song, something that has become a hallmark of the series. At the conclusion of each episode, one of the characters delivers another voiceover, typically contrasting or following up on the initial one. Cast, characters, and appearance period Cast table Main cast The five characters who are first introduced in the series premiere as surgical interns, are Meredith Grey, Cristina Yang, Izzie Stevens, Alex Karev, and George O'Malley. They are initially mentored by Dr. Miranda Bailey, a senior resident who becomes the hospital's Chief Resident, and later an attending general surgeon, in season 6. The surgical program is initially headed by Richard Webber, the Chief of Surgery, who has a pre-existing personal relationship with Meredith, having had an affair with her mother when Meredith was a child. In Webber's employ are attending neurosurgeon Derek Shepherd, dubbed 'McDreamy' by the residents, and attending cardiothoracic surgeon Preston Burke. Shepherd is introduced as Meredith's love-interest, while Burke begins a relationship with Yang. Introduced in the show's second season are obstetrician-gynecologist and neonatal surgeon, Addison Montgomery, plastic surgeon Mark Sloan (nicknamed 'McSteamy' by the interns), from New York, and orthopedic surgeon Callie Torres. Montgomery is Shepherd's estranged wife who arrives in Seattle seeking reconciliation with him, Sloan is Shepherd's former best-friend, who aided the breakdown of his marriage by having an affair with Montgomery, while Torres is introduced as a love-interest, and eventual wife for O'Malley. The penultimate episode of season 3 introduces Lexie Grey, Meredith's half-sister who unexpectedly decides to pursue her internship at Seattle Grace Hospital after her mother's sudden death, and begins an on-again, off-again relationship with Sloan. Burke and Yang, having been engaged, endeavor to plan their wedding, while Montgomery departs the show in the season 3 finale, relocating to California, seeking a new life. The season three finale also shows Burke's exit from the show, after leaving Yang at the altar on their wedding day. Grey, Yang, Stevens, and Karev are all promoted to residents in the season 4 premiere, while O'Malley is forced to repeat his internship year, following his failing of the intern exam. Subsequently, Torres and O'Malley divorce one another, due to him having an affair with Stevens, initially concealing it from Torres. Early in the fourth season, cardiothoracic surgeon Erica Hahn becomes Torres's love-interest. During the fifth season, Hahn departs from the series after a disagreement with Torres, and O'Malley retakes his intern exam, passes and