Scott McCloud's The Sculptor opens with the following scene, where Meg asks David to whisper her something:
The scene is expanded at the very end of the novel, where Meg explains David that he can choose something the world won't remember about him - a secret that will die when Meg gets old and dies.
What did he whisper? At first I thought it would be "I love you", because he's a romantic, but then I realised they'd already exchanged those words; another secret I can think of is the details of his deal with Death, but he'd explained those as well by that time. What else could he have said?
(Scott McCloud talks about this scene briefly in this interview (obvious spoilers), but he doesn't reveal what David said)