Courtney DiLaurentis was the 'Alison' who came out of the DiLaurentis house and talked to the girls - Aria, Spencer, Hanna, and Emily the morning they planned to steal Ali's piece of the flag. Earlier that morning Courtney had stolen Alison's 'A for Ali' ring in a plan to become Ali, in hope that she would fool her parents and not be sent back to the mental institution she was on holiday from. She then switched places with her twin sister, Alison, and convinced her parents that she was Alison, the real Ali tried to tell her parents the truth, but they ignored her, thinking it was just Courtney being crazy. Courtney then came back to Rosewood and left Alison's friends for Aria, Spencer, Hanna, and Emily - since she only knew them, and wouldn't be able to fake being friends with Ali's old friends. They all quickly became friends while Courtney continued to pretend to be Alison. Courtney knew each of her friends' secrets which she liked to hint about in conversation. Courtney also was the cause of the blinding of Jenna Cavanaugh by lighting a firework which hit her in the face, however Courtney and Jenna had planned to light the firework to get her molester of a brother Toby Cavanaugh out of the house, it backfired however and Jenna was blinded. Jenna and Courtney had a bond over "sibling issues". Courtney was having an affair with Ian Thomas, who was way older then her, and was already dating Melissa Hastings. In her diary, Courtney wrote that she was offering Ian an ultimatum: either he break up with Melissa, or she would announce to the world that they were going out.
One night, when all five girls (Courtney/"Ali", Aria, Spencer, Hanna, and Emily) were all hanging out at Spencer's barn, Courtney hypnotized them. She then got in an argument with Spencer and ran off to her house. Outside, Courtney and Spencer got in a fight over Ian (Courtney's secret boyfriend). Spencer shoved Courtney and then walked away as Courtney ran off, looking for Ian who she planned to meet.