Through seasons 2 and 3 of Grey’s Anatomy, Dr. Isobel Stevens, played by Katherine Heigl, had an ongoing romantic relationship with a heart transplant patient named Denny Duquette. He had been diagnosed with viral cardiomyopathy, which caused his heart to fail, and he had been a long-time patient of Preston Burke, Seattle Grace’s cardiothoracic surgeon.
Denny Duquette was portrayed by The Walking Dead‘s very own Negan, Jeffrey Dean Morgan, who rose to prominence after appearing on Grey’s Anatomy. Morgan, who’s now 57, is also known for his roles as John Winchester in CW’s Supernatural and Jason Crouse in CBS’ The Good Wife.
During season 2, Denny returns to Seattle Grace on the promise of receiving a new heart, which was supposedly lined up for him. It was during this time that he met Izzie, with whom he formed an instant connection. They flirted often, but it was Izzie who ultimately had to break the news to Denny that he wasn’t getting a heart, therefore he was discharged.
He returned to Seattle Grace at a later time due to heart failure. He was tachycardic and in order to slow his heart, the doctors needed to shock it, which stabilized him. While on an LVAD, Denny got to know Izzie better, eventually becoming her boyfriend. This all came to a head when Denny’s next donor heart fell through, so Izzie exacerbated Denny’s condition to move him further up the donor list. She did so by cutting his LVAD wire, causing him to flatline.
As a result, the heart getting shipped to Mercy West Medical Center to be used by Dr. Erica Hahn was redirected to Seattle Grace for Denny. However, after Denny’s surgery, despite the heart functioning properly, a blood clot at the incision point caused him to have a stroke and he died. This left Izzie heartbroken, never able to fully recover from his loss.
After his death, Denny continued to appear as a hallucination to Izzie until the cancerous tumor in her brain was removed.