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. Blanche begged Dan to put both her and Nora up for adoption, but he refused. They ended up compromising and while Dan was allowed to keep Nora, she was able to put Margo up for adoption. 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 and left a trial of death in her name. While posing as a babysitter named Michelle, she discovered George Smith(Josh's younger brother) had witnessed her stealing valuable jewels from his mother's safe. Realizing she was trapped and that George will likely testify against her in court, Margo killed him and stole the money from the safe. Josh found out while coming home from Police Academy and trailed her to California. Margo eventually arrived in California and learned about the Wakefield twins. She also hired James to seduce and murder Jessica upon suspicion. However, he betrays Margo and planned to tell Jessica the truth. Realizing her problems were getting worse, Margo intercepts the call and confronts James for the betrayal. When he refuses and tells her that Jessica deserves to know the truth, Margo murders James to cover her trail. She decided to kill and replace Elizabeth, but she was foiled by the unlikeliest of people, Todd(who found out her true identity) and Jessica(who realized Elizabeth was in danger). Their friends intervened and Margo crashed through the glass, and she was dead. With everyone believing Josh, he explains how Margo managed to cover up her tracks.
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. Nora explained why she should be Jessica and revealed her horrible relationship with Blanche whom wanted her to be like Elizabeth. As she continued to explain, Margo still tells her that with the longer sleeves, Nora could hide the Gemini tattoo as Elizabeth. She also forewarns that they can't screw up knowing that there could be someone out there to expose them like Todd and Josh did last time. Eventually, Nora comes to believe that Margo will kill anyone, even her. Thus Nora realized that she must kill Margo in order to stop her evil ways.
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.
Unlike Nora who was remorseful for her actions, Margo proved to be selfish, cunning and manipulative. Her stepmother, Blanche, was often afraid of Margo because of her troublesome ways and wanted to put both her and Nora up for adoption.