Dennison Duquette, Jr., more commonly known as Denny Duquette, is a fictional character on the ABC television series Grey's Anatomy. The character is portrayed by actor Jeffrey Dean Morgan, and was created by Shonda Rhimes. He first appeared in the second season episode "Begin the Begin" as a patient of Seattle Grace, and returned later in "What Have I Done to Deserve This?," when he started a relationship with the intern Izzie Stevens; the two became engaged in second-season finale, "Losing My Religion," but Denny died later in the episode.

He appeared again in the third-season episode "Some Kind of Miracle," when Meredith encountered his spirit in her near-death state. He also returned in the fifth-season episode "Rise Up," appearing to Izzie over a seven-episode arc that led to a monumental revelation regarding Stevens' character in "Stairway to Heaven."

Fictional Biography
Denny first enters the hospital in the middle of Season 2, in the episode "Begin the Begin", hoping to get a heart since he is on the National Donor List. He leaves after he doesn't get it, but not before flirting with Izzie. He asks her personal questions, like if she is dating Alex Karev, a fellow intern at Seattle Grace Hospital, and when she says she is not, he gives her reasons to go out with him, instead of Alex. When Denny leaves the hospital, Izzie looks back at him and the look conveys her feelings for Denny.

Denny returns to the hospital a few episodes later, causing Izzie to break up with her now boyfriend, Alex. As Denny is in the hospital, he and Izzie get much closer as each day goes on. Izzie spends the majority of her time with Denny, on and off duty. Later on in Season 2, an opportunity arises where Denny has the chance to get a heart from another hospital, Mercy West. However, there are two claims to the heart and Preston Burke, head of cardiothoracic surgery at Seattle Grace, finds out that Denny is 17 seconds behind the other person on the donor list.

In an attempt to ensure that Denny gets the heart, Izzie cuts his LVAD wire hoping that Denny's condition worsens to the point that Denny gets the heart, but Burke is shot on his way back to check on Denny. Denny proposes to Izzie and Izzie agrees to marry him. Denny gets the heart when he is moved up to 1A status due to his condition worsening after his LVAD is broken. After his strength returns, Izzie offers him a chance to take back the proposal, because "you thought you were dying, and I was the one saving you, so..." but Denny tells her that his possible death wasn't why he proposed. He said that he finally had a choice, and he chose Izzie to be his wife because he was in love with her.

However, Denny dies of a stroke shortly after Izzie accepts his proposal. Sometime later, Denny's father arrives to question Izzie, telling her that both he and Denny's mother believed Denny to be long dead. He questions Izzie's quitting the surgical program after agreeing to marry Denny. He insinuates that Izzie was nothing more than a gold digger, looking to get rich off of Denny. When Izzie finally explains the situation, she discovers that Denny has left her $8.7 million and that shortly before his death, he contacted his parents for the first time in years, leaving a message on their machine asking them "get on the next plane down here and meet my girl... the wonderful, talented, and incredibly stubborn Dr. Isobel Stevens, who's getting me a new heart and has agreed to marry me." After a long debate on what to do with the money, Izzie uses it to start the Denny Duquette Memorial Clinic in honor of her dead fiancÚ.

In the fifth season, Denny returns as a ghost that only Izzie can see. At first, Izzie believes she is going crazy and he is no more than a hallucination. However, she eventually accepts that he's real, sharing a kiss and ultimately having sex. However, Izzie was in the midst of patching things up with Alex and continues to do so, making Denny incredibly jealous, sparking several arguments. In a final argument, Izzie realizes that Denny has returned for her because she is sick and dying. Unable to accept this epiphany, Izzie demands that Denny leave (supposedly to return to Heaven) and they share a final kiss before he vanishes.

Izzie later discovers that she has metastatic melanoma and believes Denny to be a hallucination caused by the cancer.

Season 5, Episode 22. "What a difference a day makes", Denny is present after Izzie and Alex's wedding. They talk for a brief moment and then Izzie tells him to leave so she can be alone with her husband. A clearly upset Denny leaves soon after.

In the season five finale episode, Izzie again has another hallucination of Denny. This time the two of them are on a beach talking about Izzie's future life with Alex.

Information from Wikipedia