When Cassie and Rob need more help on the Devlin case, Rob calls in Sam O'Neill because of his government contacts. Sam, being the opposite of Rob, rounds out the team pretty well.
How is Sam the opposite of Rob? Well, he's honest, for one thing, with "tendency to directness that [Rob] always found slightly startling, in a detective" (5.45). He's also kind. And intelligent. And an all-around better investigator. Although, to be honest, we'd rather have Iggy Azalea as a detective than Rob Ryan. But anyway.
Rob kind of looks down on Sam for his innocence. He describes him as "earnest" at least three times (10.15, 15.39, 18.41) because of his wide-eyed naivety. Sam has fond memories of his uncle, Redmond O'Neill¸ and dude is even part of Sam's earliest memory (18.118). When Red ends up being a sketchy politician (is there any other kind?), the revelation throws Sam's ethics into a tailspin. The poor guy is actually really surprised.
Rob seems to cruelly relish the moment Sam loses his innocence, observing, "That kind of absolute faith is one of those things that, like virginity, can only be lost once" (18.41). Ouch, right? But it's Sam who wins out in the end. In the novel's last chapter, we learn that Sam and Cassie are engaged, and neither of them is talking to Rob anymore. Sam may be earnest, but he's not naïve enough to think that Rob could possibly be a decent friend.