 Introduced in the show 's second season are obstetrician @-@ gynecologist and neonatal surgeon , Addison Montgomery , plastic surgeon Mark Sloan ( nicknamed ' <unk> ' by the interns ) , from New York , and orthopedic surgeon Callie Torres . Montgomery is Shepherd 's 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 three 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 at the conclusion of the third season , relocating to California , seeking a new life . The season three finale shows Burke 's exit from the show , after leaving Yang at the altar on their wedding day . 
