Born in Tucson, Arizona, DeFer began her career in musical theater at age 8 and continued through high school with the lead role of Kim in "Bye Bye Birdie." In 2003 she traveled to Los Angeles to pursue her love of acting, and in 2005 she booked her first series regular role in the Fox production "The War at Home." She was a series regular on CW's hit show "Gossip Girl", playing the role of Ivy Dickens/Charlie Rhodes. Ivy was a struggling actress hired by Carol Rhodes to pose as her daughter Charlie to gain access to the real Charlie Rhodes' trust fund. Shortly after "Gossip Girl" ended, Kaylee met her husband, musician, Michael Fitzpatrick (Fitz and the Tantrums), and took the next several years off from acting to have and raise their three boys: Theo, Bash, and Remy. She returned to acting in 2021.