Kathleen Louise was the second child born to Elliot and Kathy Stabler, conceived not long after Elliot returned home from the Marines while Maureen was a toddler. Elliot was unemployed but managed to get his B.A. from a community college during night time and attended the Police Academy. While in training, Elliot was at a shoot-out that was televised- Kathy saw it and the shock triggered labor a month early. Luckily Elliot managed to witness and help deliver the birth. When she was little, Kathleen and her Grandmother Bernadette were partners-in-crime: they took Grandpa Joe's fedora and his last Cuban cigar for their snowman, which pissed him off.
By 12 years old, Kathleen was on the soccer team; an older friend quit school and soccer because she was pregnant. It affected Kathleen that prompted Elliot to talk to her about sex. A few years later, Kathleen got into trouble in school regarding her grades and was grounded.
After 20 years of marriage, Kathy had enough and left Elliot, taking the children and moved in with her mother. Elliot saw his kids from time to time and at one time got worried when he found Kathleen's old boyfriend on a list of HIV patients, but Kathleen never slept with him. Because of that, he broke up with her. However, not long after Kathleen went to a party and did an illegal U-turn. Patrol pulled her over and smelled booze. She was arrested for Driving Under Influence. Kathleen didn't tell anybody about her father (she was scared of what he would do) until one officer found his courtesy shield in her wallet while she was arrested. Elliot managed to have the arrest 'go away' but it haunted them. Elliot relied on Kathleen to help with a case about high-school kids and alcohol- how easy it was to get access and progress reports from their social network profiles. Elliot realized his mistake of covering Kathleen's DUI arrested that she never suffered the consequences, so he burned her driver's license as there's no statute of limitations of being a bad parent. During the trial of Darius Parker, Kathleen's DUI arrest came to light and Elliot came close to perjury. Judge Elizabeth Donnelly resurrected Kathleen's paperwork and charged her. Kathleen was re-arrested while Elliot had to face the IAB. Kathleen was sentence to community service: picking up trash on the roads.
A year later, Kathleen stole Elliot's credit card, claiming to have bought books for college. But Elliot didn't believe her from the charges of her spending spree. Kathleen was caught taking a shower at the Fosters' home, escaping. Elliot and Olivia found Kathleen at the playground and took her home. She claimed to be high from pills, not knowing what they were. The next day, Kathleen snuck out and was found overdosed from the pills and with a boy she had just met. Elliot and Fin rushed her to the hospital where her stomach was pumped. Kathleen was diagnosed with Bipolar disorder. The Fosters wanted Kathleen arrested for their missing jewelry. So, she was charged with Grand Larceny 3 and Burglary 2. Kathleen claimed that she wasn't sick and refused help. Elliot tried to turn to his mother for help, but she refused to tell the Judge that she was sick, so he turned in the stolen necklace so Kathleen could be forced to get help. Olivia convinced Bernadette to see and talk to Kathleen in jail to face her problems. Kathleen agreed to get help, pleading guilty and to enter in-patient psychiatric treatment and take lithium while the Fosters agreed to the lesser charge of misdemeanor. Kathleen made improvements with her life- she helped Olivia to get through to Kim Garnet, who was in a physical abusive relationship with her boyfriend. Kathleen no longer drank so she became a buddy to escort fellow college students who drink and to be sure that they get home safety. But one night when Kathleen left for a few minutes to meet up a guy she liked for coffee while her buddy got raped by Chuck Mills. Kathleen helped with the Take-Back-the-Night rally where college girls share their stories. Kathleen's boyfriend got a hold of Chuck's disciplinary folder and she delivered it to her father. They both got into trouble as it was believed that Elliot told Kathleen to steal the folder but he didn't. ADA Sonya Paxton wanted to subpoena Kathleen but Elliot stopped her as Kathleen would have been kicked out of college. So, she was listed as an confidential source.