Since his debut, Kiefer and Kristina have been dating, but he has been proven to be violent towards Kristina. In May 2009, Kiefer got into a bar fight with Kristina's 17-year old brother Michael Corinthos, who recently got out of a coma. Kiefer constantly harasses Michael, leading to the point where the two fight at the Port Charles Country Club.
Months later, Kiefer proves to be untrustworthy when he begins flirting with another senior while dating Kristina. Kiefer helps Kristina cover her tracks in July 2009 when she ran over a pregnant Claudia Zacchara Corinthos, who later suffers a miscarriage. At the time, Claudia was pregnant with Kristina's half brother.
Kiefer has also voiced his opinion on Sonny Corinthos, but is proven to be a coward in front of him. Kiefer becomes jealous of Kristina's friendship with Ethan Lovett, which turns into a crush. After Kristina ditches Kiefer for Ethan, Kiefer beats Kristina up. To protect him from Sonny's wrath, Kristina tells everyone that Ethan was the one who abused her. Sonny's lieutenant and best friend, Jason Morgan, realizes that Kristina is lying and convinces Detectives Dante Falconeri and Lucky Spencer to investigate further. On April 2, they finally discover that Kiefer was the culprit.
Out of rage, Kiefer beats up Kristina again but this time, Kristina's mother Alexis witnesses Kiefer exiting the house right before finding Kristina unconscious on the floor. While driving to the hospital, a distraught Alexis hits her daughter's abuser with her car. Kristina's sisters Molly Lansing-Davis and Sam McCall find Kiefer in a ditch and take him to General Hospital, where he later dies of his injuries. Alexis hides her guilt for a while, but eventually confesses. Meanwhile, Kristina is questioned by Dante and finally identifies Kiefer as her attacker on both occasions. The truth quickly becomes public knowledge, and Ethan is cleared of all charges and given a clean slate.
Kiefer appears one last time on June 2. Kristina has a nightmare about Kiefer being next to her bed, telling her to get up so they can go to the beach together. Kristina refuses, and Kiefer quickly becomes violent and slaps her right before Kristina wakes up.
Keifer's parents, Warren and Melinda had a hard time dealing with his death. Warren went on an emotional rampage, shooting Mac Scorpio, Ethan Lovett, and attempting to kill Michael and Kristina. He then gets shot by Mac and dies.
In 2019, nine years after Kiefer's death, a woman named Kendra Lennon became Alexis' personal trainer, but unbeknownst to Alexis, Kendra was out to get her. It was later revealed that she had a photo of her and Kiefer as children, meaning she could be Kiefer's previously-unknown sister. Their brother-sister relationship was confirmed on October 16. On December 2, Kendra's last name was revealed to be Bauer, not Lennon, and she was eventually killed in a car accident.
Victim of a hit-and-run; Alexis Davis hit him with her car, while she was under emotional duress and he later died of the injuries he suffered (his injuries included internal bleeding (his spleen) and a fractured leg and hip) [Apr 2010]