Dennis pretended he had a sick grandmother to get Caylee interested in him. He then took her on a fake failure date which ended as pizza and a DVD on his bed, which led to sex. He nurtured her dependence by making anonymous threatening calls, but stopped showing up so she became an emotional mess. He then told her he was afraid to love and that she cured him, and they had passionate sex again. He then slipped out of the window, never to see her again.
She left him an angry voicemail that he played for the rest of the Gang. He then explained his D.E.N.N.I.S. system to them, after which Dee told him that he couldn't get her back. He accepted the challenge and returned to the pharmacy, but Caylee had figured out that he was faking his grandmother's prescriptions. Mac then took her to the fair, where Dennis had brought Gladys to pretend to be his grandma. His plan fumbled due to Glady's poor acting and the rest of the gang's yelling. She left with Ben Smith. ("The D.E.N.N.I.S. System")
Caylee is portrayed by Jill Latiano, who is married to Glenn Howerton (who portrays Dennis). She is a model, actress, and dancer, and has appeared in a handful of movies, including Epic Movie and Fired Up and episodes of TV shows including Community.