Margo Chapelle was born to Dan and Emmeline (Carter) Chapelle and had a twin sister, Nora. After their mother's death, their father married Blanche, who found the twins to be just like their mother, except that Margo was worse. After Blanche put Margo in an orphanage, she grew up in abusive foster homes, and the schools she attended reported various behavior problems.
Her last foster home was with the Logan family with a foster sister named Nina. She set fire to the house, and Nina died from smoke inhalation. Margo then took off, leaving a trail of deaths. She eventually arrived in California and learned about the Wakefield twins. She decided to kill and replace Elizabeth, but Jessica and friends intervened and Margo crashed through the glass, and she was dead.
Unknown to everyone, Margo escaped death by pretending to be dead and even used the shard glass that was caught in her throat to kill the paramedics in the ambulance. Afterward, she remained in hiding and eventually visited the cemetery where she attacked a girl visiting her grave. Margo is stunned when the girl introduces herself as Nora Chapelle, her twin sister.
Margo was happy she had a twin sister, and believed they could both kill and replace the Wakefield twins. She and Nora argued about who should be Elizabeth and Jessica. Eventually, Nora comes to believe that Margo will kill anyone, even her.
Margo kidnapped Jessica in the House of Mirrors at the carnival and tied her up at the school and then slept in Jessica's bed that night. Nora stabbed her to death, thinking it was Jessica. After Jessica's funeral, Nora eventually finds her way to the school and fights Jessica, thinking it's Margo. Elizabeth rescues Jessica and Nora is charged with assaulting Jessica and killing Margo.