From the moment we first met her, Addison Forbes Montgomery Shepherd was one of the most compelling characters on the show.When viewers were first introduced to Addison, a.k.a. Mrs. McDreamy, she was the villain, arriving in Seattle just in time to ruin Meredith's relationship with Derek - Addison's husband. It turns out Derek left New York for Seattle after catching her in bed with Mark, a plastic surgeon and his best friend - a fact he conveniently withheld from Meredith.While she gave Derek the chance to sign divorce papers and end things between them for good, he declined. They slowly began to patch things up, although much guilt and resentment remained. The reappearance of Mark, who tried to lure her back to New York, did not help matters. Addison also hated living in Derek's trailer, lifestyle and the fact that he adopted Meredith's dog.Their relationship ended for good at the end of Season Two, when Derek had an affair with Meredith and decided his heart was with the intern, no matter how much his brain wanted to make his marriage work. Their divorce became final at the end of Season Three.Addison specializes in surgery involving extremely young children, including those still in the womb. She first became a mentor to Izzie, and a commanding presence throughout the traditionally male-dominated hospital. Later, she took Alex under her wing, and romantic sparks started to fly. However, Addison became jaded as both Mark and Alex spurned her or fell short of her expectations.Determined to start over at the onset of a mid-life crisis, Addison tendered her resignation to Seattle Grace and left to join a Private Practice in Santa Monica, Calif., where some of her friends from medical school live and work.