At the beginning of the book, Dane comes out of his lair in the basement, smokes a cigarette, makes a sarcastic remark to Gigi… and then disappears. We may never find out exactly where he ran off to—aside from the fact that he definitely doesn't melt—but while he's physically not present, he's never absent from the story. As Miracle's only surviving parent, Dane plays a significant role in her journey toward coming to terms with her past.
As the son of the highly controlling Gigi, Dane's a pretty sad case. Miracle may see him as her father and a child prodigy, "the great Dane McCloy" (1.7) who published his first novel at thirteen, but the truth isn't that squeaky clean. In reality, Gigi forced him into his career and made sure he was "chained to his chair day and night writing" (26.30). Yikes, right? It's no wonder he rebelled and started sneaking out at night to see Sissy, Miracle's mom.
What this all adds up to—the tragic end of Dane and Sissy's relationship, plus a lifetime spent under Gigi's thumb—is a whole lot of resentment toward Gigi and abandonment of Miracle. As we see in Miracle, growing up with Gigi doesn't leave a person with a whole lot of tools for handling reality or building her own life (more on this over on Miracle's page elsewhere in this section), and since Dane makes it all the way to adulthood under Gigi's watchful eye, we can only imagine that he's even worse for the wear than our main girl is.
Beyond this, though, there's an awful lot of gray area in the book regarding the nature of Dane's character. Some of Miracle's descriptions of him sound almost manic—for instance:
Sometimes he'd say, "Listen to this," and he'd stand up and pace and read me a part of the story he was writing […] I never understood what he read to me, and sometimes he didn't seem to be reading words at all, just sounds. (3.13)
The guy seems a little unhinged, right? He's not even using words at points, but communicating in sounds to his daughter. Plus, his habit of covering the floor in wine bottles with candles in them and lighting them has a sort of creepy feel to it as well, though in fairness, his mother does commune with the dead. So.
Just like with Miracle's character, it's hard to tell how much of Dane's dysfunction is his and how much of it stems from life with Gigi. Still, when you add it all up, Dane's decision to leave his family makes a lot of sense, even if abandoning his daughter isn't justified. "In their last argument, Dane blamed Gigi for messing up his life," Miracle says. Maybe he left a big mess for someone else to clean up by leaving Miracle, but on the other hand, Gigi "messing up his life" sounds like the understatement of the year.