Grey's Anatomy

Upon her first appearance in the series it is established that Hahn is a long-time rival of main character and fellow cardiothoracic surgeon Preston Burke, dating back to their days at Johns Hopkins University School of Medicine, where she graduated second after Burke. She is introduced as a cardiothoracic attending surgeon at Seattle Presbyterian Hospital when she and Burke fight over a donor heart. Hahn reappears in season three when she is contacted by George O'Malley for a consult on his father's valve replacement.

Hahn transfers to Seattle Grace Hospital in the season four episode, "Haunt You Everyday", after performing a successful heart transplant at the request of the Chief of Surgery, Richard Webber. She takes on the role previously held by Burke, Seattle Grace's Head of Cardiothoracic Surgery. Hahn cultivates an antagonistic relationship with Burke's former protege Cristina Yang, refusing to let her scrub in on surgeries, and constantly criticizing her overly enthusiastic behavior. She later confesses to Addison Montgomery that she is purposely hard on Cristina, as she reminds her of herself as a Resident.

The character deflects the romantic attention of fellow attending Mark Sloan, admitting that she finds him attractive but wishes to keep her private life separate from her working life. She also develops a friendship with Callie Torres which becomes temporarily strained when Callie is led to believe Hahn has romantic designs on her. Although Hahn laughs off the notion, she goes on to kiss Callie in front of Sloan, to prove that he couldn't handle a threesome with the two of them, leaving Callie stunned. In the season four finale, Callie initiates a second passionate kiss, which Hahn reciprocates.

The two attempt a romantic relationship, but while Erica comes to terms with her sexuality quickly, Callie is not so sure of herself, and sleeps with Mark Sloan to "make sure". Erica initially forgives her, but eventually decides that she does not want a relationship with Callie while she is still so unsure about her sexuality, and so Erica leaves her post at Seattle Grace.