Dr. Erica Hahn (Brooke Smith) is a long-time rival of Dr. Preston Burke in the field of cardiothoracic surgery, dating back to their days at Johns Hopkins Medical School, where she graduated second in her class - after Burke.
Erica was a cardiothoracic attending surgeon at Mercy West Hospital in Seattle. Back in Season 2, she and her old adversary Dr. Burke fought over a donor's heart.
Because of faulty information from Izzie pertaining to Denny Duquette's condition, he was given the donor heart, much to her dismay. But in an ironic twist, Dr. Hahn was asked to perform the surgery on Denny after Burke was shot outside the hospital.
In Season 3, Erica Hahn was contacted for a consult on Harold O'Malley's valve replacement after George lost confidence in Burke's condition.
She and Burke have a love-hate relationship, and while she would die before saying it in front of him, Dr. Hahn respects and admires Burke tremendously.
She told Cristina Yang - at the time Burke's girlfriend / fiance, that if she became half the surgeon Burke is, that's pretty damn good. In Season Four, with Burke having left Seattle, Dr. Hahn was hired by Chief Richard Webber to head cardiothoracic surgery efforts at Seattle Grace.