Denny Duquette, the doomed patient who fell in love with Izzie Stevens in Season 2, died in that season's final episode after suffering a post-heart transplant stroke. He subsequently appeared in ghost form during Meredith's near-drowning and for his and Izzie's truly mind boggling brain tumor-induced ghost sex storyline.
Likelihood of returning (again): Not very good. With Izzie out of the picture and Morgan occupied playing Negan on The Walking Dead, we're guessing Denny has checked out of the hospital permanently.
Photo Credits: The CW; John Paul Filo, CBS; Ron Batzdorff/NBC; George Kraychyk/Hulu; NBC, Peter Kramer/NBC; NBC; Gregg DeGuire/Getty Images; Jason LaVeris, FilmMagic; NBC, Chris Haston / NBC; Chris Haston/NBCU Photo Bank/Getty Images; Bill Records/NBC via Getty Images; Gilbert Carrasquillo, FilmMagic