelings towards Johnny even after his death. Kaitlin then returns to school but is seen again shortly when she calls Marissa for help with some trouble she has gotten into at school.
Kaitlin comes home again for good in the third season finale, under the pretense that she has come for her sister's graduation, but she then reveals that she has come home so that she can rule Marissa's former high school, Harbor.
Kaitlin is the only character who seems mostly unaffected by Marissa's death (most likely due to the fact that she and Marissa did not have a very strong relationship), but she has begun a very wild and unpredictable stage, including smoking pot, stealing clothes from a charity, blackmailing Dr. Roberts into buying her expensive leather boots, stealing her mother's car, throwing parties without permission, flirting with older men and other forms of typical teenage rebelling. Her best friends at the Harbor School, where she started attending following Marissa's graduation and death, are Eric and Brad Ward, the younger twin brothers of Marissa's ex-boyfriend, Luke.
Soon Kaitlin is attracted to band geek Will Tutt, but after being accused of making fun of Will's personality by a rival band member called Lucy, she breaks it off.
Kaitlin grows attached to Gordon Bullit, a man who is trying to date her mother, Julie. She convinces Bullit that Julie likes him. While he is away, Kaitlin emails Bullit pretending to be her mother. When he comes back Kaitlin finds out that Bullit is going to propose to her mother. She tries to tell them about the emails but it is too late, Bullit proposes. Julie says no, but she will date him as she can see how attached Bullit and Kaitlin are. Later it becomes clear that Julie is seeing Ryan's father Frank Atwood. When Kaitlin finds out she tries to stop the relationship.
Eventually, the character of Kaitlin, matures, deciding to stop with the little girl's games. In addition, the friendship between Bullit and Kaitlin grows, and he continues to be a part of his life. She also comes close to Frank, Ryan (thanks to some love tips) and Taylor, ending up making friends with all three. Kaitlin also becomes an older sister, with the birth of Julie and Frank's son.