Kaitlin grew up in Newport Beach, in the house next door to the Cohens, spending her time riding her favorite pony named China. During the first season Kaitlin was mostly seen going to events with her family, or coming or going to ride her pony, but she was not a major character. Her life was then disrupted by her father's embezzlement scandal and she continued to live with her mother in her childhood home. Then when her mother married Caleb Nichol, it is assumed she moved into the home Caleb had built for her mother.
In the second 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 when 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 matured 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.
In the fourth season Kaitlin has become a title character, living with her mother in Summer's house with Summer and her father, Dr. Neil Roberts. 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, black mailing Dr. Roberts, stealing her mother's car, throwing parties without permission, flirting with older men and other forms of typical teenage rebelling. Soon Kaitlin is attracted to band geek Will, (played by singer Chris Brown) but after being accused of making fun of Will's personality by a rival band member, 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.
Kaitlin started off as some sort of a bad influence but in season 4 she isn't to bad. She grows close to Gordon Bullit, Taylor Townsend and Summer. She even gave Ryan some love advice in season 4.