Attending West Point Academy with her twin sister Shannon, Erin became friends with the distant and sociopathic Bruce Wayne. Due to her status as on of the elite of Gotham City, Erin ended up marrying rich as well. Her husband, however, was a member of the Irish Mafia, and following his untimely death, Erin was forced to take over. However, Harvey Dent was quick to place Erin and Shannon in prison, with their escape resulting in the death of Shannon.
Deciding to leave Gotham, Erin found Dent, disfigured him and killed his wife. The incident left Harvey insane and the GCPD forced McKillen out. Upon returning to Gotham several years later, meeting with the Irish Mafia, it was decided to get rid of the freaks who had taken over Gotham's crime. Erin was told to rid Gotham of Two-Face, Harvey's disfigured persona. However, the GCPD quickly learned of Erin's return, bringing her into direct confrontation with the Batman. Though she escaped from prison, Erin was quickly captured by Two-Face, now one of the top mafioso of Gotham.