Season 2: Denny Duquette
Denny Duquette is a charming and sweet guy, albeit one with a weak heart. Izzie takes a liking to him, and he even proposes to her after she cuts his LVAD wire to get him a heart. He dies anyway, leaving poor Izzie in shambles. Anyway, Denny's character is important because he helps us understand Izzie better — especially what boundaries she's willing to break for the ones that she loves. Plus, his case brings the five interns standing together as a front after the whole LVAD wire-cutting debacle.