Adam's dad, David Farmer (formerly Anthony Delmonte) is described by Adam for the most part as a lighthearted guy. He sings in a funny voice (sometimes while dancing [5.6]) and does impressions of old movie stars (8.13). Despite all this, he doesn't really emerge as having a distinct personality. Adam himself says that he always thought of his dad as "a cutout figure whose caption said Father" (8.9), and that the only time he showed his personality was when he was talking about books. Because we know that David was a journalist before the family's relocation, we get the feeling that he was a totally different guy as Anthony Delmonte. He probably had more of a personality – maybe because he was doing what he loved instead of selling insurance.
We also know that David Farmer is a decent guy. He reported government corruption when he saw it, sacrificing his own job and his family's safety for the greater good. And once Adam figures out that something is up, David is straightforward, not condescending, with his son. We know that he's humble because he says, "I'm not the hero type" (23.15) – despite everything he's done. Later on, when Adam thinks that Amy might think of him as kind of a hero (28.5), we appreciate even more the humility of Adam's father in rejecting that title.
David Farmer's good-natured ways lead him to follow Mr. Grey's advice, no matter what. He doesn't put up much of a fight on any account, and he always seems cool and collected, even in the most dire situations (like in Chapter 30 when he thinks he and his family are being followed). This calm temperament makes Adam's neuroses and fears seem even more extreme by comparison.
In the end we find out that Adam's father is dead. We don't know how, exactly, he died, but we know he's gone. It seems the men who killed Adam's mom in the car accident ultimately hunt him down.