During high school, Kirsten dated Jimmy Cooper but she broke up with him when she was admitted to the University of California, Berkeley--at least, that was her excuse for the break-up. The truth was that she was pregnant but she decided to abort (a secret she never revealed to Jimmy, but instead revealed it to Sandy, but only years later when she was pregnant with Sophie Rose). She married husband Sandy Cohen sometime during her rebellious twenties (she states that when they fell in love, she "lived in a mail truck and stank of patchouli"). Since marrying Sandy and giving birth to their son Seth Cohen, Kirsten joined her father at the Newport Group, becoming somewhat of a workaholic.
During the first season, Kirsten has to deal with suddenly having another child. At first she steadfastly resists having Ryan stay in the Cohen house, but in the third episode, she and Seth rescue Ryan from juvenile detention. After Ryan's biological mother, Dawn Atwood, embarrasses herself, her son and everyone else at a society event, Kirsten and Sandy agree to officially adopt Ryan.
Kirsten also plays a large role in her father's business. She is very successful, and on numerous occasions this causes Sandy to feel a little insecure.
Her true problems began during the second season when she fears that her husband is "cheating" on her with his old fianceé, Rebecca Bloom. Sandy and Kirsten's marriage begins to deteriorate, and she turns to alcohol to compensate. Her alcoholism is fueled by working closely with Carter Buckley, whom she has a crush on.
While Sandy is away with the boys in Miami, she invites Carter over and they share a passionate kiss before he decides to leave town. After Carter leaves, Kirsten's drinking problem becomes worse. She drives drunk and gets into a severe car accident, which fortunately leaves her unscathed.
When her father, Caleb, tries to talk to her about her drinking issue, Kirsten says extremely rude things. This is the last time she sees her father; he dies later that night when he suffers a heart attack and drowns in the pool at his mansion.
At Caleb's funeral, Kirsten makes a scene by drinking far too much and screaming at Sandy in the middle of the reception. After an intervention from her family and friends, including little sister Hailey, she goes to rehab.
Kirsten begins the season at rehab, where she is detoxing. Thanks to the support of the Cohens and their friends, slowly, she is returning to being the old Kiki, the one, who didn't need to stick to the bottle to feel good. But things get " complicated ", when Kirsten meets Charlotte Morgan, a con artist who claims to want to be her friend, but who really just wants her money. Fortunately, Charlotte fails in intent, thanks to the intervention of Julie, then Charlotte leaves.
She returns home to Newport and creates a high-class dating service with Julie.
In the season finale, she is encouraged by Sandy's decision to return to the public defender's office, giving up all interest in Newport Group.
At the beginning of the season, Kirsten, Sandy, Seth and Summer bring the deeply depressed Ryan back to the Cohen house five months after Marissa's tragic death in an car accident caused by Kevin Volchok. Kirsten and Sandy then venture to Mexico to chase down Ryan and Seth, who have gone in search of Volchok in Mexico. On Thanksgiving Day, Kirsten helps Taylor Townsend overcome the shock of the fact that her mother : Veronica Townsend, kicked her out of the house, just during the Thanksgiving holidays, inviting her to the Cohen house. Kirsten and Taylor, decide to cook together, with the help of Summer Roberts and Seth.
Kirsten is working on New Match, an agency for romantic dates. This season, the friendship with Julie, suffers a setback, when Kirsten, discovers that Julie, used their agency, as a kind of brothel for wealthy women, cause of this Kirsten, initially, she decides to break her friendship with Julie. Subsequently however, Julie decides to make amends and between the two returns the friendship, also they decide to continue working together. This time, however, in the right way.
Later in the season, on her 40th birthday, Kirsten finds out that she's pregnant. This reminds her of the time when she dated Jimmy Cooper. During the great earthquake, she is injured with bruise, but fortunately, her unborn daughter has not been hurt, the girl will be born then, in the finale. Also in the same episode, Kirsten together with Sandy and little Sophie Rose (named in honor of Sophie "Nana" Cohen and the late Rose Nichol), will move into the couple's old house in Berkeley.
Julie: I'm impressed. You're sneakier than I thought, Kirsten Cohen.
Kirsten: Well, being around you all these years, it's rubbed off.
Julie: I tried taking your advice, KiKi. And quite frankly, it sucked. From now on we do things the Julie Cooper way.
Kirsten: Julie Cooper, be careful.
Sandy: Who would have thought the Newport Group would be where I go for to a moment of Zen?
Kirsten: You know, sometimes this job makes you do a lot of hard thinking.
Sandy: I want this hospital more than I've wanted anything in a long time.
Kirsten: Then you can't let Matt's relationship with Maya Griffin stand in your way. For all you know, she could be using him too. Or they could live happily ever after. The point is, if you want this as bad as I think you do, you have to go for it.
Sandy: So where is the line?
Kirsten: I'm not worried about you knowing where the line is. You always do. It's who you are.
Kirsten: I'd like to propose a toast. To Sandy Cohen. Who has told me countless times over the past year how important this hospital is to him. You know, they say that when you grow up you marry your father. I thought I'd escaped that.