Detective Cassie Maddox has an impressive resume. She's been:
Plus, she has Rob as a partner and doesn't kill him. If you're not clear on why this is impressive, be sure to read up on Rob elsewhere in this section.
Cassie's amazingly professional about all she's accomplished and survived. She isn't the type to brag, and Rob is impressed by her attitude—he says, "having worked hard to perfect an air of easy indifference, I recognize the real thing when I see it" (1.58). In other words, Rob's a big ol' faker, and Cassie is the real deal.
Because of her past experiences, Cassie is able to profile the killer with almost perfect accuracy in Chapter 8. She also is able to put herself into Katy's mind, which she does when she talks about the time the school janitor tried to molest her: "I thought that might just be my chance to be one of the children who go through the wardrobe into the other world" (10.252), she says. Her capacity for empathy is astounding, especially compared to Rob, who has none, and is still practically walking around with the mind of a child.
Cassie has personality and attitude, making her the opposite of the brainless girls that Rob usually goes for. Rob does end up going for her, though, but Cassie handles their relationship as casually as she says "I knew a psychopath once" (13.153) about her college boyfriend. Her laid-back nature doesn't mean she's not taking it seriously, she just doesn't turn into the melodramatic jerk Rob does.
Cassie is only relegated to the role of Rob's sidekick because he's the narrator. If this story were told in third person, Cassie would be the star. After all, she pretty much does everything and deserves all the credit for solving the case.
Unfortunately, Rob destroys their friendship, and it's hard to work with someone who acts like he can't stand you. In the end, Cassie leaves the Murder Squad and transfers to Domestic Violence. She also gets engaged to Sam, who is the opposite of Rob in many ways—genuine, nice, interesting. We're not sure if we'll see Rob again, but Cassie is the heroine of the book's sequel, The Likeness, so we haven't seen the last of her. While Rob settles in for a lifelong pity-party, Cassie's just getting started.